Re-using the heat in bathwater in Britain | Hacker News Reader

Re-using the heat in bathwater in Britain

dynomight.net

172 pointscolinprince249 comments

I tried this once on a cold winter day: I let the bath cool down to room temperature (which was <20C) before letting it drain, with the bathroom door open. The result was a lot of condensation on the windows in the surrounding rooms. I figured the high humidity was going to cause more damage than it saved money, so I scrapped the idea.

Although higher humidity should make it feel warmer given the same ambient temperature. So if you're increasing the temperature mildly using this method AND increasing humidity, it may compound one another positively.

Although, it is a minor efficiency gain either way.

I thought high humidity made it feel colder, when the temperature was relatively low?

Hot damp air feels hotter than hot dry air, but I've had the idea that cold damp air feels colder than cold dry air. Is that a misconception?

I agree with you. Water facilitates heat transfer, and the surface of skin is about 95f, so it seems that below that temperature, and assuming you aren't already sweating for the evaporative cooling effect, humid air should move heat out of your body more effectively than dry air.

I think “dry” and “damp” are two extremes. I feel warmer in 45% rH at 67°F in winter than at 30% rH same temperature, but the air doesn’t feel damp (OTOH 60% would start to approach feeling damp).

That being said, I think at normal conditioned air temperature ranges more humidity virtually always feels warmer. I’m not exactly sure where the cutoff is, but I’d guess below 60°F might be a decent guess?

I think you're right and actually the extra moisture in the air would also mean that raising the temperature of the house requires more energy input since the specific heat capacity of water is higher than air.

I'm in the UK, extra humidity is the last thing I'd ever want in winter.

At the moment, I've got two beefy dehumidifiers running and I'm still at between 70-85% humidity in each room. I don't have any damp or leaks, it's just that the outdoor humidity has been 80-90% for the last few weeks, so ventilation is futile. And because of the weather at the moment, we're drying clothes indoors.

What is the temperature outside? Humidity is measured relatively (to how much water air can hold at the current temperature), so even 100% humidity outside at a moderately chilly temperature outside can still translate into uncomfortably dry (e.g., less than 35%) humidity inside.

Currently about 17c in the day, 14c at night. At the time of writing, outdoor is 13c and RH is 95%. It was 22c the other day, however it is quite unseasonably warm right now. Once it gets into proper winter weather, normal daytime climate is around 10c-ish and 80%-90% humidity. I've never seen my indoor humidity lower than about 50% without a dehumidifier except after a really long dry spell in the summer or a really cold day in winter.

It doesn't often get that cold here, just very damp. Global warming is making winters even wetter. The UK actually has a fair bit of "temperate rainforest" and is classed as a humid temperate oceanic climate.

What region of the UK are you located in?

Inner London, which is going through a bit of a damp period right now after a really dry summer. I'm not sure if those two things are related, but technically we are still in drought even though my garden is basically a swamp right now.

My tropical plants are enjoying it though.

You could place a plastic mat on top to limit evaporation.

You could also switch to a water conserving mist shower and reduce the amount of heated water used for the shower.

You could also reclaim the water from the shower for use in the toilet by turning the supply to the toilet off and manually transferring it to the toilet tank.

There are lots of things you could do to save resources but most things are not done and are not going to be done because it is inconvenient and takes time, and time is money.

Edit: now that you have me thinking about this idea, it would be possible to have a container in the bathroom attached to a plug that has a hand pump and a tube going to the drain that could be used to reclaim and hold the water that could be higher than the toilet supply and the pressure from it could be used to have it self fill the toilet tank. The container could be highly conductive material like stainless steel and it could be relatively sealed to avoid humidity issues. This would of course be a vector for illness in some cases.

Combo that with toilet sinks and a lot of water would/could be saved per year/person.

The same tank could be connected to the sink too, turning any toilet into a toilet sink.

Britain is not suffering a water shortage, nor is it likely to in any foreseeable future. Not that I'm encouraging people to waste water, but conserving water doesn't address the unfavorable economics of energy.

If they used less hot water they would use less energy heating the water.

That was the original thought until I went off on the tangent about saving the hot water in a separate tank from the bathtub after it is "used" for the bath allowing the heat to be harvested and the water to be re-used.

The original post is about leaving the hot water in the bath to use the energy to heat the house or help heat incoming water. The idea of an external tank made out of conductive material would eliminate some of the problems and also allow the water to be re-used for the toilet.

I see your point but I am not sure about this. I suspect a lot of people use just as much water but spread it out longer. In California there are a lot of devices mandated to reduce consumer use (flow restrictors and so on), but interior water use often demands fixed quantities. For example if you want to have a bath rather than a shower then people will use the capacity of the tub, likewise if you boil water for cooking a flow restrictor just means it takes longer to fill the pot to the required level. So people use just as much but just suffer lower pressure.

Toilet cisterns arguably do reduce usage but then many people complain about less effective flushing which requires additional water; I don't have data to know if these objections are just a cliche or reflect an actual problem. My understanding is that lawn-watering is the biggest waste of water by consumers, to the point that some local authorities will subsidize people reorganizing their gardens to be more suitable for a dry climate.

As for the issues flushing large, ahem, amounts of waste with the low-water toilets, it's mostly just cliche now. When the restrictions were first added, the toilet manufacturers weren't ready. Probably a lot of the complaints were from experiences with these early models.

Now, low-flow toilets have an amazing amount of research and design put into how to meet the guidelines while still performing as well as, and often better than, old 4+ gallon flushes.

A few (ill-informed) complainers aside, this actually seems like a win all around. Toilets don't cost more than they used to, they use less water, and perform better than ever.

What California really needs to do is ban alfalfa farming for export. It is infuriating how much water goes to that crop alone for absolutely trivial amounts of return, compared to what the water consumed could have been used for. Water rights in the American West are an absolute cluster.

Hosepipe bans are pretty common in the UK due to water shortages

I think that has more to do with under-investment in infrastructure. Britain does have recurring droughts but they're pretty shallow.

> I think that has more to do with under-investment in infrastructure.

We have a lot of leaky pipes. I don't get around as much as I used to but it's not uncommon for me to see a different leak every time I go for a walk around SE London.

Saw this a couple of years ago in the Grauniad:

https://www.theguardian.com/environment/2019/mar/18/england-...

[redacted for cynicism]

> There are lots of things you could do to save resources but most things are not done and are not going to be done because it is inconvenient and takes time, and time is money.

This is radically underselling how much most people are willing to do for 50 pounds. If people are being squeezed out of their lifestyle due to energy costs (which seems quite likely, looking at the aggregate stats) they'd spend the time to conserve energy. Assuming someone shows them how to do it.

It remains a mystery why everyone was so calm walking in to this while tightening the screws on fossil fuel industries to stop them investing int he future. The globe needed a supportive regulatory environment for more nuclear power 20 years ago. Now there appears to be a global crisis in progress.

> It remains a mystery why everyone was so calm walking in to this while tightening the screws on fossil fuel industries to stop them investing int he future.

Because the only half-solutions we got for mitigating climate change were drafted by people who are largely free market types, and they can only envision free market solutions - which require end users figuring out how they are going to deal with more expensive energy on their own.

That does, of course, result in quite a bit of pain along the way.

The German situation is pretty illustrative of what would have happened to anyone stupid enough to try and build more power plants in Western Europe - nuclear stations would have been shut down, fossil fuels have been staring down the barrel of the burgeoning Green movement for at least 20 years and are somewhere close to being hit with legislation designed to make them unprofitable.

Renewable haven't been cost-effective (or technically feasible in some cases, renewabled seem to have been a warning sign for incoming grid instability so far) and all the alternatives have been blocked by politics. I don't know about Europe's environmental legislation but it would be no surprise to discover even most renewable plays were facing legislative challenges too due to the area they take up.

For a market solution to work somebody has to be allowed build something and let it run for 20-50 years to reclaim the capital costs and make a profit. Speaking as someone who would quite have liked to be investing in energy plays about 5-8 years ago when the problem started to become apparent, the legislative environment in the west is too fraught to risk it. We weren't going to start doing anything useful until a crisis happened to get some political cover for the unpopular energy sources. Here we are, so now maybe something will get done.

On the contrary: everyone who invested in Europe in building any sort of capacity was handsomely rewarded (incl. nuclear).

Energy prices are so high because too many guarantees were given to too many parties to entince investment.

They aren't building new capacity in most forms of energy generation. Eg, in the UK they've basically only building renewables [0], and not in sufficient quantities to maintain their electricity production. The story is similar in most EU countries last I checked. Compare to, say, India to see a healthy approach [1].

Sure the market is signalling that anyone who can bring energy to the table will be rewarded, and trying desperately to keep people in the game who are already playing. But there'd be capacity built and the aggregate numbers wouldn't look so bad if new construction hadn't been politically blocked for years now.

They're choosing de-industrialisation rather than letting people use nuclear or fossil fuels. People really should be panicking about that, they are going to take a massive lifestyle hit. There will be trouble.

[0] https://en.wikipedia.org/wiki/File:UK_electricity_generation...

[1] https://en.wikipedia.org/wiki/File:India_electricity_product...

It is a bit weird to call building of fossil plants "healthy" when it is leading us down to environmental destruction.

Nuclear on the other hand is game over everywhere except China. It won't come back within the next 20 years. After this renewables and power grids optimized for renewables (across countries and with lots of decentralized storage based on electric cars) will make any type of energy generation which isn't wind and solar infeasible.

Nobody is choosing de-industrialization. Cheap gas from Russia has been keeping energy cheap in Europe. People are choosing solidarity with Ukraine even if this means some harder years.

The global crisis is not because we under invested in the future of fossil fuels. They must have no future. The current crisis is because we relied on Putin.

Interesting thought on graywater. It really does seem easy enough for shower/washer/non-kitchen sinks to drain to toilets.

The problem is how to keep such a system clean, as then there will be a bunch of foreign gunk floating around in your cistern.

Yeah and bacteria would build up in the reserve tank.

An inline filter of some sort could be used for particles but that would require maintenance.

There would also be soap scum build up and hair conditioner products would gunk up the works I'm sure.

In Japan they don't seem to have a problem to put a sink (for handwashing) that drains to the reserve tank:

https://imgcp.aacdn.jp/img-a/1200/auto/global-aaj-front/arti...

I've noticed that when I put my shower on mist I have to turn the water temperature up quite a bit to have the same felt temperature. The small water droplets seem to cool a lot faster. I'm not sure that it is actually more energy efficient.

On one of my visits to japan my hotel had a toilet with a sink on top of the tank. Really loved the idea of flushing and washing my hands at the same time. The water used to wash your hands ends up being used for toilet water anyway. So you're not really wasting water at a completely separate sink.

It could also be that your windows are not insulated enough. For 20C and 40RH the windows need to be colder then 6C to get condensation.

The relative humidity of a bathroom after a shower must be much higher than 40% - I would imagine getting towards 100% (hence room temperature mirrors steaming up). Where I live (UK) I don't think I've ever seen an indoor humidity as low as 40% - it's typically in the 60-70% range.

Moisture has to go somewhere even if you don't see it visibly condensing on windows.

Either it's being absorbed or you have something removing moisture from the system (venting, etc).

House code in Finland requires mechanical ventilation with vents in the bathroom. I only see brief condensation on my bathroom windows when I leave the sauna door open in the winter (approx. 100 celsius, 80%RH).

With good building codes, you won't have condensation issues.

Alas, the UK has the twin problems of poor building codes and lots of old housing stock that nobody puts much thought into maintaining. In the latter case, people will take single skin brick buildings and apply "vapour control" materials outside the building, thus ensuring moisture is locked into the bricks. Then they wonder why they have damp, etc.

Alas, we are a very silly country.

What kinds of material is added to exterior brick? Insulating panels?

Sand and cement in my house's case. Rubbish insulation, utterly impermeable - combined that means the dew point (where water vapour condenses) is within the brick. The freezing point is too, and bricks are not exactly flexible when saturated with expanding and contracting substances.

Some insulating panels are also dreadful, but on the whole a semi-competent installer will do the thing right. The problem is, some aren't even that good - we're talking a minimum bar of an NVQ level 2, and that's if people don't decide to just.. go self employed and do it without training. No regulation at all, and there can't really be any until it is a licensed occupation.

As a bather, I also tried this idea a couple of years ago. I thought condensation would be an issue but the house I live in seems to have low humidity. The bigger downside for me was having to expend more time and effort cleaning the sides of the bath.

This summer, I tried taking cold showers as an alternative energy conservation measure. So far, this Autumn has been very mild in Ireland so I've been able to keep up the habit (I still treat myself to a bath once a week or so).

It occurred to me to try this for the first time last week and the extra cleaning thing swung it for me too, which is a pity as there's a good 40 pence worth of heat in that water at current prices. Not sure the wife was going to be too pleased with my used bathwater hanging around all day anyway. Also, probably not a very good idea for anyone with young children in the house.

Living in Germany and getting both my heating and hot water from natural gas I regulary track my gas consumption throughout the different seasons. With 1-2 showers per day in the household my daily natural gas consumption in winter is about 20x higher than in summer, so compared to heating the house the energy in the hot water is a drop in the bucket.

Same here (I live in Poland). 2 + 2 family with small kids. Hot water is 5% of my overall gas bills. Rest is heating.

Note that a portion of that increase in gas consumption is still due to the hot water usage, even if the number and duration of showers stays the same - tap water varies in temperature through the year, getting colder in winter and warmer in summer. In my case, it varies between ~5 and ~22C. Germany has milder winter temperatures so likely not as much variation, but still a significant fraction of the temperature increase imparted by your water boiler. Additionally, if you have a hot water tank (as opposed to on demand), then the tank has to work harder to maintain temperature (assuming you maintain the indoor air temperature differently in winter than summer).

All combined, that might account for a 2 to 3 fold increase, the rest of it would be caused by space heating.

Well water stays very consistent year round in most areas.

Some municipal systems vary more depending on where they get it and how they store it. Water towers used to smooth out water demand for example can noticeably impact water temperatures.

Temperature at the well stays consistent, but it can vary at the tap. At a family member's secondary property, tap water will vary from 2 deg in winter up to 15 in summer (because the line between the well and the house is only ~3-4 feet deep, and surface temperature changes a lot). If we were drawing more or less/line was different length/different depth, the variation would change.

(I know all these values from homebrewing, where water temperature plays a big role in how long it takes to cool)

Very rarely used property you can get temperature swings underground next to these pipes. But in normal domestic or municipal settings the regular water through the pipes is going to equalize temperatures year round to within a fraction of a degree at 3-4 feet underground outside of extremely long runs.

Also of note, ~25feet of one inch pipe holds 1 gallon of water. So there can be quite a bit of lag involved, but water is unlikely to spend that long in these pipes to change temperature. Thus the ground temperature would need to be extremely different to make a significant difference once the temperature equalized.

The various municipalities I've lived in require 5 to 8 feet minimum bury depth for water pipes to avoid freezing. The seasonal property I mentioned earlier being at 3-4 feet depth is likely helped by the fact that snow is not cleared above the run (additional insulation) and it is in one of the milder climate areas where a municipal spec water run would only need 5 feet of bury. It may also have half inch of foam wrap for insulation, I'm not sure.

Agree on the pipe volumes.

That’s likely overly cautious. https://www.hammerpedia.com/frost-line-map/

That said, municipality’s will often be overly cautious and they do need to consider property siting abandoned.

> That’s likely overly cautious

I'm not in the country shown on your map. The area of Canada I'm in would be somewhere roughly around the 90" line on that map. It's between the 2000 and 2500 lines on this map [1] (the seasonal property is around a 1500 contour)

The municipalities pay for the main to be installed. They wouldn't spec it to be 8' (2.4m) deep if it didn't need to be.

edit: the contours in my source are not mm of freezing depth, but degree-days of freezing. However, 2500 DDOF corresponds[2] with 70" of frost penetration, while the 1500 DDOF corresponds with 55". So the required bury depths are slightly but not significantly conservative

[1] https://www.urecon.com/applications/images/canadian_map_lrg....

[2] 1962 source: https://nrc-publications.canada.ca/eng/view/ft/?id=15f6f5eb-...

I agree they need to bury it nearly that depth but while IPC 2018 section 305.4 puts water pipes at least six inches below the frost line many places increase that to 12” for water and or sewer.

I don’t know much about how Canada does things.

Yeah, you really don't want those sewer lines freezing...

I'm very confused at the lash back you're getting. Have you all never seen the pipes freeze? Or worse: burst? They wouldn't do that if they didn't swing in temperature. I'm not convinced of SEC's 2-3x estimated increase but the claim that water temperature delivered to your taps vary in temperature is an objective fact. Of course different areas have different infrastructure and can be more resistant to freezing and rupture, but turn on the cold water in the dead of winter and you'll find it is a lot colder than in summer.

I agree water temperature at the tap can vary quite a bit. My parents place has the water go through an unheated crawl space and they would regularly keep the tap on to avoid the pipes freezing when temperatures got unusually cold. Further many municipalities extract water from rivers which experience significant seasonal variation etc.

My comment was simply in reference to well water which has very consistent temperature and will warm or cool the soil surrounding pipes at any reasonable depth if you are using water regularly.

The original point was that a shower would have to use more energy to heat shower water in winter. Not sure now how any of your comments relate to that - do you agree with it?

The relationship is many people don’t experience that. Aka it’s a you might experience X not a you will experience X.

A poster mentioned failing to get a cold shower in Singapore, but Singapores ground temperatures are much higher than we are used so well water is going to be ~25C year round and tap water can be significantly warmer than that.

People don't experience it because the water heater output is a consistent temperature. There's nothing to experience with increased consumption of electric/gas/heating oil, especially when those concurrently increase for home heating

That assumes the water going into the water heater changes temperatures which as I just said isn’t always the case. Some people take “cold” showers year round and the tap is very consistent.

Trivially people in the tropics don’t have a winter, but well water can similarly have very consistent temperatures independent of seasons.

> I'm not convinced of SEC's 2-3x estimated increase

It's a simple calculation, but very location specific. In my particular case, if the water heater is set to 50C, and tap water is 22C in Summer, it has to impart a ΔT of 28C. If your tap water is 4C in winter, the ΔT is 46C. Not quite double, but close to it. If your water heater is in a poorly conditioned space, then it would lose considerably more heat to the surrounding space when outside temperatures are -15C than when outside temps are 35C. All told, hot water energy usage for my particular situation is between 2x and 3x. Other situations would vary, but still significant.

Once in Singapore when I was invited by a friend in a highrise flat I wanted to have a cold shower. However the cold water was still very warm and I didn't feel refreshed. I think the tropical sun warmed up the water in the pipes in the walls.

Where do they store the water? In NYC it's not uncommon for a building to have a holding tank on the roof of the building and then use gravity to supply "pressure" to the units in the building. If Singapore does something similar, it would be logical that the water would be warmer.

I lived on the top floor in a newer building in Dallas that pumped water the entire way and the pressure was noticeably less in my apartment than in the gym on the ground floor. The water was "cold" though!

22 degrees is the coldest tap water? That means you basically need ice cubes or fridge storage before you can drink it in the summer?

Strangely enough, drinking water is served at temperatures up to 100C in certain preparations (eg tea)!

Less tongue in cheek, yes, people will often chill their tap water in the summer before drinking. But it's perfectly drinkable at whatever temperature it comes out of the tap. The water supply in my locality comes from a surface reservoir, leading to the higher summer temperatures.

I just never reflected on the idea that cold tap water in summer is quite luxurious. I have that but I have no idea why tbh.

Indeed. Having living in SE Asia where you can drink the tap water, it’s rough getting a glass of water from the tap and it’s 25C.

There is no “cold” tap, just warm and hot taps.

> SE Asia where you can drink the tap water

A very verbose way of saying Singapore.

Your way is more verbose, by two characters to be exact.

You can drink it other countries! It's risky, but you can drink it!

Taiwan, Japan, Singapore. Only countries you can safely drink tap water in Asia AFAIK.

Also South Korea, Brunei and Hong Kong (apparently).

But I have friends who rinse their mouth, brush with tap water in Vietnam and haven't gotten sick. It obviously varies a lot by which city (or even within the city), but the water is tested and obviously highly chlorinated (by smell).

I mean, I wouldn't drink it regularly because I don't think they look for non-biologic toxins like metals.

I've rinsed my mouth in most countries in SEA. But never drink it. In Taiwan you can drink tap water but vast majority of people boil it because apparently alot of the pipes in homes are old so they boil it first. Sometimes I'm lazy and don't want warm water so I just fill my bottle with tap water.

In Arizona, "cold" water at the tap can easily be in the neighborhood of 100F/37C all summer. We kept a water filter pitcher in the fridge for drinking water, so it wasn't really a problem there, but it did make it impossible to take a truly cool shower or follow a recipe that instructs you to rinse something in cold water.

I grew up in AZ.. just run the water until it cools down. Unless your showering from an outside hose it'll be 50 degrees just from being underground. If you aren't getting that then something is very wrong with your plumbing I'd recon. Like, the tap intake comes into the house after the swap cooler on the roof kind of thing. Plus the evap after the shower.. that'll cool you off.

You sure it'll be 50 deg? that's significantly colder than summer tap water in my Canadian city, which is closer to 70F at peak.

Not sure where you grew up, but most of the population gets tap water from the CAP, which means the water ran for hundreds of miles in canals across the desert before going into a water treatment plant. It’s 90F+ easily in the summer in any modern builds in Tuscon/Phoenix.

Cap water is recharged into the groundwater aquifers.

Also you’ll notice no water towers.

And it’s Tucson… not Tuscon.

> Cap water is recharged into the groundwater aquifers.

Not as part of the “to tap” process.

> Also you’ll notice no water towers.

Not relevant. CAP boosts elevation many times. A water tower is needed if you don’t have a high pressure water source like the CAP canals and the salt river reservoirs.

> And it’s Tucson… not Tuscon.

Don’t emotionally lash out with irrelevant shit. Focus on the content, not the presentation.

>> And it’s Tucson… not Tuscon.

>Don’t emotionally lash out with irrelevant shit. Focus on the content, not the presentation.

If you can't even get the spelling of a major city in Arizona correct, your content is... as the kids like to say, "sus".

You’re still struggling to grasp any of the content. Your whole reply is only focused the response to your unrelated spelling dunk.

Having such superficial interactions with the world is congruent with believing the meme that all of Arizona drinks well water though.

I hope you do take some time some day to learn about the state you grew up in. The water projects are quite impressive engineering because there is no other option. The rural holdouts that pumped their groundwater dry learned that lesson the hard way.

Look dude it is you who doesn't understand how it works, at least as far as it concerns Tucson: https://storymaps.arcgis.com/stories/70f626809fd84e228c0f32a... https://www.tucsonaz.gov/files/water/docs/coloradoriver.pdf

They tried "to the tap" and it pissed everyone off because the river water leeched out all of the rust in people's pipes.

Water lines in Arizona single family homes are very commonly routed through the attic, where they can get very hot, so your imagined scenario isn't really that far off from reality for a lot of people.

I don't know how many gallons I would have to waste to get the tap to start delivering water at 50°F. I could run the faucet for several minutes and still get water that was body temperature or warmer.

> Strangely enough, drinking water is served at temperatures up to 100C in certain preparations (eg tea)!

It's prepared at 100C, and _may_ be served at 100C, but it does not enter your body at 100C or you will burn your mouth.

> But it's perfectly drinkable at whatever temperature it comes out of the tap. The water supply in my locality comes from a surface reservoir, leading to the higher summer temperatures.

It may be drinkable, but temperatures above 25C will be a cesspit for legionella.

But did it even do anything in the first place? Because if the temperature wasn't measurably raised, it's not even worth considering the 'how to deal with humidity' question that people are discussing.

If you really wanted to do this and remove the moisture problem you could probably cover your tub full off water with a plastic sheet to cool off.

Simplest use case would just to have a container in the basement / garage through which the water flows, assuming you are heating it at least a little. You likely keep lower temp there so it is much more efficient. Although in practice drain from the shower or bath quickly connects with the one from the toilet so it's not that easy to implement.

In your case efficiency is also slightly lower due to evaporation. If anybody has any idea how significant it is I'd love to learn. I was surprised one day in winter to find that my evaporative humidifier has temp below 10C on it (room temp 23C).

I think ground heat pumps could have an exchanger around the main drain pipe which should work nicely both in winter and during summer.

My smoke alarms go off.

You might have benefited from MVHR, dMVHR, or an EAHP:

* MVHR extracts air from "warm" rooms like kitchens and bathrooms, runs it through a heat exchanger to warm new supply air, and then pushes the supplied air into other rooms in the house. It stands for Mechanical Ventilation with Heat Recovery. These systems can significantly increase the efficiency and comfort of your house, provided you also air tighten.

* dMVHR is similar, supplying fresh air but reclaiming heat, except the "d" is for distributed. They're like extractor fans but they don't waste the lovely heat.

* EAHP is an Exhaust Air Heat Pump. Usually they sit on top of an invented (eg pressurised) hot water cylinder, and they'll take hot, humid air out of whatever rooms you duct them to, heat you water for a quarter of the cost of an immersion heater, then blow cold air out of your house. This is a great option IMO (I have one) but unlike the first two options it depressurises your house and draws cold air in from outside through drafty bits of your house unless you have a corresponding system to warm incoming air at a similar rate to it being expelled.

tl;dr there are options to recover the heat while expelling the moisture!

We run a humidifier in the winter (Eastern US) because of dry winter air made even worse by the heating process, so this would be a benefit if there is some air circulation to move that moisture throughout the house.

I appreciate this attitude:

> Should you do this?

> I render no judgment!

> But here’s a thought: It’s OK to use energy for convenience. This is true even in a time of crisis. After all, only a small fraction of the energy we use is necessary to stay alive. We could live dirty and shivering in dark rooms, eating oats and only moving around on foot. But no one does. So the question is how much energy you want to use for how much convenience.

Too many articles or conversations about energy usage seem to take an attitude more like, “if you’re not doing the thing we say is the solution then you’re the problem”.

I couldn't agree more.

That type of sentiment is pervasive, and it's absolutely baffling, because we as a society can keep all our excesses and comforts while making the consequential energy expenditures completely green. It seems like people are just incapable of conceiving of solutions that don't involve morally shaming others into being less happy. Or a more cynical take: It seems like a lot of people enjoy "solutions" that stick-it to some other part of the population.

"That shower you enjoy uses too much water, you should take shorter and weaker showers." "Shame on you for flying to see your distant relatives, you should take fewer flights." "How dare you use a comfortable car to get around? Unless you're unhappy in a bus with at least 10 other people, you're a bad person."

Without even considering other sources of green energy, there is more than enough solar energy reaching the Earth to allow all 8 billion of us to enjoy consumption levels typical of Western nations.

I won't be needlessly wasteful but I'm not going to inconvenience myself over a corporation's marketing dept. trying to avail its company's tremendous carbon footprint.

I totally agree. However, if you have the means to spend a bit more on appliances (with payoffs 5-10 years out), then you can easily reduce energy / water (ignoring grass/crops) usage by about 75% vs 10-20 years ago with zero lifestyle impact.

Not doing that is needlessly wasteful.

In the particular case of showers, get a High Sierra shower head. They have more apparent flow / pressure than a full flow one, but use a small fraction of the water. They cost under $50 and are extremely reliable. (Never had one clog, and never did any maintenance on one, even with extremely hard water.)

The article estimates scavenging the bath water heat would reduce total energy consumption by 1.2%. This is much better achieved by lowering the thermostat, or showering less often in the winter. (Or buying a low flow showerhead -- see above!)

Note: No affiliation with High Sierra. Happy customer for a decade, across multiple homes. We currently have a 1.5 GPM, but I throttle it down with the trickle valve. The 1.25 GPM is totally fine; the trickle valve might make sense for it too. Note that you can customize colors, etc:

https://www.highsierrashowerheads.com/shop/classic/

Many of these things are holdovers from an earlier era. Have you ever used a shower from the 30s? It's like a wall of water. Using something like that compared to a generic showerhead today is wasteful. But now we're at diminishing returns territory, eking out every fractional water savings.

I think the answer is actually neither "you are bad for doing x" nor "doing x is fine", but rather that intentionality is important. Taking time to think of your impact on others and saying, "I'll sacrifice here and put a burden on others there" with intention is fine. I'm flying to my grandmother's funeral. I recognize this pumps tons of carbon into the air. I'm intentionally conscious of this and acting in a way I think others should be allowed to (funerals are important!)

That said, driving a car by yourself is pretty bad for many, many reasons (safety, efficiency, pollution, road damage, parking needed, noise, tire and brake dust, and about a million other things.)

It's ok to make bad and inefficient decisions, imo, if you are doing so intentionally rather than just because that's the default.

It's about thoughtfulness, awareness, and intentionality.

You shouldn't let warm water stay in a place with air contact. First it makes air humid and the affected rooms prone to mold, and second evaporation takes energy away, and while condensation might give the energy back you will have wet conditions. It's not very sanitary: warm and wet conditions for some time tend to encourage unwanted life.

I armchair thought about a scheme: collect the bath and shower water in a tank in the cellar and use a heat exchanger. However the tank will turn disgusting very quickly. Perhaps a better solution is using a large, long pipe with a flat floor with little inclination, such that warm water does not stay for a long time but flows slowly. Cool the floor of the pipe with a heat exchanger. Perhaps have a contraption to regularly have the pipe brushed from inside and flushed with cold water. Instead of a heat exchanger, use cold water, but this has the problem that fresh cold water needs to flow at the same time as the used warm water.

It is worthwhile?

In the winter, the air is very dry in my house and we use humidifiers. By leaving the bathwater in the bathtub, we could kill two birds with one stone.

Everything you say about ventilation is right though. I just don't know if calling this unsanitary is universally true. Maybe in climates where humidity is a problem to begin with, adding more humidity leads to less sanitary conditions.

I agree, sometimes I do that, too. However the whole October was warm and humid and I never let the water stay in the bathtub. I carefully check for humidity: it should be between 40% and 60%. Sometimes I hang washed clothes in the room to get more humidity.

I have been considering a grey water solution that would achieve two things. The bath in my house is upstairs, and I have an additional toilet downstairs, almost directly underneath. So, I could let the bathwater drain into a tank above the toilet cistern, from where it can slowly release its heat, and then use the water to fill the toilet cistern for flushing. That way, I save on heating and on water usage, and the water wouldn't be evaporating into the house causing humidity problems.

I did the calculations. A 120l bath has water costing £0.53 and heating (by gas, if we assume the incoming water is about 10C and heating it to 40C, about 4.2kWh) costing about £0.44. We probably get out of the bath and drain it when it's about 35C, so if the house is at 22C then there's about 1.8kWh of energy that would get transferred as heat to the house, about £0.19 worth. So, for a very modest cost of installing a tank and a little bit of plumbing (probably less than £200), we'd be saving £0.53 each day (assuming all that water gets used) plus £0.19 each day that we would have the heating on, probably half the year, which is around £230 per year. It'd pay itself off very quickly.

The main concern is that the holding tank doesn't become a bacterial breeding ground, so it may need some chemical discouragement every now and again, but that's hardly difficult.

That's a clever solution.

In a different vein, I thought about dismantling the mobile air conditioner and putting it in a cold water bath instead sucking air to heat to blow to outside. The lower part is wet anyway from condensate.

This summer I did something different and simpler: instead of a hose to outside, I made a wooden frame fitting into the window and strapped the air conditioner to it such that it sucked the air from outside instead from inside. I think it worked better this way because it didn't create negative pressure in the flat anymore and because it could blow the hot waste air directly to the outside. The fans didn't need to fight against the negative pressure and the long hose.

I was actually looking at heat recuperation systems for bathrooms recently and stumbled upon https://ecodrain.com/en/ - if we hadn’t already remodeled our bathrooms I’d have loved to have installed this.

For now we make do with a heat pump water heater which is very efficient.

I wonder how different it would be to use an off the shelf heat exchanger, e.g. this one? https://www.ebay.co.uk/itm/263272702860

You'd probably have to filter the shower water before sending it through, but beyond that I wonder how different the principle is. Maybe being easy to clean is a significant factor in drain showers.

There may also be problems with sanitation rules, only a minimum of water should sit in the pipes, and then ideally hot our cold but not luke warm.

Got a link (or explanation) for that temperature stipulation?

Temperature window for bacterial growth. Above about 120F harmful bacteria commonly found in water won't proliferate, but will survive. As you get hotter from there, they're killed off increasingly rapidly. Above 135 or 140 or so they don't survive long. In cold water they won't die off but also won't multiply quickly. The dangerous range is in the middle.

Yes, as tempestn explained this is to prevent bacterial growth, especially Legionella. Otherwise this can be a problem in areas where tapwater is not chlorinated.

I think doing adequate filtration of the full waste water flow without losing most of the heat would be a challenge. And remember you're also sending soap suds down the pipe, which will pass the filter and leave deposits inside the exchanger.

Also, if you're just using gravity to push the waste water through, what flow rate would you get on one of those units, and how many would you need to handle the full flow?

The most persuasive waste water heat exchangers i've seen are based around the a large-diameter pipe, with conical sections connecting it to the input and output, where the waste water flows down the inner wall of the pipe in a thin layer, and the cold water flows up the outside. It's not as space-efficient as a conventional heat exchanger, but it avoids the problem of blockage.

That would also be fairly easy to improvise, i think, given that you can buy copper pipe, or even buy copper foil and wrap it into a large, thin-walled cylinder.

There are numerous manufacturers of these things, eg:

https://powerpipehr.co.uk/

https://showersave.com/vertical-wwhrs/

https://recoupwwhrs.co.uk/

Ultimately they're just cleverly arranged copper pipes, so the barrier to entry for new manufacturers is low!

I am keen to get one - my shower sits directly above a cupboard on the floor below, where it would be simplicity itself to fit.

This article suggests the energy savings are not particularly big:

https://www.thegreenage.co.uk/tech/waste-water-heat-recovery...

... but there are several comments reporting the opposite!

It depends heavily on how much your house showers, what the water temperature is at the inlet to the house, and how ideal your plumbing is. The efficiency is highest with a vertical unit at least five feet in length, and that’s really just down to your house’s layout.

Note the manufacturer of the GP's device claims a 33% energy savings. You can get a hybrid heat pump hot water heater with a coefficient of power of 4, for a 75% savings. If your water heater is in a garage, it is likely a simple retrofit. (If it is in a small closet, it will require a duct).

Ignoring installation, the heat pump water heater costs more than those devices. With installation, I imagine it is cheaper.

Given the coefficient of performance of heat pumps is a function of the difference in temperature between the two mediums, if you can use heat recovery on the shower water you can boost your COP further by having both. I'm in the midst of this exact system being installed: it'll be extracting hot, humid air from the bathroom and receiving pre-heated mains water via a reclamation unit.

The main thing with EAHPs is finding a plumber who is cognitively flexible enough to fit one. I'm on number six and it is finally going in next week.

Newer heat pump bath system in Japan supports to reclaim the bath water heat into heat pump bath system's tank. Though it won't reclaim from shower. https://www.mitsubishielectric.co.jp/home/ecocute/function/h... https://kaden.watch.impress.co.jp/docs/news/523254.html

I just built an off-grid cabin, and built heat reclamation into our plumbing in multiple locations, as it seemed silly not to.

Currently we’re using butane for our hot water, although I’m switching to biogas in the spring - the first stage of reclamation is a simple loop of hot water line around an insulated hutch in which the gas bottle lives - keeps it warm, as butane is useless in cold weather.

The second stage of reclamation is exactly what he proposes - our cold water line comes in, spirals around the waste water line for about 5/40 meters of respective length, in an insulated sleeve, before hitting the boiler.

Our input water is often barely above freezing - it’s the difference between having a cold/lukewarm shower and a scaldingly hot one.

I read a lot about district heating systems yesterday after seeing an article on NYC's steam system on HN (https://news.ycombinator.com/item?id=33388473). District heating systems allow you to capture waste heat at the power plant (called co-generation), and use that to heat & cool (via cooling compressors) many, many buildings at once. The wikipedia article about it is very informative as well: https://en.wikipedia.org/wiki/District_heating.

Usually district heating systems route heat into your building's pipes using a heat exchanger - so the network's fluid never actually intermingles with your building's fluid lines. These systems also use the same heat exchanger to return excess heat back to the network. District heating systems require a large initial investment, but over the long term are very efficient.

The mirror image of this is combined heat and power (CHP). That is, instead of burning fuel to heat the home, use the fuel to power a generator of some sort, and using the electricity to power the home. The waste heat (most of the energy from the fuel ends up as waste heat) is then captured to heat the home It's basically free electricity, if you were already heating the home by means of combustion anyway (e.g. oil, natural gas, propane).

The problem with district heat schemes in the UK (at least) is they are becoming mandatory for new large housing developments but are 100% unregulated every step of the way. The builders do the cheapest job possible because the efficiency targets they're supposed to hit are never measured or verified post install. And unlike the rest of domestic energy in the UK, pricing is totally unregulated. The residents can (and are) charged high rates because it's impossible for them to change supplier. Any potential savings are undone by these two things. The energy regulator has so far refused to do its job because they supposedly don't want to stifle growth in this new market.

It is a good idea but it's been sabotaged by greed.

It seems sabotaged by incompetence on the part of regulators. You can’t make something both 100% mandatory and unregulated and then go shocked-Pikachu when the monopoly grantee takes advantage.

Charitable of you to assume it is incompetence rather than malice :/

> You can’t make something both 100% mandatory and unregulated and then go shocked-Pikachu

The shocked-Pikachu face is the face they make when trying to figure out if anyone will notice the bribe they took to gift the contract to their buddy joe.

Why not feed that heat back in to the power plant to squeeze more electricity from it?

I'm no power-plant engineer, but vaguely remembering back to thermodynamics 101, in a perfect heat-engine efficiency is relative to the temperature delta.

You can't just turn heat to electricity, so below a certain threshold it's only good for warming spaces.

They exist but you're right that you can't just use the waste heat directly. You have to pump in some extra heat to make it worthwhile.

They are called co-generation power plants. The one I used to work at took the waste heat from gas turbines, added more heat with HRSGs (heat recovery steam generators) and fed it into a steam turbine. It adds a significant amount of efficiency.

I live in a post-Soviet country that has district heating systems, and rather quickly the demand for heat outstripped the demand for electricity, so in my city this is now mostly fulfilled by 'heat plants' burning gas. The price you pay per kWh of heat is quite a bit higher than you would pay for having a gas furnace or heat pump at home, so I'm not sure how efficient it is.

Also the heat plants are not that far from residential areas, so if they were to burn more polluting fuels (apparently when they were built, they burnt bunker fuel) it wouldn't really be any better than burning at the point of use.

If you have some kind of geothermal source (proper geothermal, not a ground source heat pump) it seems like it would make sense, but if not I think heat pumps at the point of use are a much better solution. In apartment blocks you could have a centralised system for the building, you don't need each unit to have its own system.

District heating here in The Netherlands is a great way to be screwed by a supplier. We have pretty good competition between suppliers for gas and electricity(). Households that get their heat from district heating can only say yes or no to one supplier. They will generally pay the maximum price regulation allows.

() This competition has led the way in greenifying electricity usage for Dutch households. Dutch electricity was mostly coal and gas generated, but there were energy suppliers that guaranteed green electricity. Green-minded people could use market mechanisms to increase carbon neutral electricity(*) usage. I've always liked that personal power in market mechanisms.

(*) Similar contracts exist for gas usage, but it's mostly based on "compensation", which at best isn't very good, and at worst completely useless.

It is worth pointing out that a more modern approach to district heating will use ground source heat pumps on-premise in people's homes, and will circulate fairly low grade heat (20C or less). Top-ups in system temperature can come from refrigerated warehouses inputting into the system, or solar photovoltaic-thermal setups that cool PV with circulating water, or just the ambient heat of the ground (10C most of the year far enough down).

See: https://www.cibsejournal.com/technical/take-it-down-low/

These networks are advantageous because the heat pumps using them can still produce enough heat even if the system temperature drops - they just do so less efficiently.

Someone (Luu?) coined, a while back, the phrase "cocktail party idea".

This is exactly that. Something that sounds like a great idea to people standing around chatting, but in reality is full of fail.

Energy poverty can be sorted out by better insulation and investment in cheaper and more sustainable sources such as nuclear, wind and solar. Instead of working around this issue with clever hacks this needs to be a national policy. I think all roofs should be mandated to have solar panels installed, as a start, and all new builds should have proper insulation.

I think a step further is required. I really like the idea of using solar panels as an integral part of the roofing structure. There are people in the UK doing this as integrated solar panels which use a plastic tray underneath. I think with some slight adaptions we could even do better.

We had a few of these projects in the Netherlands. It is nice, until it isn’t. The solar panels are more vulnerable than tiles, they will degrade or need repairs/replacing. The mechanism for sealing it up will leak. The format of the panels and or the inverter was proprietary. Or the connection was and you couldn’t retrofit another inverter. It wasn’t a good experience.

Sounds like there are a few things being mixed in there. I'm not really that convinced that solar panels are any more subject to damage than they would otherwise be. Maybe you gain a little in terms leak proofing by having a normal roof underneath but it's a lot of material for a marginal gain. Typically roofs these days are constructed with an impermeable plastic sheeting which is waterproof in its own right but requires protection from the elements (particularly the sun).

It's impressive that they managed to make a DC electrical connection proprietary though!

If I was planning national energy infrastructure, I wouldn't plan on dividing the equipment into a million tiny pieces and installing it on roofs, of all places. It seems much simpler to manage a solar farm than 10,000 roofs.

Why not both tho? Energy issues arent limited to capture and storage, it involves distribution as well. Plus you want energy to be dirt cheap and accessible. So the more the better.

Solar farms are more economic to build than rooftop solar: https://www.nrel.gov/news/program/2021/documenting-a-decade-...

For individuals the calculation might be different as rooftop solar makes you less dependent on price trends you have no influence over, but as a society, utility solar is less expensive.

> Solar farms are more economic to build than rooftop solar

For utility / central planners the cost of rooftop solar is ZERO. The owner of the roof pays it. Even better they get to buy the excess for 5p and sell it on at 35p. So they make money on every panel installed.

The utility pays for the solar farm and it costs $.

Rooftop solar adds 'antifragility'

Waste water heat recovery should be mandated. I'm not sure if you included that in your "clever hacks", but it's definitely fantastic value for effort.

> I think all roofs should be mandated to have solar panels installed, as a start, and all new builds should have proper insulation.

I have good news for you: New construction has been required to have significant insulation in most jurisdictions for a very long time. The insulation requirements have steadily increased over the years, too.

States like California are already requiring new construction to have infrastructure to enable solar panel installation.

Not discussed: hot water in houses increases humidity. In modern well sealed houses, that can result in unfortunate mold growth.

I've always experienced extremely low humidity in winter months, so I would think the net effect might be a reasonable positive if this saves me needing to run a humidifier. Am I missing some detail? Or is that just unusual?

You’re in the uk?

No, but I was given to understand that winter => cold => dry air was driven by the basic physical properties, so I would be very curious if it was different there. Is it different in the UK, and if so, why?

The UK climate has moderate temperatures and a lot of rain. Excess humidity is a common problem here, and excessive dryness is rare.

It tends to often be humid here, even in winter.

Right now, it’s 81% humidity. Our laundry has been taking two days to air dry.

Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up with will have very low humidity, but if the outside is a drizzly 10°C and you are only heating to some 18°C, it can be a problem.

someone else can explain better than me, but.. humidity is lower when it's cold but relative humidity is higher. Being outside in the uk when it's cold is often like being inside a cloud, with 100% relative humidity levels. Combine this with the old, badly insulated, difficult to heat housing stock, and you can guarantee that there is one part of the house (a north facing wall, say) that is at or dangerously close to the dew point for some part of the day/night. You wouldn't want to leave a steaming bath for longer than necessary.

It depends massively on your climate and how air in your house is managed.

If you live somewhere which is significantly colder outside than in (e.g. 0-5C out, 21C inside) heating outside air to comfortable temperatures will greatly decrease humidity. Couple this with a system which circulates air, or even exchanges inside&outside air and you will have incredibly low humidity inside.

I live somewhere which often sees 10-15C @ 80% humidity over winter, and the only air circulation in my house is from opening doors/windows. 70% internal humidity @21C is incredibly common. It takes a bit of moisture management to keep it below 60%.

Ventilation with heat recovery sounds like it could work in those conditions, but just barely. 80% @ 15C would turn into 55% @ 21C.

Unfortunately, very few homes in the UK have any air ducting. I'm looking at installing a MHRV system at the moment, but planning the ducts is a nightmare. Brick walls, lintels and structural joists everywhere with no headroom between floor and ceiling.

We absolutely should be doing it in new builds though, and we're not.

New build flats have some air ducting. At least, mine did in two rooms (it wasn't for AC). But maybe you meant new build houses.

That sounds rough. Consider putting in air to air mini-splits instead, and leaving window cracked open, etc. As a bonus, each room will get its own thermostat, and cooling / heating.

The mini-split will only require coolant lines, and will have a sufficiently high coefficient of power that this + whatever ventilation exists might use less energy than heating with natural gas (or resistive electric), even with the loss of efficiency from not using heat exchangers.

(Doing both things would be much better, of course. Retrofitting is hard.)

Define “massively.” Sounds like a Reddit-esque waffle word like “wildly” or “dramatically.”

Moisture is the #1 destroyer of buildings in its various forms.

https://www.buildingscience.com/bookstore/books/moisture-con...

Moisture control and mitigation is a very important part of building to last, and building to minimize maintenance and energy usage (simplistic "insulate everything and seal all air leaks" can result in a doomed structure).

Why do you say "not discussed"? That's point #4 in the article.

From the article:

> Extra moisture in the air might cause problems with things like mold. Use your judgment. It’s probably fine as long as things don’t stay damp all the time. For many, extra humidity in the air would be welcome. (Maybe you can stop using that ultrasonic humidifier.)

It's because the house builders don't do the job properly and install MVHR, which not only removes the excess humidity, but also recovers most of the energy of vaporisation.

This strongly depends on the climate. In some climates like where I live, we run a humidifier in the winter to deal with the super dry winter air. It is built into the central HVAC for our modern well sealed house.

Turning your thermostat down by one or two degrees per week and getting acclamation to cold is a better alternative. 5 degrees on the thermostat would be a far better measure than tub heat.

Short showers, or 'sea showers', to conserve water and heating would likewise be a far better option than a 50 gallon bath.

And not just short showers, but managing the amount of time the water is actually running. Here’s my strategy for an efficient shower:

1. turn on the water, get in, rinse down, soak a wash cloth

2. turn off water

3. lather hair with shampoo

4. lather wash cloth with soap and scrub body

5. turn on water and rinse down

The majority of the time we’re standing in the shower, we don’t actually need the water to be running.

It gets really cold without the shower water running. Our indoor temperature in the day is 16-18°C in Winter (UK).

Ufff. I’m not doing that when it’s 4c outside and because I turned the heating off at 9pm my house is 14c. I need the shower to warm me up and stop me turning the heating on early in the morning

Turning off your heat is costing you money. You don't want to cycle all the heat out of the building and then have to warm it back up.

It's called heat capacity and the materials themselves require so much heat before you feel warm.

TL:DR Turn your heat down, but not off. Save money

What's a "sea shower". Do you mean when you literally just spray water on yourself, then soap all over, then rinse off?

https://en.wikipedia.org/wiki/Navy_shower

Tom Clancy loved to collect details like this. When Jonesy detected the Red October on sonar, in the book he was rewarded with a “Hollywood shower.”

In addition to things like heat exchangers or reducing the amount of time/water in a shower, is there any work being done around optimizing the amount of time a hot water heater keeps the water at high temp? I’ve thought about this for extended absences but this could also bring potential benefits if it just doesn’t run for certain portions of the day or middle of the night.

People have done that (some recent post on HN was about using a RaspberryPi to do just that, including a "get hot enough to remove legionaries' disease").

Likely for most people the tankless heaters would be a better option, or using in-floor water-based heating instead. Something like this: https://www.radiantcompany.com/system/opensystem/

I'm not sure if things are different outside Australia, but it's probably been 15-20 years since I've lived in a house with a hot-water storage tank. As far as I know most places seem to have instant/continuous systems that heat the water on demand as it passes through the line.

Solar-heating definitely makes storage make more sense, but I don't know how the numbers actually work out on that one.

Or for heat pump water heaters, which have unbeatable efficiency. Still not cost-effective in many areas compared to gas-burning heaters, but they use much less energy.

Pretty much everyone in the UK overfills the kettle and boils it 4+ times per day, I'd imagine if we could just get folks to boil the correct amount of water here the energy savings would be far more significant

Years ago I was renting a house with a small in-law building in the back which was rented separately. I'd get the electric bill and then split it 75/25 with the guy who lived there. Over time the electric bill kept getting higher and higher, until it was like $500+ a month. I talked to the landlord and the other renter about it and none of us could figure out what was drawing so much electricity. I called the electric company several times, and they'd asked if I had shut off my pool, which I didn't have, or if I had been using the jacuzzi a lot, which I also didn't own. Then they asked about power tools, welders, and all sorts of other things, and when I finally convinced them something was wrong they sent someone out.

The electric guy did the obvious thing: He started flipping breakers to see which circuit was drawing so much electricity. I had done that as well, but had no idea what to do from there. He was able to follow it out to the in-law house, which was in complete darkness as it had zero electricity inside - except for a tiny water heater installed out back. It had somehow gotten filled with sediment, so the heating element was basically trying to keep a barrel full of sand warm. (The guy who lived there just assumed the hot water tank was small and took fast showers as a result). The landlord replaced it and suddenly the bill went down to less than $100. Crazy.

The point to this story is that there's probably a lot more that could be done before worrying about recouping the heat from your bath water.

Tangentially related: has someone found something less energy intensive than taking a bath while bringing the same degree of warmth?

I've been looking at heated blankets but they're all made of polyester which is not comfortable due to the moisture being trapped under the blanket. Heated seats in some cars are the closest sense of comfort I've found but that technology doesn't seem to exist for regular seats.

Maybe just one or two hot water bottles? [1] We use them in Winter to warm our feet in bed. But they are very versatile and can also be used to alleviate any kind of tension.

I also use one while I code: I put it inside of a small (baby-)sleeping bag wrapped around my feet. Sounds weird, but it's so comfortable!

[1] https://www.google.com/search?q=hot+water+bottle&tbm=isch

We use this trick on winter backcountry camping trips to good effect. After dinner, melt snow to a boil and fill up all our water bottles and bring them in the sleeping bag. Every little bit helps!

Went for https://www.yuyubottle.eu/ after a bit of googling due to the cotton fabric + the strap, will test it this winter.

Another trick is to put lots of dry rice grains into a sock, tie the sock and warm it up in a microwave. Just be careful, as the rice heats up Really fast in the microwave.

https://solar.lowtechmagazine.com/2022/01/the-revenge-of-the...

Electric heating pads?

Heated blankets never really did the trick for me either. Electric foot-warmers do. They're cheap, don't consume too much energy, and will get your feet nice and toasty in no time.

I have an electric blanket. The way it works is that you put it underneath the sheet, on top of the mattress. Moisture getting trapped under it isn't really a concern, because i lie on top of it. From the sheet up, the bed works in the normal way.

I turn it on for 30 - 60 minutes or so before getting into bed, to warm the bed up, and turn it off once i get in. I have tried leaving it on all night on the lowest setting, when it is very cold, but i wake up dehydrated.

A heated mattress cover has been a game changer for me. You can use your existing comfy blankets and it only takes a very small amount of heat to really make a difference. It definitely pays for itself in being able to reduce temperature set point at night.

Heh get a dry suit (like a wetsuit but sealed) and then fill it with hot water? Should only use a few gallons.

Isn't bathing the worst heating method for energy usage perspective? I don't think it's heating method but cleaning.

Maybe, but wet warmth like a bath has excellent thermal properties when it comes to human perception. Warm air doesn't have the same feeling.

As the article points out, only some 9% of domestic energy goes towards heating water for showers and baths. Instead of trying to force yourself to do cold showers, you could save much more just turning down the thermostat.

Exactly. Space heating is the most energy-intensive an expensive item on the list of things you can save on in a detached house - especially that you only really need the heat to be where you are.

It heavily depends how they use bath, and how cold. In Japan, 25-40% (vary depending on research) of energy at home is used for heating water because people prefer bathing. Heating water is expensive.

Japanese houses are more likely to be cooler but have local sources of heat, from what I understand. For example a kotatsu is a warm space.

Heating locally is such an interesting idea to me.

The local heating "idea" is now low valued because moving between cold and hot area isn't good for health (called "heat shock"). It is recommended to make house insulated well and heat all rooms, for new houses. https://japantoday.com/category/national/agency-issues-warni...

Fascinating!

I've done this. It works surprisingly well, and (as Might points out) humidifies the house in a way that radiators and resistance heaters do not.

The evaporating water does convert (to use the traditional terms) sensible heat into the latent heat of water vapor, and you might think that this heat should be discounted from the heat you're getting out. Sometimes it should, but not other times, because if the water vapor condenses again inside the house (for example, near the surface of the plaster in cold walls) it will release this heat again as sensible heat. In other cases, it's worse — I've had the displeasure of staying in a cold guest room where the blankets were all slightly damp from such condensation and so failed badly at keeping me warm.

In that case, the water vapor had been produced not by evaporating standing water but by burning natural gas. Be careful.

On the latent heat problem, I wonder how the math works out if you put a dehumidifier in the bathroom to counteract the effect of leaving warm water standing around.

What's interesting about that is that you'd effectively be using the dehumidifier as a heat pump, indirectly pumping the heat out of the water and into your home, leaving you with cold water in the heat pump's reservoir. You're adding energy, but that energy also ends up as heat, so you'd still certainly end up ahead compared to using electroresistive heating (but not necessarily compared to a traditional heat pump).

My guess is that it would be a lot more effective to _directly_ pump the heat out of the waste water, but you can't do that with off the shelf appliances.

I've noticed that people wildly overestimate the environmental impact of things they personally touch.

This, I theorize, is why sub-rounding error things like plastic straws, plastic bags, cups, etc get such disproportionate attention.

Bathwater that heats every pore of our naked bodies is probably even more susceptible to this bias.

The plastic straws were for virtue signaling. A bar near me put up signs in the bathroom with pictures of sea turtles advertising that they no longer use plastic straws. Outside the bathroom, the AC units were running (completely on and running; not just the fan) along with household electric space heaters. Outside the building, it was snowing.

The plastic bag laws are because municipalities don't want to pay to haul as much garbage and so seek to save themselves money.

I don't know what "cups" is but so far, none of it is honest.

You may not be wrong - however, what's your suggestion then?

Not doing these things?

IMHO, every little bit counts towards a livable future.

Most people not in the Nordics / Germany / Switzerland would most likely see way more energy savings per year by simply spending the money used to install a waste water energy recovery system to instead just insulate their house.

Plastic straws are too minor and alternative is too crap, so it's outlier. Heating water is somewhat a bigger thing.

Problem is you're not just adding heat, but humidity. Normally while having a shower you're going to run an exhaust fan in the bathroom. So if you want to conserve heat, the better approach is just to make the shower as short as possible.

That seems like a more realistic suggestion to me. If you want to take this further, switch to taking Navy showers: get wet, turn off shower, apply soap, turn on shower to rinse off.

What, all my life I thought that was the normal way of showering. So how do you people actually shower? You leave the tap on while soaping?

Yes, I believe that is the common way of taking a shower. Turn on the water, do whatever you need for approximately 10 minutes, then turn off the water at the end.

>get wet, turn off shower, apply soap, turn on shower to rinse off.

Isn't that just a regular shower?

Also, cold showers are better for the skin, and good for mental health. Take a bit of getting used to though, particularly in the middle of winter!

I've been doing this, except I have a bath. It finally dawned on me that heating water and leaving it to cool was exactly the way the house is normally heated, so it was costing me nothing to bath as long as I liked. I live there now.

It's very likely be more cost-efficient to just use a standard heat-pump ("air-conditioner") set on the 'heat cycle' instead of intermittently mucking around with bath-size amounts of water.

A good way of reusing the heat from a shower is to have a heat exchanger to the cold pipe coming in. This has the advantage of also meshing nearly perfectly when you want to heat water.

The article mentions this about halfway down, with a diagram.

This reminds me of a "re-circulating shower" concept I was looking into recently. By filtering and re-using shower water, it saves both water and heating energy. There is a company in Sweden doing this: https://orbital-systems.com/.

There's a bit of an ick factor, but with filtration, it seems this should be more hygienic than bathing? My family remains unconvinced though ;)

Most people are fine with hot tubs aka spas, which are essentially this. (In fact most spa designs don't even filter all the water on each re-circulation, as there are returns both at the bottom and in the skimmer, and only the latter is filtered.) Re-circulating shower sounds like a great idea to me.

But you don't wash in a hot tub, and you usually shower before entering them.

"Extra moisture in the air might cause problems with things like mold. Use your judgment. It’s probably fine as long as things don’t stay damp all the time. For many, extra humidity in the air would be welcome."

In climates like in the UK, this is a great way to create mold in your bathroom and adjacent rooms if your house is well insulated and does not have central ventilation.

I just want to put in a word for standard microeconomic theory. If our water (or energy) use in aggregate suggests to the govt that our consumption needs to go down, then raise the price of water (or energy) across the board till it does.

Individuals can choose which of all your amazing water (and heat) savings ideas they feel would best suit them.

Doesn't this approach hurt the poor a lot more though? Like, if my mum suddenly got charged more for her heat, she might have to live in a cold house, which would make her deeply uncomfortable or even sick.

The problem in reality is that people have wildly different amounts of disposable income and that there are more and less important uses.

Rich people use drastically more resources. If you want to curb their resource usage with price you would need to increase prices so much that poor people can’t afford the resource anymore.

E.g. you want to stop people watering lawns during drought, so you make water so expensive that rich people cannot afford it anymore, but then poor people cannot afford to wash hands, shower or flush the toilet anymore.

One solution is to have the first x units of the resource cheaper, similar to progressive taxation.

Where we live in Portugal humidity is a big problem, and we actually spend 1kW 24/7 running dehumidifier throughout the house. If we didn’t, mold would grow and we would get severe allergies. So… this seems like a very bad idea even for the UK, considering it’s very rainy there in the winter.

A house with plenty of air exchange (either mechanical or because of a leaky air barrier) in a cold climate will tend to suffer from too low of humidity (cold air enters, and as it gets heated to room temperature the relative humidity drops).

In Japan at our place we always do this. After the last person has used the bath water, we let it sit there overnight, and then use the water for the first wash cycles of the washing machine.

The less efficient part of this story is that everyone showers before entering, and that does go down the drain.

Having read this, I'm strongly considering routing my bath and shower wastes through a poorly insulated hot water cylinder in reverse (new [warm] waste in at the top, old cold waste out the bottom). Hot water will float in there. Cold will be displaced first.

It only has to hold the volume of "a shower" and we'd keep that energy within the envelope of the house, without added humidity or state-change heat loss.

The only major downside is the "what happens when it clogs" question. To work normally, it would require the discharge to rise to the same height as the top inlet before falling to the drain, but there could be a secondary purge for pressure-flushing it once a month.

Rarely does plumbing make so much sense to me. I suspect I'm overlooking something. Answers on a postcard.

Something like this could be interesting with active heat transfer, for example using it as the source side of a heat pump water heater. As long as the outgoing water remains above freezing, of course.

In my experience, there a many lower hanging fruits in British houses to save energy than this. Uninsulated and draughty old houses seems to be the norm here (at least if you're renting).

I'm surprised the government is not pursuing this more aggressively.

Even if you're buying, the most popular houses are ancient Victorian ones which, in spite of their charm, are very badly insulated.

They're often listed and require fitted wooden windows, so it's not exactly easy to fix.

Vanishingly few houses are actually listed. Some are in conservation areas, which have similar but less strict restrictions on what changes you can make. These are the areas in my borough - almost half the built-up area, but i would guess proportionally less of the homes, because they are lower-density old buildings:

https://www.haringey.gov.uk/sites/haringeygovuk/files/haring...

You can coax a London-wide map of conservation areas out of this, and see that most of the suburbs aren't covered by them:

https://apps.london.gov.uk/planning/

It probably is true that the most desirable houses are in those areas, but being in a conservation area helps preserve them and keep them desirable.

Anyway, even within those conservation areas, there is plenty you can do to improve insulation. I had internal wall and ceiling insulation fitted, and replaced the window sashes with double-glazed ones.

This is a thing in some newer construction, e.g. in Vancouver BC, the Southeast False Creek Neighbourhood Energy Utility recovers waste heat from a wastewater treatment system to supply heating energy and hot water to mixed-use buildings.

If you let dirty water fill up your tub while you shower that'll result in a dirty ring around your bathtub which you'll have to clean... with more water (and possibly chemicals)... it'll cost you extra time to clean as well.

All together the total costs of the extra cleaning resulting from this may exceed the cost savings from letting the water heat your home.

Or simply take shorter showers, then this system wouldn't really benefit because before the drainage pipe warms up enough to transfer any heat to cold water, you will be done. Grey water tank systems with recuperation are already known tech, but it requires extra maintenance unfortunately.

I do a version of this when cooking without knowing if it actually has a significant effect: In summer I pour leftover hot water from cooking down the drain right after I finish, while in winter I leave the hot water sitting in the pot so the heat can warm the kitchen.

I reuse my bathwater to start a new load of laundry...that seems a lot simpler

How do you get the bath water to the laundry machine?

I do my laundry in the bathtub :-)

Not if you have a frontload washer. :)

Along with the problem of getting the water into the machine, the soap from a bath would damage the machine.

Front Loaders require low-sudsing soap, and you have to be very careful on the type of soap, because many types tend to precipitate onto the inside of the machine.

How does the plumbing work for that? I can't imagine it passing plumbing regulations in most countries. Even having a grey water system is tricky under most regulations (the reason for this is the slim chance someone could misconnect a potable system.

> 90% of the heat in that water currently goes down the drain.

I think this number is very flawed, at least if we talk about bath and not shower. Kids sometimes play in bath until the water is "cold". Even if they just wash, the resulting water is far cry from what it was when "inputted" into bath.

Moreover, all that temp won't be recovered to heat air. Some energy goes to heat bath itself, some goes to heat space under bath and some goes to heat air. So I suspect the impact numbers to be far less benefit. And the result will be slightly warmer bath, but no effect to overall heating.

But it is true that water is hungry for energy. My avg energy usage is 550kWh and I have electrical water heater. It takes exactly half of total energy usage per month.

I’ve looked into installing a system like this in Australia. It only barely makes financial sense as part of a new build. Maybe in the colder UK climate it would make more, but surely not as a retrofit.

Why not limit toilets to only flush every 8 hours to allow better heat capture from human waste?

These diagrams imply that the incoming water to the heater is heated by drain water from the bath.

But if you're filling a bathtub there's no warm water going down the drain. So the exchanger is useless here. And then when you're done and drain the tub the exchanger gets hot but the heater isn't filling any more.

This device only really works in a shower situation, it seems.

The only "practical" installation I've seen (besides ones in massive multiplayer showers like you'd see at a college or YMCA) was for a massive house with a huge shower designed for 30+ minute showers.

And at that point you probably can "save the environment" more by not building a 10k sq ft mansion. But what do I know?

i did hear about some shower drains that reuse the heat like this one: https://www.youtube.com/watch?v=s5bJ61fCA2E

This seems like a lot of work to save £0.13.

As someone who buys a lot of bathwater on Onlyfans, this is how I break even.

Put on a hot heater when taking a shower.

In theory you'll turn the water heat down which stops a lot of loss, 1 kg air is ~1000 liters, 1 kg water is 1 liter. It'll also flow back into the house.

And I get out sooner because it's not scary outside.

This person has installed two drain-water heat recovery systems, and measured it https://www.tiktok.com/@johnsavesenergy/video/70465726671214... This is the proper way forward.

This is so much more reasonable than that …other… Modest Proposal.

The other one would have a bigger environmental impact;)

What other proposal?

That would be A Modest Proposal For preventing the Children of Poor People From being a Burthen to Their Parents or Country, and For making them Beneficial to the Publick by Jonathan Swift: https://en.wikipedia.org/wiki/A_Modest_Proposal

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