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Comment by wcoenen | Hacker News Reader

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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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!

My smoke alarms go off.

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