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Comment by ak217 | Hacker News Reader

I'm amazed at how little district heating is used in northern climates like Canada and the northern US. In dense areas, district heating using waste heat from power plants should really be required by urban planners, or at least it would make a huge carbon footprint difference if we invest in it.

On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building. (Edit: I guess they do exist - https://www.homedepot.com/p/Power-Pipe-4-in-x-48-in-Drain-Wa... - but I've never seen one in the wild)

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It's a cost thing. Prices for energy have been so cheap that investing in recapture capex didn't make sense. With energy prices going up, it does make sense.

A funny anecdote (or sad?), after gas prices spiked in much of Europe due to the war many manufacturing facilities were looking to cut energy use, they found a lot of waste. Many places were able to reduce their gas usage by as much as ~33% if I recall correctly.

I can't find the original source, but here's [1] one that claims 23% lower consumption across Germany in 2022 adjusted for temperature.

1 - https://www.nature.com/articles/s41560-023-01260-5

Reducing consumption as a response to increased prices doesn’t mean the difference in consumption was originally “wasted”; it only means it was not worth the higher cost.

From the POV of capitalism, yes. From the POV of the environment, noooo.

Or even from a non-fossil capitalism point of view, a carbon tax that equaled the externalities would lead to higher efficiency and give people a permanant financial incentive to find these efficiencies.

If people can’t afford to heat their homes and freeze to death in the winter, is their continued survival wasteful from the POV of “the environment”?

I think you're misreading me. The fact that there was that much waste that could have been found and eliminated earlier is an affront to the health of our planet. At no point did I suggest freezing people in their homes in the middle of winter to die.

Cutting consumption because the price went up doesn’t necessarily mean the difference in consumption is waste. If you keep increasing the cost of energy, eventually people are going to freeze to death in their homes because they won’t be able to afford the heat. That doesn’t mean the energy required to keep them alive was wasted; it just means they couldn’t afford it anymore.

Germany is heading into a recession because the high cost of energy is destroying their manufacturing sector. They aren’t just making the same amount of stuff with less energy; they’re making less stuff and becoming a poorer country.

Which could be a coordination problem, though. Where initial high costs are due to a lack of scale whereas if everyone participated, costs would come down fast.

I remember reading on a plumbing blog (or YouTube or something) about someone who installed one of these - high end house, massive shower, owner who liked to take hour long hot showers or something.

In that case, it paid for itself pretty quickly.

For "normal use cases" the cost savings over time just isn't there, when you compare the added complexity. You're better off saving heat with on-demand heaters or HVAC recirculators, etc.

Places where it could make sense already use them (I believe car washes recycle water and heat in some locations).

That someone just needs a sauna.

So how does one keep all the showered-off gunk from clogging up the works ?

There's nothing to clog up. The drain is a normal drain; the innovation is that the cold water source pipe winds around the drain, transferring some of the heat from the outgoing water to the incoming.

Heat transfer requires time, which is why I've always assumed that such a system would have to use coils somewhere.

In the system under discussion, all the heat-transferring parts had better be metal, and it won't be worth a damn except when handling a hot shower or a draining bathtub.

> In dense areas, district heating using waste heat from power plants should really be required by urban planners

Power plants generally aren’t located in dense areas…

> I'm amazed at how little district heating is used in northern climates like Canada and the northern US.

District heating and cooling are available in both Mpls and St Paul in the downtown core. This is the coldest large metro area in the United States.

St. Paul: https://www.districtenergy.com/

Minneapolis: https://cordiaenergy.com/our-networks/minneapolis/

> On an individual household scale, I've always wondered why nobody has come up with basic drain heat exchangers to capture most of the heat coming out of shower and washing machine wastewater before it leaves the building. (Edit: I guess they do exist - https://www.homedepot.com/p/Power-Pipe-4-in-x-48-in-Drain-Wa... - but I've never seen one in the wild)

$851 just for the heat exchanger, not including piping and labor. It will take a lot of showers and laundry to pay that back. I’d probably add one to a new house build but would pass on retrofitting one into an existing home unless I was doing a complete plumbing remodel. Similar to daylight harvesting LED lighting, it’s possible you’ll never see a payback on the equipment and labor.

I was going to mention the Minneapolis system, I work across the street from this plant.

I have had one of those installed in my home recently. A QB1-16 from Q-Blue https://www.q-blue.nl/en/products/q-blue-showersave/ Supposedly it saves about 50% of the loss heat. Since I dislike hasty showers, I figured that would save me plenty.

> I'm amazed at how little district heating is used in northern climates

In my country we can pick our own electricity and gas provider, but for district heating you're tied to the provider for your area. Consequently, people with district heating get fleeced. When buying a house, I gave preference to the ones without district heating.

David MacKay discusses this in Sustainable Energy Without the Hot Air. He concludes that, while it's on first glance appealing to want to use waste heat from power generation, heat pumps are strictly superior to combined heat and power except for in a few specialized circumstances (e.g. industrial uses that require high temperatures). The book is fifteen years old now, so I suspect the math even more strongly favors heat pumps than when it was published.

See the section, "Heat pumps, compared with combined heat and power": https://www.withouthotair.com/c21/page_147.shtml

But district heating is a distribution mechanism, not a heat generation mechanism. Cogeneration plants are one way to power it, but you can also use waste heat from industrial processes, very large scale heat pumps, or burning household waste. All of these are in common use in places where district heating is common. The public district heating utility in Stockholm claims to have the world's largest heat pump installation, which has the capacity to extract 225 MW of heat from treated sewage, in the process generating both heat that's sent into the district heating network, cooling that's sent into the district cooling network, and finally a small amount of electric power by releasing the treated water into a lake via a turbine. This facility opened in 1986. The utility also has various other facilities, both cogeneration plants that burn biofuel (mostly byproducts from the lumber industry) or household waste, as well as other heat pumps that extract energy from seawater.

District heating/district cooling has three advantages: you can change the heat source centrally without having to refit every single dwelling, there are economies of scale in the heat generation, and you can take advantage of heat or cooling that's just in the wrong place and transport it to where it's needed.

Seems to me like the electric power grid is an already existing, efficient energy distribution mechanism. Converting waste heat to electricity is probably a lot cheaper to retrofit than adding a new distribution network to places that don't already have one.

To my knowledge there's no really effective way of converting low grade waste heat into electricity. Basically all heat-based electricity generation today relies on being able to boil water into steam to spin a turbine. But with heat pumps and district heating, you can recover useful amounts of energy even from quite small thermal gradients. That's why things like room temperature sewage and waste heat from data centers can be useful to these systems even though they couldn't be used to generate electricity.

Also, if you rely on local heat pumps, you still need to take the energy from somewhere. Taking it from the air is fine in warmer climates, but in colder climates the efficiency of air-source heat pumps starts to become an issue when it gets colder. You can extract heat from deep underground, but that doesn't universally work in densely populated areas because the ground doesn't contain infinite amounts of energy and it's possible to extract more than it can sustain. So, in cold climates there's definitely a use case for distributing heat using water pipes, with or without local heat pumps in every building (see the sibling comment about 5th generation district heating systems).

Thermodynamics prevent you from turning low grade heat into electricity with anything resembling reasonable efficiency. Moving the heat around on the other hand it reasonably simple.

5th generation district heat systems can transport water at roughly the same temp as the underground and use distributed heat pumps to bring them up to the required temperature at point of use. This prevents heat losses along the way.

https://5gdhc.eu/5gdhc-in-short/

I’ve often wondered if late if rather than a “wet wall”, if houses should have a “hot wall”. Could I run a thermal loop past the fridge and a couple other appliances and either shunt it outside in the summer or distribute it around the house.

Total BTU aren't that great. Would (somewhat) work for a very small apartment, for any big house you would have a better experience making a proper thermal insulation.

Given that people are pointing out that heat recovery from washers doesn’t really work because they alternate between filling and draining (so there is no counterflow except incidental from other house activities), I’m not so sure that would be true.

Particularly if you use heat pumps instead of basic plumbing to achieve the transfer. Dump heat into a glycol line with a reservoir.

Arguably, decentralized approaches have other advantages when it comes to redundancy, gradual upgradability and expandability.

Yeah; if just used for hot water, this doesn’t make much sense compared to a hybrid heat pump water heater. Here in California, with a well insulated/partially passively heated/cooled house, our hot water is well under 10% of our electricity bill (ignoring our EV).

In places where it’s really hot or cold, it would be an even lower percentage. The money that it would cost to trench and maintain the hot water distribution lines for a house would probably be better spent on heat pumps, architectural features or upgraded insulation (Even just $2000 of extra insulation does an amazing amount of good vs. bare minimum code insulation, and I doubt you can get district hot water installed for less than that.)

Heat pumps move heat so the heat from the waste water is still useful to them.

> I'm amazed at how little district heating is used in northern climates like Canada and the northern US.

It is used on most if not all Canadian university campuses. Canadian university buildings are also far less sprawling because they're all connected to a steam generation plant.

Toronto has district heating.

https://www.toronto.ca/services-payments/water-environment/e...

In the reverse direction, Lake Ontario is part of a cooling system as well.

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

Similarly, I know Harvard and MIT, and also parts of Boston have centralized steam heat systems. Harvard uses theirs extensively still, at least in the old buildings, but I'm not sure about MIT. Boston's is fed by an old commercial steam plant on the outskirts of Chinatown. I think it covers a pretty limited area though. I never lived anywhere with it.

The problem is adding the infrastructure where it doesn't already exist. District heating is great when building a new city, but we don't do a lot of that anymore. Just imagine the cost of laying new pipes under an entire city. I'm sure someone smarter than me can do the math, but I'd suspect it'd take many decades to recoup the environmental and economic costs of installation.

> I'm amazed at how little district heating is used in northern climates like Canada and the northern US.

District heating is very expensive to build and difficult to retrofit. Doesn't seem very surprising to me. Any time a piece of road has to be dug up, costs skyrocket and people are inconvenienced for what seems to be an eternity. As far as I can tell, this is universally true among western nations.

> Any time a piece of road has to be dug up, costs skyrocket and people are inconvenienced for what seems to be an eternity.

Former trench digger here. The cause are usually two problems... first, the "underground line maps" ("Leitungskataster" in German) are usually not up to date because not everyone bothers to update them. Quite the "fun" if you suddenly hit a cable at 50cm when the map says you should be clear up to 2m!

The second problem is that Western societies almost always choose the cheapest bidder, which means no work on weekends or, heaven forbid, 24/7 as that would be too expensive. That means everything is slow as molasses.

Why is it cheaper to have one shift work on a road project that takes months, compared to three shifts working around the clock making the project take 1/3rd of the time? I've always vaguely wondered that when driving through construction zones that are seemingly devoid of workers most of the time.

I assume the workers are paid hourly, so wouldn't the cost be the same? Are second and third shifts more costly?

Overtime is more expensive. Shift differentials (paying more for 2nd and 3rd shifts) makes those shifts more expensive. There are more workers willing to work in the 7A-4P window than in the 9P-5A window.

Second and third shifts are more costly, yes. Many fewer people want to work them, so supply is much lower, so prices are higher.

Also in evenings, road crews have extra safety precautions (there’s a higher chance of running one over in the dark) and that’s labor too.

I guess paying outside normal working hours it's extra money that adds up?

> I assume the workers are paid hourly, so wouldn't the cost be the same? Are second and third shifts more costly?

At least for Germany, yes - by law, night and weekend/holiday work must both be approved by the authorities and compensated. Most union contracts here go way beyond the vague notion of the law and require significant compensation.

I think it's only cheaper because people aren't being compensated for the annoyance caused by road works that take forever. Months of detours, traffic jams, dirt, people struggling to get to their homes/shops/whatever, probably even extra accidents. If we were to put a monetary value on those, maybe the cost of working odd hours would be compensated.

It might not even be necessary to work 24/7. My street was recently dug up over its entire length to install new water mains and sewage pipes. It took nearly 2 years. Only 1 crew of some 5 people were working on it at any given time. Given that the street is roughly 1km in length, at least some of the work could probably have been done in parallel with multiple crews.

But that's just my armchair constructioning.

> Given that the street is roughly 1km in length, at least some of the work could probably have been done in parallel with multiple crews.

This is routine in Asia. Just look at this video remodeling a railway station in just a few hours [1].

For that to work though, the construction companies need to have the backing of the government that there will always be budget to keep these people employed - even for supposedly "low skill" work such as road construction, recruiting and onboarding costs are significant. But when government appears to be in budget crisis to everyone reading a newspaper, no reasonable construction company will hire more workers than they could also use for private projects (e.g. home construction, garden remodelings, ...) if the government decides to undergo yet another shutdown.

[1] https://www.youtube.com/watch?v=wIbZqqLra9k

Late 2022 my partner needed to go to hospital here in the UK at something like 4am. On the way there we got slowed slightly by roadworks to replace a (normally) very busy roundabout. The signs all stated that the working period for the rebuild was something like 11pm to 5am for that single night.

It was an eye opener for me, it had never occurred to me that such late night roadworks was carried out to reduce daytime disruption to traffic.

There again, this is in a city that is currently expanding the district heating system that they installed when they built a new waste incinerator - so that the process of waste is: 'burn the waste, heat water to turn turbines to generate electricity, the waste steam is then fed through pipes in the city to hospitals, council owned buildings, and some select high rise apartments, then the water is returned to the incinerator building to be reused. Solid remaining 'slag' from the burnt waste is used to make non-fossil asphalt. And the incinerator building also has wind turbines and solar panels covering the roof.

The builders of my house (who were angling for, and got, PassivHaus+ certification for it), built a heat exchange for the drains on the second floor that warms the intake to the hot water heater. It works, but it's pretty limited, in practice it does pass some of the heat from a bath or shower or dishwasher run back, so that's something. Of course the timing matters, water drains pretty quickly, even from a bath, the there's no coordination to guarantee the water heater runs while that happens (other than the good chance that if we just pulled hot water out it will be running to replace it, but sometimes that isn't immediate).

The DoE quoted a figure of 30% savings on showers, which is pretty impressive. All houses should be built so this is standard. It's so much more expensive to retrofit it later especially if the layout doesn't have the shower over the water heater.

The water heater is always refilling. Otherwise you’d have no hot water pressure. It’s just that the new water goes to the opposite end of the tank from where the output is attached.

How twin heating element water heaters work:

https://youtube.com/watch?v=Bm7L-2J52GU

Hmm, good point, I guess I hadn't thought about it, but that does mean whatever residual heat is in the water draining does get transferred to the water coming in to the tank, which if anything means the heater is less likely to come on.

For showers definitely. But as someone astutely observed, washing machines (clothes and dish variety) pull a bunch of hot water, and then dump it out a half hour later, and then may or may not pull more hot water afterward, but never while they are draining. I guess there's an expensive reservoir for that and that sounds like more water damage waiting to happen.

So the only heat transfer you'd get there would be whatever heat capacity the exchanger has. And since the thermal conductivity is high, all it's probably done is heat up the air inside of your walls a little bit.

The best alternative to me seems like heat pumps, although you might also be able to scavenge heat to feed into your radiant heating system, or the cold air exchange on your furnace. The former seems more likely not to cause problems on hot days.

I was just about to post that same link. I suspect the reason you don't see them much is because installing one will often require more plumbing and more labor. Residential installations in the US are particularly sensitive to up-front cost, and tradespeople in the US tend to be conservative in choosing 'new' technology.

And also exacerbate an already challenging shortage of trades.

When I renovated my house, I installed such a heat exchanging pipe. It reduces hot water usage (=fossil gas) by about 40% and with my usage payed itself back in about two years. It's not a drop-in replacement, since you should use it only for the shower drain.

So the shower drains through it and the 'wrap around' pipe is the input to your hot water tank?

No, it preheats the cold water connected to your shower. Warmer cold water means you need less hot water to achieve the same temperature in the shower.

Ah yes, that’s much neater and more localised. Thanks

$851?!

Well, at second glance, it's definitely larger than it appeared to be in the picture. Home depot says 35 pounds, so ~$100 worth of copper.

In Sweden most of the population is covered by district heating. I think it is mostly a matter of population density, in Sweden most people live in 3-6 floor residential buildings clustered together close to public transport hubs. In Canada and US you have urban sprawl and a lot of rural population, so running district heating pipes to every single house is just not worth it.

To be fair people who live in detached houses in Sweden often don't have district heating either, but the % of the population living in detached houses is tiny.

I guess if everyone did that (residential heat exchangers on your house drain), there could be a real risk of the sewage cooling and congealing in the downstream pipes, causing a blockage.

This would work as a sort of energy storage - you could overheat your water reservoir during cheap power times and then transfer the extra heat to the entire city the rest of the day.

You would need a lake sized insulated reservoir for that to work. Too much water is used already by those systems, they don't typically want to add another storage. Typically they are gigawatt scale systems burning something because just using electricity is too much strain on grid, unless you have a power plant nearby (like cogeneration, which uses waste heat from electricity generation from fossil fuels).

Why? Just overheat the water circulating through the district water system when you can. People's thermostats will simply turn off sooner, leaving more heat in the water.

Then later, you underheat the water, and the city's various thermostats turn on a bit longer.

The risk is that you underheat too much, and people compensate by plugging in their own heaters during underheating. And underheating will occur during expensive energy periods, so this will exacerbate periodic energy shortages. But overall this could work to store a ton of energy - there's a huge volume of water in a district heating system's pipes.

Typically water coming out of your heating plant is already at max temperature. Any more and it will start boiling.

56 Million litres, about 2,5GWh: https://group.vattenfall.com/de/newsroom/pressemitteilungen/...

Some places already used flooded mines or other underground reserviors for this purpose. As an added bonus, they're already warm and continually have heat replenished by geothermal heat.

Atleast in BC, our power plants are hydroelectric dams far from the urban center so there's no waste heat to collect.

The nice thing about district heating is the extra space you get and no fire/Co hazard.

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