[NoMoreNicksLeft]

Can substantial heat be recaptured? The sink drain's often pretty far away from the hot water heater in most homes. There might even be multiple faucets (not to mention shower/tub drains) in the mix. Seems less than feasible, but my intuition's often shit at figuring this stuff.

[blitzar]

You can preheat the cold flow into the shower so it is just local within the bathroom. Your hot water tap flows at the customary ~55C and the cold feed to your shower gets a boost to 25C or something.

Or you could send all the waste heat to a big sump and extract it with a heat pump (dont think that actually exists though - or would be some form of ground source heat) - at extreme temperatures would be worth it.

[IshKebab]

No, the hot water gets heated in the boiler which is usually not in your bathroom.

[Retric]

People who use tankless electric hot water heaters in their bathrooms to avoid waiting for hot water spent quite a bit for the privilege.

Recovering significantly energy without causing plumbing issues with clogged hair etc seems difficult however.

[rainbowzootsuit]

If you recover heat to the cold supply then you need less hot supply to get to the same outlet temperature. If you have an updated fixture with a thermostatic mixing valve then I think this would track pretty much seamlessly, but I've not ever had the opportunity to use a shower equipped this way.

[btbuildem]

Something as common as automotive coolant/"antifreeze" could efficiently travel via thin lines alongside regular plumbing.

I think a big part of the challenge would be in having multiple heat exchangers. Perhaps (if plumbing codes allowed) a plain p-trap could be swapped out for an exchanger unit that also serves as a sewer gas stop.

I don't know anything about thermodynamics; how long would it take to transfer a reasonable amount of energy from waste water into the coolant?

Another issue again, running all these coolant lines back to a central water heater w/another exchanger. It does all sound pretty complicated and expensive.

[rainbowzootsuit]

In vertical stacks surface tension makes flowing water cling to the perimeter of the pipe. The supply can then be coaxial like I think you are saying.

This is one product (linked below) like this although I think plastic would be fine if a little less thermal conductivity and less expensive than this much copper.

https://renewability.com/

[pandora-health]

There are three showers and two baths in the house. All upstairs, which allows for the pipes to recover some of the heat on the way down. This excludes sinks where you most likely to use cooler, if not cold water. I take the maximum efficiency of heat recovered with a pinch of salt, but the hot water tank seems to last for longer heat-wise, compared to the one I had in the previous house, without such system.

[hinkley]

Technology Connections has a pretty good rundown of how electric hot water heaters work.

You’re using hot water at X gallons per minute while the heater can heat Y<X gallons a minute but the water is extracted from the top of the tank so it stays fairly hot until you’ve used more than half of the tank, then drops like a rock at the end when the supply is exhausted.

Anything that warms the cold water going to your shower slightly reduces the rate of hot water use, which extends the amount of time before you run out, which is more time to heat more water.

[pandora-health]

Yes, that's what I noticed in the previous house, where the water tank was connected to a solar thermal panel. Solar thermal panel heats the oil inside it and the heat is exchanged between the water tank and the panel during the day when it's sunny. That system was about 60% efficient throughout the year but I did notice that the temperatures would go down really fast and that didn't feel efficient.