Why is hot waste water (shower, sink…) not reused for heating?

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Why is it not used again and wouldn’t it be very efficient to combine with a heat pump for reuse?
Especially since water from shower or sink isn’t usually very dirty, couldn’t it be stored for a while in an isolated tank so that e.g. a heat pump can run more efficient on it than on usually colder air or colder ground water?

In: Engineering

27 Answers

Anonymous 0 Comments

I want to say it can be reused. I think I recall a product in a solar home catalog back in the early aughts that acted as a heat exchanger between grey water and the clean water entering your hot water heater. It would preheat the water entering the tank, saving you a couple bucks a month or year, depending on how much hot water you use in a day.

Anonymous 0 Comments

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Anonymous 0 Comments

There are lots of things in sink and shower water. There is hair. People shave. There is grime. If you come from a swim in the ocean lots of salt. If you come from a hike maybe some dirt. If you come from the beach there is sand. If you come from a hard day of work there could be oil, soil,… That’s of course before you even start talking about the soap, make up, shampoo etc.

There has been some experiments with re-using that water for things like toilet. But even that you need to clean and filter it.

Anonymous 0 Comments

It’s warm, but it’s not hot enough to provide much useful energy. Hot water/steam heat is much hotter than you’d be comfortable taking a shower in.

You’d need a whole separate drainage and pump system that’s only gonna run for a few minutes a day to recover the minuscule heat from lukewarm water flushed down the bath tub. Not worth the cost, and probably costs more energy to manufacture than you’d ever recover.

Sometimes the water that goes down those drains *is* very dirty too, so you’d need some way to select recycle or sewer.

Anonymous 0 Comments

The magnitudes in what you are trying to get out of that waste wouldnt be quite enough and since water is such a great transient for hot/cold most of it is already lost in someway, something more like “what if the water heater could give off some ambient heat to warm up the heating”, well thats because the energy required to warm, say a room, by that would increase to the point of complete inefficiency, and you’d be better off putting that energy directly into an actual cooling/heating mechanism.

Anonymous 0 Comments

It’d be hard to retrofit a system like that. You’d need a decent sized holding tank to work though the water as you get the heat down.

I saw a thing recommending leaving the shower drain stopped so the hot water stayed in the tub. The heat from the water then radiates into the room instead of going down the drain.

Anonymous 0 Comments

It is possible, just not many people bother. See [https://www.homedepot.com/p/Power-Pipe-3-in-x-48-in-Drain-Water-Heat-Recovery-Unit-R3-48/203456041](https://www.homedepot.com/p/Power-Pipe-3-in-x-48-in-Drain-Water-Heat-Recovery-Unit-R3-48/203456041)

Anonymous 0 Comments

Most household water only starts at 120-140 degrees to begin with. By the time is passes through the pipe in the house, runs though the air, down the drain and back to some location to be stored its not that hot any more.

Furthermore its usually going to be mixed with cold water by the user.
There is no practical way to separate cold water going down the drain from hot,so it all ends up in the same place further reducing the temperature available for the pump.

Yes you could design some complex system of pipes that detect the temperature and shunt it one way or another but the more complex you make the system the less return you are getting out of it and the more likely it breaks down. Our current water disposal systems are simple and extremely cheap. Water flows downhill. Thats it. No pumps, no storage, no insulation to maintain temperature no diversions. It flows downhill all the way to the sewage treatment plant.

Your collected water system needs to divert the water somewhere, hold it, insulate it from losing more heat and then run it through a system to get the heat from it. Much more complex than todays drainage with much higher up front and maintenance costs. The end result is you gain no benefit from capturing barely if that heated water water for use in a heat pump.

Anonymous 0 Comments

By the time the hot water hits the drain and travels down uninsulated pipes it cools down to ambient temperature. So there’s not much useful energy left anymore at that point. Now some places use ‘grey water’ from sinks for irrigation. ‘Black water’ from toilets must be on a different system so that adds to cost.

Anonymous 0 Comments

Our new home uses the hot waste water from the shower to re-heat part of the new water going up through the pipes