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
Temperature gradient, the ELI5 is heat energy always moves to equilibrium that is to a balance where it won’t move anymore, the greater the difference in temperatures the greater the function through a heat exchange. The shower water you use is only about 10⁰c hotter than your body by the time it hits the drain it loses a couple more degrees, by the time it would be collected from a drain and recirculated through a heat exchange it would only be a few degrees above ambient air temperature and would barely heat your home at all. The hot water used in water heating systems is much hotter than the water used in your shower and is circulated directly off the boiler and back to the boiler, this water stays clean. Combining the shower water with the boiler water would only lower the temperature of the water from the boiler it would also turn the hot water into waste water, this means a grey water heat recirculating system has to operate on its own loop, it’s a high infrastructure cost for an extremely low return, it probably exists somewhere but for larger scale applications than the shower in a single residential home.
It is not that difficult (if you have any plumbing skills) to divert the drain to have the option of draining into a closed holding tank, which in turn drains back into the main drain. But it is hardly worth the effort, compared to other things like sealing out drafts.
This can be done. I don’t know too much about it, but I believe it is a relatively new idea and probably slow to take off, but wherever there is a source of heat, it’s probably being looked at as a heat pump source. I think the idea is to run the heat pump refrigerant line set through the sewage pipes, so it captures that heat before it goes off to the sewer. Probably best done in a large commercial project where there’s a reliable source of effluent.
Also there this low tech domestic solution someone else pointed out that seems pretty smart: [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)
It’s easy to capture the heat from your shower, if you really want to. Just cover the drain while showering, come back and let the water out when it’s cool.
The water cools off shortly after hitting the drain. It’s not contained well or insulated so it is basically just going to cost lots of money to get a system to work the way you want. On the other hand, the heat it does radiate out of the pipe as it drains is raising the temperature of the space slightly so technically it’s already doing the work.
The HVAC system that is already used is called an “exhaust recovery ventilator” does something like what you are imagining. It has a system that uses outgoing air to pre heat/pre cool outside air in places like hotels, highrises and nursing homes that are typically closed up tight.
So the heat exchanger thats being spammed in the comments looks only mildly useful at best.
The thing ive had in mind for a while is a shower that you can switch from drain to repeat. You soap and rinse off, then flip the switch. It takes the hot water going down the drain, hits it with uv sterilizing (probably not nescessary, but still a good idea) and a heating coil, and cycles the water back up to the nozzle. Guilt free long showers sounds like a game changer to me
Practicality.
Not all water that goes down the drain is hot. Even when washing hands, or taking a shower, the water is usually “warm”. It would be unlikely that you could extract any meaningful amount of usable energy from it.
Hot water usage in the home is typically used only briefly. There would unlikely be sufficient quantity of hot water available for energy recovery.
And while most hot water sent down the drain doesn’t immediately appear to”dirty”, that’s not necessarily true for everything sent down the drain. This would require more maintenance and upkeep if you’re storing this in a tank for energy extraction later.
I imagine it would be far more practical, economical, and efficient to slap a small solar panel on a heating element and stick it in a tank of water or solution to keep it warm/hot.
A friend works in… water stuff. Apparently it’s not done, at least where I live, because of biological reasons. There’s a certain kind of biological process that happens in the sewer network that’s essential for waste water treatment. Water in the system is expected to be within a certain temperature range and, if we were to pump all the heat out of it and have 4°C water in the pipes, waste treatment just wouldn’t work.
Vancouver does this for the False Creek neighbourhood https://vancouver.ca/home-property-development/southeast-false-creek-neighbourhood-energy-utility.aspx
My instincts are that the amount of heat you could recover from the wastewater would never justify the amount of plumbing you’d have to do to set this up. Maybe if you have a tankless water heater you could preheat the city water a little with a heat exchanger… but drain lines have to be huge to avoid clogging so this thing is going to be large, expensive, and not all that efficient.
Latest Answers