4 ms·

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 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).

thinkcontext 3y ago
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.
hinkley 3y ago
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 https://youtube.com/watch?v=Bm7L-2J52GU
- blacksmith_tb 3y ago
  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.
  - hinkley 3y ago
    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.

thinkcontext 3y ago

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.

hinkley 3y ago

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

blacksmith_tb 3y ago
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.
- hinkley 3y ago
  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.