4 ms·

Agreed. I suspect one barrier to that is making a good heat exchanger that doesn't get clogged with hair etc. Another is that whatever you use it for, you will

Agreed. I suspect one barrier to that is making a good heat exchanger that doesn't get clogged with hair etc. Another is that whatever you use it for, you will likely want a higher temperature than waste bathwater so it only works as pre-heat for something ... which means you need something running that needs it.

sparselogic 7y ago
Drain line heat exchangers are built for this. They preheat the water heater supply line using waste heat from drain water. Renewability Power-Pipe [1] was the first one I encountered, but there are several in the NA market. [1] http://renewability.com/ http://renewability.com/
- twic 7y ago
  > Water falling down a vertical drain stack does not run down the middle of the stack, but instead clings to the inside wall of the pipe. In a Drain Water Heat Recovery (DWHR) application, falling drain water forms a thin film that rapidly imparts its heat to the colder pipe wall. The Power-Pipe® captures much of this heat, then "recycles" it to raise the temperature of incoming cold water. That's brilliant. And it means that if you want to increase the surface area for heat exchange, you make the waste pipe bigger, which reduces, rather than increases, the chances of clogging!
  - dsfyu404ed 7y ago
    >which reduces, rather than increases, the chances of clogging! Yes and no. A bigger pipe will have slower flow (it's not linear based on area since the pipe is not full at all times) which lets things settle.
    - twic 7y ago
      The point is that the pipe is never full, because the waste water runs down the walls. Nothing's going to settle in a vertical pipe. When the pipe leaves the heat exchanger and goes back to wiggling around the building on the way to the sewer, then sure, go back to sizing pipes based on thinking about settlement and other things.
  - frosted-flakes 7y ago
    These are required by building codes in my area, and I've wondered how effective they really are. It never occurred to me that water would cling to the side of the pipe. Some of the other trades suggested that it would make more sense to have them on horizontal runs, but that would probably be less effective.
twic 7y ago
If you're showering, then you have a need to pre-heat cold water at a rate perfectly matching the rate at which you discard warm water!

sparselogic 7y ago

Drain line heat exchangers are built for this. They preheat the water heater supply line using waste heat from drain water. Renewability Power-Pipe [1] was the first one I encountered, but there are several in the NA market. [1] http://renewability.com/ http://renewability.com/

twic 7y ago
> Water falling down a vertical drain stack does not run down the middle of the stack, but instead clings to the inside wall of the pipe. In a Drain Water Heat Recovery (DWHR) application, falling drain water forms a thin film that rapidly imparts its heat to the colder pipe wall. The Power-Pipe® captures much of this heat, then "recycles" it to raise the temperature of incoming cold water. That's brilliant. And it means that if you want to increase the surface area for heat exchange, you make the waste pipe bigger, which reduces, rather than increases, the chances of clogging!
- dsfyu404ed 7y ago
  >which reduces, rather than increases, the chances of clogging! Yes and no. A bigger pipe will have slower flow (it's not linear based on area since the pipe is not full at all times) which lets things settle.
  - twic 7y ago
    The point is that the pipe is never full, because the waste water runs down the walls. Nothing's going to settle in a vertical pipe. When the pipe leaves the heat exchanger and goes back to wiggling around the building on the way to the sewer, then sure, go back to sizing pipes based on thinking about settlement and other things.
- frosted-flakes 7y ago
  These are required by building codes in my area, and I've wondered how effective they really are. It never occurred to me that water would cling to the side of the pipe. Some of the other trades suggested that it would make more sense to have them on horizontal runs, but that would probably be less effective.

twic 7y ago

If you're showering, then you have a need to pre-heat cold water at a rate perfectly matching the rate at which you discard warm water!