Drainwater heat recovery questions
Hello, I’ve been reading some things on drainwater heat recovery and came upon this article: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/toronto-passive-some-thoughts-drainwater-heat-recovery#comment-form
In it, Lyndon Than says, “Leave at least a 12-inch run of straight drain pipe above the exchanger to smooth out the flow.”
How critical is that? I don’t have.much room to work with — about 36″ in total. I could likely get a 30″ unit in that space, but if the 12″ above is necessary, it wouldn’t be worth the trouble.
Also, why aren’t the units insulated? An insulating wrap similar to pipe insulation could increase efficiency, couldn’t it?
Thanks.
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Replies
Calum,
1. I just reviewed the installation instructions for a popular drainwater heat recovery device (the Renewability Power-Pipe), and I didn't see any requirement for 12 inches of vertical drain pipe above the unit. It's possible that the 12 inches of vertical pipe above the unit is desirable, but it isn't required. If any GBA readers have data supporting the need for the vertical pipe above the unit, please share that data.
2. Believe it or not, insulation doesn't improve performance of these units by a significant amount. That said, if you want to insulate it, go ahead. Make sure that you choose a type of insulation that isn't vapor-permeable, however, because condensation can occur on cold water pipes in humid weather.
Thanks Martin.
For units shorter than 5-6' the temperature of the potable water output of these things is only slightly above room temperature, and the convective losses to the room air are, low and only occurring the the top of the heat exchanger, where it's above room temp.
The bottom of the heat exchanger is closer to the incoming water temperature, typically much lower than room temperature. So the bottom part of the heat exchanger is taking up a bit of heat from the room, the top of the heat exchanger is giving up some heat to the room.
With smaller heat exchangers even the top of the heat exchanger may be below room temp most of the year, and insulating it would only impede the tiny amount of room air heat it might take up. With taller heat exchangers there may be more of a balance- insulating the top of the heat exchanger might move the needle ever so slightly, but it would probably be difficult to measure, and would definitely not be cost effective, or worth your time, even if the insulation were free. The duty cycle is so low in a typical house, that the "payback" would be "not within the lifecycle of the heat exchanger".
For industrial applications at higher temperatures that can be revisited, but even where they use them in car washes at a high duty cycle it's simply not worth it.
Thanks Dana.
I'll leave the insulation off for now, but I reserve the right to revisit the idea if I'm getting condensation on the heat exchanger. I've had a few people give me odd looks when they see my well tank and water softener are insulated with water heater wrap to prevent condensation... :)
The heat exchanger could theoretically get a haze condensation near the bottom while the shower is in use (though I've never observed that with mine), but unlike a tank full of cold water, it doesn't stay cold for very long once the flow stops. There is very little thermal mass in the heat exchanger- typically less than 5lbs of water in it that would be below below room temperature, and an even lesser amount that would be below the dew point of the room air. The thermal conductivity of the copper moves heat from the warmer part of the heat exchanger to the colder part, evening out the temperatures fairly rapidly after the flow stops.
If uninsulated cold water distribution plumbing in the basement doesn't get condensation, the heat exchanger won't either, since it is being actively heated by the drain water whenever the water is flowing, and will be warmer than your cold water distribution plumbing.
How cold is water from your well? Sounds like you live in the South, it can't be that cold.
Dana,
I do get condensation on my cold pipes. I've purposefully left them uninsulated in a particular section to allow the condensation to drip in a place that won't hurt anything...
Anon3,
I'm in Nova Scotia. My well water is about 5-7*C. I'm curious why you thought I live in the south?
With a 35% efficiency recovery unit (something 3' x 30" or 4 " x 30") with 7C incoming water and 38-40C drain water, the temperature of the heat exchanger 10 minutes after the shower stops will be between 15-18C as the temperature gradient evens-up, but cold water distribution plumbing would still be below 10C.
Outdoor dew points of 15-18C happen in NS, and interior dew points of 10C aren't rare basements, but if there is any mechanical dehumidification or air conditioning in the house the interior dew point won't be be as high as 15C.
Most NS is south of even the most southerly parts NL, so that's "... in the South...", isn't it? :-)
Or does only the Ontario Riviera count as "...the South..."?
Lol, fair enough, this is the south. Typically when I read that I expect it to mean the southern US.
I do have a heat pump in the basement, but I try to avoid running it in the summer to save on power. The basement has never gotten hotter than 22*C. I guess another side benefit of the DWHR is that it'll stop the condensation from happening in the walls.
How is the drain water heat recovery theoretically supposed to do anything for mitigating condensation in walls?
By your math the temperature of the cold water past that point shouldn't be below the dew point of the room. The cold water pipes don't go into a wall untill after the main stack line.
I think if you are trying to avoid condensation in the wall, a tempering valve would be a better idea. Just remember to skip it for the kitchen and drinking water.
The heat exchanger is only warm during and shortly after a shower. Cold water flows through the exchanger that occur when there ISN'T warm water going down the drain will lower the temp of the heat exchanger VERY quickly, and it will pass the ~7C water into the rest of the distribution plumbing without heating it up.
There is very little thermal mass in the heat exchanger. It's a heat exchanger, not a thermal storage device.