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Community and Q&A

Relocating drains to maximize drain water heat recovery

mackstann | Posted in Mechanicals on

I have a central stack for the bathroom, iron pipe, probably 4″ (not a plumber and haven’t measured it, but it’s big). This is where my DWHR will go, and serving the shower will give me the lion’s share of the heat recovery benefits. But I wonder if I could do better.

My kitchen has a separate stack, about 2″, which is about 12 feet away. I seemingly could connect this to the main stack (they’re both in the basement, and out of the way of anything important), and even with 1/4″ per foot slope, it would still connect to the main stack high enough to leave plenty of room for the DWHR. It would also give me one less sub-slab pipe to worry about in the future.

The washing machine is just a few feet from the main stack, and drains to a utility sink that drains into yet another small stack that connects under the slab. This one is harder… it seems that washing machines cannot pump up to the roughly 6 foot height I would need to get the water to the top of the main stack. I thought about using some kind of pump in the utility sink, but I don’t like the complexity and possible fragility of the pump, and I’m not sure how I could plumb it anyway.

I use cold water for my laundry, but we have kids and a dog, and my wife feels it is important to use warm or hot water for some of the more filthy things. We have a top loader and I think that a top loader linked to a DWHR could be much more cost effective than upgrading to a HE washer.

Any advice?

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  1. GBA Editor
    Martin Holladay | | #1

    It's OK to think about this issue, but it's time to let the washing machine go. No biggie.

    You certainly don't want to be using electricity to run a pump to move drainwater, simply in hopes of raising the temperature of a cold water line 1 F degree or even 5 F degrees. You energy input is likely to be greater than the value of the small thermal benefit.

  2. mackstann | | #2

    Martin, that is what I suspected.

    The kitchen drain, however, seems more viable... are there any problems inherent in adding a ~14' horizontal (sloped) section to it? It would even need to have a couple bends, to go behind and around the stair case.

  3. GBA Editor
    Martin Holladay | | #3

    As long as you follow basic plumbing principles for minimum drain line slope -- 1/4 inch per foot -- you can install a long run that is almost horizontal. But such long pipes are more prone to clogging than vertical pipes, and are more likely to need to be snaked out now and then. And I doubt whether the minor thermal benefit of adding the kitchen drain water to your drainwater heat recovery system is worth the disadvantages and expense of reconfiguring your pipe.

  4. Expert Member
    Dana Dorsett | | #4

    Natural Resources Canada tried to measure the additional heat recovery from smaller more intermittent draws such as kitchen and lavatory sinks, etc. and was unable to come up with anything above the measurement error noise floor.

    The heat recovery from batch draws such as tub-bathing, dishwashers, clothes washers, (or even toilet flushes, where the drain water is still 10s of degrees above the incoming water temp) are essentially nil, even on the best-case theoretical basis, since so little of the potable flow is concurrent with the drain flow. To get a reasonable return requires that the drain and potable water flows occur at the same time, for a long enough period that the thermal mass of the drain upstream of the heat exchanger isn't lowering the temp of the drain water as the drain plumbing warms up. Sending a few quarts of water from a dishwater down a drain just raises the drain a fraction of a degree if iron, maybe a couple of degrees if PVC, so it's going to be pretty close to room temp by the time it hits the heat exchanger. And even if it were 140F, the amount of water in the potable wrap is the only water that got heated up, since it's not filling and draining at the same time. Same story with clothes washers. An extended (several minutes) long rinse of hot water at 0.5-1gpm a the kitchen sink would probably yield a measurable return, but that's not how people generally use hot water.

    Showering typically counts for a third to half the total hot water use in most families, and are 1.5-2gpm flows for several minutes at a time, which is is a high enough volume and duration that the losses to the thermal mass of the drain plumbing doesn't impact the energy returned to the incoming flow by very much. The return in the first minute is lower than the return during the last minute of the shower, but the average is still pretty good.

    Bottom line- it doesn't hurt to route other drains through the DWHR unit, but it's not going to make an appreciable difference on the amount of heat recovery you get out of it.

  5. mackstann | | #5

    Dana, that's a very helpful citation. The fact that it's actually been tested makes me much more comfortable in making my decision. It sounds like re-routing the kitchen drain wouldn't be worth the trouble. Furthermore, I am a hot shower glutton (part of my motivation here is sheer guilt), and will have two teenage boys in a few years, so the shower water will drown out the kitchen water even more than average. Thanks!

  6. user-1135248 | | #6

    Don't want to hijack the thread, but does anyone have a feel
    for what percentage of DHW heat goes out of the house via the
    soil pipe vs. how much heat escapes into the building and helps
    heat it? In the absence of a heat-recovery system, that is.
    Just trying to refine my heating-BTU calculations by taking
    some proportion of DHW usage into account, and I realize that
    much will have to do with pipe insulation and length, usage
    patterns, heater type ... just looking for ballpark.


  7. Expert Member
    Dana Dorsett | | #7

    Without the drainwater heat recovery almost all the heat of tepid or hot water goes down the drain. The heat transfer rates along the horizontal sections is small- gravity keeps it running in deep narrow stream along the bottom edge of the pipe, limiting the contact area. The transfer is much more rapid vertical sections of pipe: Without gravity causing it to stay on one side of the pipe the surface-tension causes the water to cling to the surface and spread out into a thin film (the process is called "wetting", even if the fluid isn't water ). The amount of potentially useful heat that can be retained by the vertical sections of pipe are a function of it's thermal mass.

    The heat transfer from the pipe to the air is comparatively slow, primarily via radiation to it's surroundings, so once the temp of the pipe is near that of the water clinging to it's inner surface your heat capture is pretty much done, whether it got there with a gallon of hot water or 50 gallons. The thermal mass of plastic drain plumbing is pretty small- orders of magnitude less than cast iron, but I doubt the energy benefit of the higher heat retention is large enough to rationalize changing over to cast-iron drains.

    So, the amount of heat retained depends a lot on your plumbing geometry & type: A slab-on grade single-story with plastic drains would retain less drain heat than a 2 story with a full basement & cast iron drains, with the bathroom on the second story. But even in the latter case it's probably less than 1% of the total energy used to heat that water (or house, in if you're replacing stagnated 70F water going down the drain through a 65F basement, with 45F incoming water, the toilet-flush scenario.)

  8. mackstann | | #8

    I'd like to circle back to something I mentioned in passing but kind of forgot about.

    The kitchen drain has its own separate stack. It goes straight down, under my basement slab, and connects horizontally to the main drain line underground somewhere. Isn't this a liability? It's a 63 year old ~2 inch iron pipe. Isn't it going to eventually rust through or crack? Is it best to just wait until there's a problem, instead of fretting about it? When the time does come to replace it, wouldn't it be easier to run the new horizontal connection indoors instead of under the slab?

  9. GBA Editor
    Martin Holladay | | #9

    Q. "When the time does come to replace it, wouldn't it be easier to run the new horizontal connection indoors instead of under the slab?"

    A. Yes.

  10. Expert Member
    MALCOLM TAYLOR | | #10

    My mistake.

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