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Drain water heat recovery DWHR pipe installation best practice

Tanton | Posted in Energy Efficiency and Durability on

We have two 72″ drain water heat recovery pipes installed, underneath each of the showers, located at far ends of a house.

The plumber has installed already but I don’t think they’ve ever done this before. Before it’s closed up, I want to verify what is the most energy efficient method of installation for these devices. 

From what I have seen from various vendors (PowerPipe, Watercycles – the one we have, etc) there are typically three installation options:

1. Cold water enters at the bottom of recovery pipe, warmed water goes to cold water intake of hot water heater.

2. Cold water enters at the bottom of recovery pipe, warmed water goes to cold water tap of shower

3. Cold water enters at the bottom of recovery pipe, warmed water goes to both hot water heater and shower.

Apparently, the third option appears to be the preferred option in terms of efficiency. But, at least for Watercycles, they say a mixing valve is required after the warm water comes out the top of the DWHR where it splits to go to shower and the hot water heater. 

 – I don’t really understand the necessity of this mixing valve. The plumber is reticent to install a mixing valve buried somewhere in a wall.

– What is the difference in efficiency between the above three options? I want to get this right but if the difference between, say option one and three is negligible, then that is an important consideration.

Apologies if this has already been discussed but I’ve not been able to find this information here or in a wider search.

Thanks in advance. The water heaters are electric resistance Marathon heaters.

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Off the cuff reply, with no real analysis. First, I see no reason for a mixing valve. Mixing valves are used when there is a risk of scalding, or when you need to control a circuit from overheating (like with radiant floors). I don't see that either case is appropriate to DWHR. You might want to touch base with the manufacturer's tech support to find their reasoning. FWIW, I agree with your plumber that you don't want a mixing valve in a hidden location. It must be accessible for servicing, because they will eventually fail and leak, sometimes withi just a few years.

    I generally like to see the warmed water go to the water heater inlet, but your case might be an exception. You mention that the DWHRs are at opposite ends of the house. I'm guessing that the water heater is centrally located? If so, the warmed water would have to flow back to the water heater before providing any help at all. Most of the scavenged heat will be lost to the building, and there will be a long delay between warming the water and the warmed water getting to the WH. All of this reduces much of the value of the DWHR. OTOH, if you pipe the warmed water to the cold water inlet of the shower, the recovered heat is used immediately, very close to the point of generation. This is probably the most effective and efficient.

    1. Tanton | | #3

      Hi, thanks for the response. There are two hot water heaters, each located directly under the DWHR. Sorry, should have mentioned that in my post.

  2. jonny_h | | #2

    I haven't looked at Watercycles in particular, but I've looked at a couple others and never seen a mixing valve on the output of the heat recovery unit -- and to me it doesn't seem like one would be necessary, because thanks to physics the water at the output of the heat recovery unit will always be colder than the hot water you're actually using.

    A note though on the options that pipe the output of the heat recovery unit to the shower cold directly (which is something I'm looking at doing due to how pipes are routed) -- you may want a thermostatic shower valve (rather than the much more common pressure compensating valve) since during the first couple minutes of the shower, the "cold" side will change from "cold" to "some degree of less cold" to "warm", meaning that a ratiometric mixing valve will need to be constantly tweaked to maintain a comfortable temperature, while a thermostatic valve will compensate for it on its own.

    1. Tanton | | #4

      Hi, thanks for the response. I believe they are thermostatic valves in the showers but will double check.

      I've sent an email to the manufacturer (Watercycles) for clarification and will share here. I also drew a very crude diagram of what I believe is my third (and apparently, most efficient) option. I've attached it here. There is no mixing valve, contrary to what Watercycles says, but I believe there remains some confusion (mostly by me and caused by me!) and hope this diagram will help.

      Again, thanks for the responses. I'm hoping we can determine the ideal installation method so others on the forum can benefit.

  3. Expert Member
    PETER G ENGLE PE | | #5

    The sketch helps a lot. With the WH close to the shower, the plumbing you show is fine. The mixing valve at the shower may or may not be required. Most thermostatic shower valves qualify as a mixing valve for anti-scald protection.

  4. charlie_sullivan | | #6

    Mostly confirming what others have said but perhaps clarifying some as well:

    The third option is more efficient: having more water flowing through the exchanger gives the opportunity to recover more of the heat that would otherwise go down the drain.

    I agree that what the manufacturer meant is probably a thermostatic valve at the shower. I have a DWHR system without that, and as I'm showering, I notice a little boost in the temperature as the DWHR starts working. I feel no need to install a thermostatic valve, as I really enjoy noticing that pump and knowing that the DWHR is working. Sometimes I then tweak the temperature and sometimes I just enjoy the warmth.

    1. Tanton | | #7

      Thanks for the comment. I can’t really get a definitive answer about the ideal installation. The rep from company that makes the DWHR says that connecting warm water to hot water tank is best but this is contrary to their own literature (warm water goes to both shower and water tank) and other DWHR companies.

      So, I don’t feel satisfied with the answer and wonder if anyone has some research to point to on this issue. I read one guy’s masters thesis but it didn’t contemplate the return plumbing.

      So maybe it doesn’t matter that much?

  5. Tanton | | #8

    Hi, final update on this, in case it helps anyone. Couldn’t get a definitive answer from the pipe manufacturer nor from a reputable academic source of what is the most efficient install.

    However, for me, it’s moot. The building code where I live won’t permit the warmed water from going back to the cold tap in the shower. Apparently it can allow bacteria to grow in the pipe, thus releasing a disease-laden stream of water in your face when you shower.

    I kid you not. :)

    So we’re going to stick with warmed water feeds cold water supply to the hot water heaters.


  6. ibrahincomp | | #9

    I'm not versed in which method of installing drainage pipes is the most energy efficient. But I do know that you should never do it yourself. Do you know why? Because I was an idiot when I did it myself. I installed French drains at the outlet of my little farm, which is on an elevated site. There's a little hill there. Afterward, I put the black pipes in. The sewer went into the basement. And that's where the leaks started. If it wasn't for the pollution remediation and water damage company, I would have started doing repairs in the basement. I imagine how much I'd have spent laying a new concrete floor. So I am happy I found these professionals. Don't skimp on professional help!

    1. DC_Contrarian | | #10

      Spam, spam, spam, spam.

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