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Drain Water Heat Recovery (DWHR) options for shower drain in crawlspace?

Nic Smith | Posted in Green Products and Materials on

Hello all,

I am about to start a bathroom remodel and am interested in adding drain water heat recovery (DWHR) to the project. The main motivation for this is not “payback”, but rather to improve the performance of my heat pump water heater.

My home is on a crawlspace with good access to plumbing. However, being a crawlspace, there is more limited vertical height compared with a full basement. There *may* be just enough vertical height for one of the shorter DWHR units, but this depends on where the drain outlet on the DWHR needs to be connected.

I have two questions:

1. What are my options for the DWHR unit’s drain outlet connection? Does this need to connect to the main plumbing stack, or can it be connected downstream of the plumbing stack, e.g., directly to the home’s drain line?

2. I like EcoDrain’s design and efficiency, and their shortest units are 36″ (3″ pipe) and 32″ (4″ pipe). I am not a plumber, but I assume I can use either one of these options (3″ or 4″ diameter) with an adapter for a typical 2″ shower drain – is that correct? Are there any considerations on choosing the 3″ vs 4″ diameter models that I’m not aware of?

Thanks for any advice – I’d really like to install one of these units during the remodel!

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Replies

  1. DCContrarian | | #1

    The requirements for a drain are demanding but not that complicated.

    Below the drain in the shower there has to be a trap. The outlet of the trap has to travel horizontally -- which in the world of plumbing means sloped exactly 1/4" per foot -- until it comes to a tee. One end of the tee connects to a vent, which exits through the roof, and the other end connects to the waste line, which exits underground. For its entire run the waste line has to be either vertical or horizontal (which means 1/4" slope per foot). The vent line also has to always run either vertical or horizontal, which for a vent means sloping upward 1/4" per foot.

    The distance between the trap and the tee can be no more than 48 times the pipe diameter -- 6' for 1 1/2" pipe and 8' for 2" pipe.

    As long as you meet those rules you can tap into the drain anywhere on the pipe.

    The iron law of plumbing waste lines is you can never decrease pipe diameter. Doing so creates a spot that will be prone to clogging. So if you use a DWHR that is 3" diameter everything downstream of it has to be 3" or bigger.

    Can the DWHR be mounted horizontally?

  2. Bennett G. | | #2

    Why wouldn't an Ecodrain A1000 https://ecodrain.com/en/products/A1000/ horizontal 2" work between the trap and tee?

    Article here on GBA https://www.greenbuildingadvisor.com/article/drainwater-heat-recovery-comes-of-age

    1. Expert Member
      Dana Dorsett | | #3

      Return efficiency goes up with drain diameter- a 3" unit has only 3/4 the amount surface area in contact with the gravity film of water flowing down the drain of a 4" unit, resulting in less heat return. The shorter 4" x 32" VT1000-4-32 tests at 44.6% efficiency, slightly higher than the 43.5% for the longer 3" x 36" VT1000-3-36, tested under Natural Resources Canada' s standardized protocol. Fatter & longer is always better as long as it fits.

      The return efficiency of the horizontal units are pretty lousy compared to a 3 foot gravity film (must be vertical) types, though better than nothing, and the cost is higher.

    2. Alan Afsari | | #8

      I don’t think the A1000 is available.

      I have a similar situation (crawl). I called eco drain a couple months ago and was told only the vt 1000 was for sale.

      Are there any other horizontal units?

  3. Expert Member
    Malcolm Taylor | | #4

    DWHR seems as though depending on occupant behaviour you could end up with either the anticipated maximum energy savings or none at all.

    1. Expert Member
      Dana Dorsett | | #5

      True, if it were about energy savings, but it's not.

      Nic's interest is "...to improve the performance of my heat pump water heater".

      I'm assuming what he means by that is an increase in "apparent capacity", &/or reduced recovery time after showers, not energy savings. DWHR does both.

      1. Expert Member
        Malcolm Taylor | | #6

        As long as showers form a significant portion of the hot water use. If there are no showers, is there any energy savings or improved performance?

        1. DCContrarian | | #7

          The way I've seen it used, DWHR warms the cold water going to the shower so you use less hot water to get the same temperature. In order for it to work everything has to line up -- the water being used has to come in on the same pipe as the DWHR, the used water has to go down the drain with the DWHR, and it has to all happen at the same time -- if you fill a tub and then drain it the DWHR does nothing because the draining is after the filling. Also, the drain water has to be pretty hot to make a difference.

          There aren't a lot of appliances other than showers that fit that profile.

        2. Expert Member
          Zephyr7 | | #9

          I’d suspect that dish washers and probably clothes washers can also see some benefit from DWHR units. Anything dumping hot water down the drain would see at least a little, even hand washing, but showers are probably the biggest winners due to the relatively long time duration compared to the others.

          Bill

          1. Expert Member
            Dana Dorsett | | #13

            >"I’d suspect that dish washers and probably clothes washers can also see some benefit from DWHR units."

            NOPE!

            Not a bit.

            These are not heat storage devices- the incoming water has to be flowing at the same time the drain is flowing to reap a measurable benefit. That makes it great for showers, but near zero benefit for any batch draws (bathtubs, washers, etc.). Even short draws like handwashing aren't long enough to reap a measurable benefit.

            NRCAN spent some real money looking pretty carefully at these issues 13-15 years ago. One thing that came out of that research is the standard test protocol to be able to compare apples to apples between vendors & models. Several other academic, provincial & utility organizations have done in-situ measurements as well as lab experiments:

            https://maisonsaine.ca/wp-content/uploads/2012/08/B-12-GI-23Doc1-1_RepDDRSE-AQLPA_3637-2_28sept07.pdf

            http://gfxtechnology.com/NRCAN-6_29_07.pdf

            https://ecoinnovation.ca/wp-content/uploads/2019/PDFs/CCHT_2006_REPORT_DWHR_SHOWER_TIMES.pdf

            https://sustainabletechnologies.ca/app/uploads/2015/06/DWHR_TechBrief_June2015.pdf

            https://open.library.ubc.ca/cIRcle/collections/graduateresearch/66428/items/1.0108494

            So while they're pretty good at making a 50 gallon tank seem like a bigger tank for showering purposes with substantial energy savings there, no evidence of measurable benefit from other typical household water use exists. The "payback" is pretty quick at commercial laundries & car washes, but for homes where showers aren't in daily use the energy savings aren't there.

            For showering families with periods of the day when the duty cycle of the showers are high, there is some "payback" up front in being able to down-size the water heater, and in the reduced recovery times for storage type water heaters.

          2. Expert Member
            Zephyr7 | | #14

            That makes sense, I hadn’t thought of that (but should have), in regards to dishwashers and clothes washers. Hot water going in does NOT go out the drain at the same time, so there is no possibility to recover any heat to “preheat” the replacement water going into the water heater during the time the hot water is drawn out.

            In a commercial setting like a laundromat, I imagine there is a decent chance that one machine may be draining while another is filling, so you get some chance at heat recovery due to the staggered nature of the individual machines operating cycles. With only one machine in a typical residence, this isn’t possible.

            What all of this does mean is that you can get most of the benefit of a DWHR by installing it on the drain line of whichever is the most active shower in a house. It’s not necessary to install the DWHR on the main drain out of the house since most of the heat available for recovery is only coming from the shower.

            Bill

          3. DCContrarian | | #15

            >What all of this does mean is that you can get most of the benefit of a DWHR by installing it on the drain line of whichever is the most active shower in a house.

            I would go further and say you get more benefit from installing it just on the drain of the most active shower. If it's installed on a shared drain it loses effectiveness when other, non-heated water is used elsewhere in the house. If someone flushes a toilet all of a sudden that drain water is awfully cold. The highest possible benefit would be to put one on every shower. Whether that makes economic sense depends on the use pattern.

            BTW, a design that can only go vertical and requires 36" is asking a lot of the plumbing. That's normally going to be the drain going down to the lower floor, and that will typically be shared unless you plumbed it from the beginning for heat recovery.

            If you're plumbing for heat recovery I from the get-go I could see a design where all the showers use one drain that doesn't meet up with the rest of the house until the basement, and all the showers share a cold water supply that goes through the heat recovery.

  4. Kevin Henry | | #10

    I wrestled with these questions a few months ago.

    Like Alan, I asked EcoDrain about the A1000 and B1000 and was told they were not available. So it's quite possible that there are no horizontal units currently on the market. EcoDrain does have the best reported performance of the vertical models, and I ended up getting the VT1000-4-32.

    One question you'll face is where to send the fresh water after it's warmed. You can send it to the hot water heater, the cold side of the shower, or both. The last will give you the best performance, as shown in research by Natural Resources Canada. In the case of the Sanden hot water heater, though, Sanden cautioned against sending the warmed water to it, since they rely on temperature stratification in the tank and were unsure what effect that would have. I don't know if that would be an issue with other heat pump hot water heaters.

    1. Alan Afsari | | #11

      Kevin, are you using the vt1000 in a horizontal position?

      1. Kevin Henry | | #12

        No, vertical. We have just enough room for this short unit.

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