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Sizing a drain water heat recovery unit and combining DWHR with future graywater system

Keith H | Posted in Green Products and Materials on

Hi all,

I have the opportunity to retrofit a drain water heat recovery unit into my home.  I also, due to some plumbing oddities have the opportunity to plan for a grey water system.  However, that produces some conflicts in sizing the DWHR unit.

Basically, due to previous owners presumably moving the showers some decades ago (unknown reason) but not the toilets, the upstairs showers are on a separate 2″ vertical stack from the 3″ (black water) drain (they merge in the crawl space).  Basement shower is impossible to access for either purpose.

So here are the design dilemmas:
1.  If I push the easy button (always good), I put a 2″ x 72″ power pipe in and recovery ~51% of the shower drain heat from the shower water and none of the black water heat or basement bathroom drain water.  I like this option fine.  Choosing this option also leaves open the possibility to 3 way valve the grey water to exterior landscaping use during the growing season.

2.  Alternatively, I could (upsize just the 6′ of the drain recovery portion of the 2″ drain to 3″.  Ecodrain said they didn’t know if that would work and accordingly couldn’t recommend it.  Power pipe explicitly recommended against it.  I’m wondering if anyone here has actual data for how falling into an enlarging pipe would effect film heat transfer.  Certainly I could see it having a strong negative effect.  I could also imagine that if the plumbing elbow above was upsized that the difference would be modest.

3.  I could combine the two drains above the drain recovery unit (I think I have the headspace and go with a 3″ unit.  That would increase efficiency by ~10% and extract more of the low delta T heat (toilet etc).  FYI in our climate delta T between indoor winter temperature and water temperature is not nearly as extreme as the northeast.  Perhaps 16 F (66-50) so we aren’t going to get as much out of toilet bowl water as some climates.

4.  I’m wondering if anyone ever puts these units in series.  In split level homes, it is common (I think) to not have long uninterrupted drain runs like it might be in a 2 story with basement design.  Runs are potentially only 1/2 story in length.  Does anyone have any data regarding linking two vertical units?  Or linking a vertical and a horizontal unit?  I haven’t asked Renewability about this yet.

5.  Is there a point of diminishing returns?  If the goal is to recovery heat from a hypothetical 10 minute shower (let’s call it 15 gallons) without the assumption of 4 back to back showers, at what point are you leaving too much hot water in the power pipe.  I can’t believe that the unit’s efficiency is truly insensitive to total minutes of back to back hot water usage and other usage details but I haven’t seen anything on any manufacturers site indicating those kinds of details.

I’m thinking of just doing #1 but am wondering if I could get away with #2.  #3 isn’t that appealing although it would be maximum recovery.

Anyways, thanks for any subtle wisdom on these units.

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Replies

  1. Trevor Lambert | | #1

    I think the amount of heat recovery you're going to get from toilet water is close enough to zero to just call it zero. Keep in mind that the only time you get any recovery from the DWHR is when drain water coincides with hot water use. So if you're taking a shower or the washing machine is filling with warm water, and someone flushes the toilet, for the two seconds it takes for that water to go by is all you're going to get. How often is that even going to happen? I also wonder whether it's even going to be a positive effect rather than a negative effect. If you've got hot shower water going down the drain, the relatively cold water is going to lower the average water temperature. I don't think it matters that the toilet water is slightly above the incoming cold water temperature, the lowering of the deltaT is probably going to have a bigger effect.

    I think you could fairly approximate the effect of two units in series by looking at the differential increase in recovery efficiency between a single unit and a larger one the size of the two you plan on using. The law of diminishing returns is going to be big here, and it's unlikely to be worth the effort, even if you got the second unit for free.

    "If the goal is to recovery heat from a hypothetical 10 minute shower (let’s call it 15 gallons) without the assumption of 4 back to back showers, at what point are you leaving too much hot water in the power pipe"
    I'm not sure what you're asking here. You're never leaving hot water in the power pipe. Regardless of the length or duration of your shower, the hot water is going down the drain. The water that goes down while hot water is being drawn will exchange some of that heat. That will continue indefinitely. The only variables are that it will take a brief period to heat up the power pipe, and once the shower is over there will be some heat left over. So in theory, back to back showers are more efficient than ones separated by 5-10 (or more) minutes. I wouldn't waste my time worrying about that, we're talking less than 1% total efficiency.

  2. Expert Member
    Dana Dorsett | | #2

    With the heat recovery units in series the pressure drop with flow on the potable side goes up. That may or may not be an issue in your application, but it's probably never going to be "worth it" unless you're in "endless shower" or car-wash volumes territory.

    Usually the biggest one that fits reasonably is the "right" one. If going with a larger diameter, yes, make the diameter change on the horizontal section, prior to the turn down toward the heat exchanger.

    Recovering efficiency drops with flow rate. At toilet bowl flush rates (a couple of gallons in less than 5 seconds) flows you get effectively zero heat coming back- effectively impossible to measure, since the gravity film from which the heat is being transferred is long-gone well before the first quart of water hits the toilet tank. A heat exchanger is not an energy storage device, and the thermal mass of the copper is fairly low, retaining very few BTUs.

  3. Keith H | | #3

    Thanks for the rapid replies Trevor and Dana. I appreciate the confirmation that I'm not missing an opportunity and the likely best I can do is 2"x72" (~51% efficiency). Still seems like a worthy retrofit.

  4. Nadya01 | | #4

    Hi Keith,

    My name is Nadya from ThermoDrain (ecoinnovation.ca)

    Here are some answers that might help you:

    2- Depending on your plumbing code for the state you are in, you might be able to do this. But definitely, check with the plumbing inspector in your area.

    3- By doing this, you would be able to recover heat from everything that goes through the drain. Example sinks, dishwashers, ect. You also won't need to go as long as with a 2" drain for the same efficiency.

    4- 2 units in series will perform essentially as an equivalent length single one. Ex: 2x36’’ will equal 1x 72’’ length

    5- • In real-world testing, the unit takes about 30 seconds to reach a steady state. Therefore, on a 10 minute shower, assume 30 seconds to get the shower running to temp and reach the DWHR unit at 0 recovery, 30 seconds to reach steady state and 30 seconds worth of water that does not reach the DWHR unit. That would mean that of 10 minutes shower time, 8.5 minutes was recovering heat at the full potential, 30 seconds at no potential, 30 seconds at partial potential ramping up from 0 to full, and 30 seconds at anywhere from 0 to full potential depending on when the next hot water demand may happen as there is a heat storage potential..

    If you have any more technical questions on regards to the Thermodrain -DWHR, please don't hesitate on contacting us 1-888-881-7693 EXT204

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