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