Drain Water Heat Recovery system design

[MTcummins]

Normally, this would be pretty simple, but my system is not that simple, so here goes. I have an open radiant floor heat system, so its combined with my DHW. Yes, I'm well aware of everyone's concerns about legionaires, etc, and have addressed them all with redundancy in both temperature and circulation, had it approved by one of the strictest plumbing authorities in the country, etc... so lets talk about adding a DWHR system to the mix.

I currently have my system set up where in the heating season, all the cold makeup water from any draw of hot water flushes through the entire floor system, but cold water draws go straight to the fixtures. In summer, I switch 2 valves, and all the water in the house (with the exception of kitchen cold line) goes through the flooring first to further eliminate any chance of stagnation, as well as give a slight cooling effect (giant heat sink for floors), and preheat my DHW with unwanted waste heat from the house.

Now I'd like to add the DWHR system. I'm thinking it would be nice if I could minimize the dumping of cold water into my warm floors in heating season as much as possible, while retaining the water movement, cooling effects, etc of the current system, and adding the benefits of waste water heat recovery from any sources, but mostly showers. So I'm thinking of a somewhat complicated system, and wanted to see if I'm missing anything as far as how this might go wrong. I'm not concerned about selling the house or next owners etc, and I'll have things well labeled just in case (and so I don't have to head scratch again every season change about how to switch it over.

Anyway, the first image is my current setup. Note that I drew it spread out to make things clearer. In reality, all the valves etc are within a couple feet of each other, so no significant stagnation zones... There are some other details excluded, etc, but I think this covers all the most relevant items. Also note that I have a home run manifold system with every fixture having its own line all the way from the manifolds.



Second image is what I'm thinking of doing. All of the drains in the house except my old (still occasionally active) laundry room and a rarely used basement tub go into this drain stack. Note the shower cold supplies are moved to after the DWHR as well, rest of cold supplies remain as they were. That image is pretty hard to follow with all the valves, notes, etc...



so third image shows only the closed valves on each mode separately to try to show the water pathways for each mode.



4th is same as 3rd, but I drew in water pathways in blue lines for cold water, orange for DWHR tempered water, and red for fully heated water. Hopefully this all makes sense.



I think I've thought through every possibility of how the system could short circuit somewhere etc, but if anyone sees anything I might have missed, would greatly appreciate the input.

Also, any specific experiences with different manufacturers of DWHR units? I see several that seem to be out of production, some that seem to be only in Canada (I'm in PA), etc. I've been looking at the Power-Pipe (seems best design to me, but not sure), and Thermopipe (mostly because I found a 42" for under $500 shipped, way cheaper than the Power-Pipe models, so ROI would be much faster). I can only fit a 42-45" without totally shoehorning it in (then maybe a 48, maybe), so seems like the couple hundred bucks price difference is more substantial than any modest gains I might be able to get with a different model. But I'm open to suggestions.

I know Dana used to chime in about these a lot. I'd love your (and everyone else's) input!

 

[Dana]

As long as the input of the drainwater heat recovery unit is getting only cold water, unmixed the floor's heating water, and is supplying both the cold side of the shower and the water heater (whether it's through the floor first or directly to the water heater) it will do the right thing.

EcoDrain's VT1000 series is a 4th generation design with both higher return efficiency and lower pressure drop than the PowerPipe series. (I have an R4-48 PowerPipe at my house, installed 10+ years ago, still doing fine.) The VT1000-4-32 is only 32"long, but at 44.6% recovery has nearly the same return efficiency as the PowerPipe R4-48's 47.5% or the 42" ThermoDrain TD442B at 46%. It should be easy to make it fit in the allotted space if you were contemplating a 42 incher. (I assume you were looking at the 42" Therm0Drain, not ThermoPipe, which doesn't seem to exist.)

 

[Sponsor]

 

[MTcummins]

Hi Dana, thanks for the reply.

The winter mode would have straight cold water from the city going to the DWHR. Summer mode would have the water go through the floors before it gets to the DWHR unit, but no heating will be going on, so it will just be slightly pre-heated from whatever is absorbed from the house. I figured this would be more optimal than dumping 70-80 degree water from the DWHR into my floors in the summer - both for keeping load on my mini split AC down, and for reducing system noise. I wanted to keep all incoming water going through floors (other than kitchen cold) to be extra careful about keeping water moving in the off season. As its an open system, I didn't need O2 barrier pex or Pex-Al-Pex... unfortunately that also means I get a lot more expansion and contraction noise than I'd prefer. Part of the thinking in this whole system is also to reduce the temp swings when you pull hot water from the fixtures in the winter (dumping 40 something make-up water into 140 degree floor water adds to expansion/contraction noises). I think this system will handle those issues as well as mostly maximize energy efficiency of all systems, and reduce load on the water heater a bit. Certainly open to suggestions of improvement etc though.

I'm considering swapping out 2 of my pumps for smarter pumps down the road - try to modulate flow rates a bit more to reduce the up/down swings of pipe temperatures, both for efficiency gains and for expansion/contraction reduction... but not sure if that will really achieve my goals or not. Just something I've been kicking around. The pipe movement noise was annoying at first, now I hardly notice it, but would be nice to at least reduce it more. Solar water heating is another potential add on down the road, but my climate and orientation are both sub-optimal for it, so might not make much financial sense until prices come down a bit more.

I know I could simplify a bit with a couple of 3 way valves to replace 2) 2 way valves... but the cost is much greater (and I have lots of 3/4" pex valves on hand) and leaves me a little less flexibility in trying to minimize lengths of pipe which could become stagnant. But using separate valves, I can move the valve 6 inches from the pipe intersection so there's only 6 inches of "dead zone" on each side, rather than a foot on one side. Very short sections like that should naturally change out their water slowly over time when they're in off season, then will be fully active again in other season. Its a total of 6 valves you manipulate to change seasons in the design I posted (3 open, 3 closed for each). I figured I'd color code them and write the seasonal settings on the wall right there for my quick reference and for any other future potential owners/service techs. I can't figure out how to eliminate any more of the valves without opening a short circuit somewhere, where heating system could bypass water heater, for example. While in some ways this system is a little "complicated," I actually really like the simplicity of it... no complicated electronic controls, very little moving parts, etc. Just some valves and the rest pretty much handles itself with the city pressure.

Thanks for info on EcoDrain. I can't find anywhere with pricing for the VT1000, or anywhere to order one. I messaged the company yesterday to inquire about it, as its claims are pretty solid. I'm def not an expert in such things, but the design seems to me like it would be far less efficient - the straight up the walls design rather than the 4 pipes wrapping it like the PowerPipe. I can definitely see the lower pressure drop in the shorter, but greater number of exchanger tubes in the VT1000... is more conduits over shorter distance with seemingly less surface contact with the drain pipe (VT1000 seem to be round, not rectangular, tubes, so less contact with drain) really more efficient? I see the data on it, just seems hard to believe.

If thats the case, then wouldn't a simple pipe in pipe (or double pipe) design be the most efficient? Full surface area contact, should be minimal pressure drop, straight run up the side of the drain pipe, simpler design... I saw a youtube video of a guy building them for 1.5" shower drains out of copper drain and PVC outer sleeve - interesting, but not sure I'd want to trust that.

Efficiency is def higher priority than pressure drop for me, within reason. I operate at 80 PSI (pressure from street, has been stable at that pressure for a long time, so I don't bother with a PRV), so as long as I'm not losing a lot, still plenty of pressure. Most of my pressure losses are from my master shower being on 3rd floor, but its not really been a problem with the 80 PSI system pressure.

Sorry, yes, I meant ThermoDrain. Brain fried from working out that system of valves last night

 

[MTcummins]

One other question... not sure if I'm missing something, but it seems like EcoDrain's V1000 (previous generation, I assume) efficiency ratings are higher than the VT1000. According to http://oee.nrcan.gc.ca/ and my search range of 3-4" diameter, 32-42" length, the 2 highest efficiencies are the V1000 3-36 and 4-36. Any idea why, or what benefit of going to the VT1000 series is? Also, the VT 3-32 seems to outperform the VT 3-36. Seems odd...

 

[Dana]

The link you gave goes to a main page, not the drainwater heat exchange search page.

I think you're probably mis-reading it. VT 3-32 does not outperform the VT 3-36. Search this page by brand.

The VT-1000-3-36 runs 43.5%, the VT-1000-4-32 runs 44.6%- it's 4" shorter, but an inch fatter. The VT-1000-3-32 runs 42%, which is not greater than 43.5%.

What isn't visible in the exterior pictures of the V-1000 and VT-1000 is the internal helix structures in the potable tubes inducing turbulence, which is what is responsible for the higher heat transfer efficiency.

If it's on the NRCAN site it has to have independent third party testing data, tested to NRCAN's protocol.

You're making the plumbing too complicated and possibly overthinking it. Install the cold supply to the house to the cold end of the drainwater heat exchanger, and nowhere else. At the output of the heat exchanger, first make a tee off to the cold distribution plumbing to the whole house, then a tee into the radiant floor's loop. That way there is only flow through the heat exchanger when water (hot or cold) is being drawn, and the cold water is only pre-heated when there is hot water flowing down the drain, and flow in radiation never flows through the heat exchanger.

 

[MTcummins]

I couldn't link the actual search results, but here's the search page... http://oee.nrcan.gc.ca/pml-lmp/index.cfm?action=app.search-recherche&appliance=DWHR Using parameters 3-4" width and 32-42" length, I get the results mentioned above. Maybe the search is pulling data incorrectly or something, I dunno. Certainly isn't the most logical results...

That certainly would be the simpler piping arrangement... Your suggestion definitely simplifies the system, but doesn't necessarily maximize efficiency, which is kind of the whole point of the DWHR in the first place, isn't it? Also, the wife would not be happy if I was showering and she was pulling 75 degree water out of the cold tap. She's not thrilled about the slightly warmer water from running it through the floors first in summer, but she's accepted that part.

I had also come across one other small manufacturer online, so emailed them asking for more info. Company called Swing Green, and their unit is called the GreenFox. https://swing-green.com/drain-water-heat-recovery I got the following info back today in an email...

"We have the 3" models in stock, both 42" & 48" length with a single 3/4" tubing wrap & squared.
We have test homes in Colorado, whereas the 48" is raising the incoming water temperature on average from 45 degrees to 80 degrees with 100 degree water in the waste. We've seen as high as 65-70% eff., with less than 1% drop in pressure.
The 42" model is slightly less performance (approx. 50-55% eff., with same drop)."

If these numbers are remotely accurate, they seem impressive. Pricing is also competitive. But I've never heard of these guys before or seen much about them other than on their website... anyone else know anything about them?

 

[Reach4]

You're making the plumbing too complicated and possibly overthinking it. Install the cold supply to the house to the cold end of the drainwater heat exchanger, and nowhere else.

I expect that was not worded quite as you intended.

I think you meant that the cold end of the drainwater heat exchanger only connects to the cold supply of the house, and nowhere else.

 

[MTcummins]

Dana said it right. Its a very viable way to do the piping for my system - simple and efficient as far as piping and day to day operation (no messing with valves to change seasons, etc) goes, but sacrifices some of the other benefits I'm targeting for that simple piping and hands off nature. I'm cool with flipping a few valves twice a year, as well as doing a little more piping and adding a few extra valves to get the added benefits.

I think you guys are both saying the same piping arrangement, just with slightly different wording. I already have a setup more complex than that w/o the DWHR, it would actually be almost as much work to undo that and go back to the simple design he's suggesting as it would be to go to the more complex one I'm proposing due to the existing setup.

Regarding the GreenFox, here's an article about them (written by one of the cofounders, fwiw) in a publication. I know nothing about the publication, but maybe it could lend some credibility to the company? The little chart in the article shows the 3-42 model at 43% efficiency, which is right in line with other units I see out there. The email I got seems to overstate substantially, but he did say it as the one test house is getting results up to that amount, which could be true under whatever ideal circumstances etc. Hard to say. I'll probably give the guy a call and just talk through it a bit more, but thought it was at least interesting. Smaller, newer (founded 2011) company, less track record, etc, but might be better customer service, pricing, etc to go along with that? Or the opposite. That's why I wanted to throw it out here and see if anyone had heard of them, etc.

 

[Reach4]

I think you guys are both saying the same piping arrangement, just with slightly different wording.

I think the incoming cold water should go to the hose spigots and water softener if there is a softener. If no softener, then also cold for kitchen sink, and cold other places such as toilets, as well as the cold input to the heat exchanger. I expect that is you assumed. I am confident that is what Dana prescribes.

 

[MTcummins]

Yeah, I have home runs to every fixture (just about), so allows a lot of flexibility like that. For example, one of my down the road ambitions is to do some rainwater harvesting for things like toilets etc that don't need to be cleaned to full potable spec... will not be that hard to pull those from the manifold and add them to a different source.

Currently I pull cold for kitchen sink before anything else, but everything else is on the manifold. After this DWHR system goes in, I plan to pull the shower colds off and feed them off the output of the DWHR all the time, but not feed the remaining cold fixtures off that pre-heated water. That pre-heated water will also go back into the system for make up water for the water tank. With the need to constantly flush the open hydronic system, I have several extra layers of complexity, but also several opportunities to gain some minimal free cooling in summer (as well as some extra pre-heating of water), so I think my design maximizes on all those opportunities, but am open to any further suggestions. I don't have a water softener. Probably could use one, but been avoiding it... don't really care for them and our water isn't that hard. I hadn't really thought about pulling the hose bibs out separate, but I think its not worth the changing of the system in my case for that right now - If I ever get around to the recycled rainwater, would def pull those into that system as well. I hardly ever use the hose anyway, though that might change as my little one gets older...

 

[Dana]

I expect that was not worded quite as you intended.

I think you meant that the cold end of the drainwater heat exchanger only connects to the cold supply of the house, and nowhere else.

The cold water entering the house goes only to the drainwater heat exchanger's intake, with nothing else teeing in between the water supply to the house and the heat exchanger.

The output of the heat exchanger feeds the cold distribution plumbing to the house (ensuring that it will supply the cold side of the shower mixer), and feeds the radiant floor (ensuring that it will supply the cold side of the water heater.)

Whether a valve isolated bypass for the radiant floor in summer get's added in is up to you- it will work either way, as long as the heat exchanger feeds the bypass when it's active.

 

[Reach4]

The cold water entering the house goes only to the drainwater heat exchanger's intake, with nothing else teeing in between the water supply to the house and the heat exchanger.

To be clear, you would supply the outdoor hose spigots, the water softener input if present, and toilets etc via the heat exchanger?

That would seem to be wasting recovered drain heat for cold supplies that don't need, or even want, warming.

 

[Dana]

To be clear, you would supply the outdoor hose spigots, the water softener input if present, and toilets etc via the heat exchanger?

That would seem to be wasting recovered drain heat for cold supplies that don't need, or even want, warming.

That's right, but only when cold water is being drawn while somebody is in the shower. In most homes the fraction of showering time coincident with other water draws is extremely limited.

How much additional plumbing & valving are you willing to install to perhaps save another $2 in fuel use per year, or to avoid having to brush your teeth rinsing with tepid rather than truly cold water?

 

[EcoInnovation]

Mike,

Kudos for prioritizing efficiency!

EcoInnovation is the manufacturer of ThermoDrain. Dana referred to our TD442B model earlier in the conversation. At the end of January 2020, we released a new product line that is more compact called the ThermoDrain High Efficiency Series. Our higher efficiency per inch comes from our 3/4" coil profiled in a “D” shape to maximize heat transfer and minimize pressure drop.

You can review our TDH series 3" drain specs here for efficiency and dimensions: https://www.ecoinnovation.ca/wp-content/uploads/2019/PDFs/3in-SpecSheets-TDHSERIESR1.pdf.

Our 42.1% unit (33.5" long) and 57.2% unit (62.0" long) are also available online at https://www.supplyhouse.com/ThermoDrain.

For the maximum model length, keep in mind that you will need extra space on the drain stack for drain couplings so we usually recommend to measure the total available length of the vertical drain line and subtract 2 1/2". The result will be the maximum model length.

If you have 45" length on the drain stack, you should go for a 42.5" long model or shorter. Note that our 42.5" long unit (model #TDH3425B) offers 48.0% heat recovery. That is the minimum efficiency verified to CSA B55.1.

You also referred to potentially having enough space for 48". If you have 48" - 2.5" = 46.5" unit. Note that our 46.5" long unit (model #TDH3465B) offers 50.2% heat recovery. That is the minimum efficiency verified to CSA B55.1.

We offer plain copper tube or crimp PEX fittings for an easier installation.

Let's discuss further by phone or email. You can reach me here:
laura@ecoinnovation.ca
888-881-7693 ext. 207

Thanks and best of luck with your project!

- Laura

 

[MikeKenmore]

Adding onto this thread (for better or worse).

We have this model:

https://www.homedepot.ca/product/power-pipe-r4-72-drain-water-heat-recovery-unit/1000698147

New house. Every time we run the dishwasher, or drain a bath tub, we get a lot of clangs and bangs, presumably (to me) from the DWHR (that's what it sounds like to me). Is this normal? They're pretty loud and annoying.