Spotted this 'invention' while browsing for something else.

http://www.contest.techbriefs.com/c...le/399

It's someone's idea for a 'geothermal' that steals heat from incoming city water (which is usually around 55 F) and puts in into the hot water tank, the idea being that it would make your house cold water colder, but in exchange heat your hot water.

In theory, it sounds like a good idea, but I keep looking at it and thinking I'm missing something.

It could work, up to a point. If cold water use exceeded hot water use by a factor of 3 or 4, it could work. Trouble is, much cold water use is tempered with hot water. Other issues include sweating pipes and fixtures. I don't imagine wanting to sit on a toilet with 35 degree water in its tank and bowl. Another issue would be timing. Cold water use would have to precede hot water demand - big tanks would be needed to bridge demand gaps.

More promising might be an arrangement where heat was stripped from wastewater. This has the appeal of both being warmer (mixture of hot and cold streams) as well as coinciding with demand - take a shower, then recover heat from shower drain water and store it for the next shower.

Agreed - my first thought was that you would need way too much cold water, but in the example he presents, the numbers check out. The main problem I see is that it would be highly regional - where I am the incoming water gets cold enough for a good part of the year that there is very little heat to be extracted from it before freezing.

Curt, I like the idea of a heat pump recovering heat from wastewater - this would greatly increase the amount of heat you could reclaim compared to a straight drain hx. I'm still hesitant to implement one based on maintenance - even gray water pipes get pretty gunked up over time.

I dimly recall reading of a Copper drain line wrapped in copper tubing intended for this service

Yep, it was discussed here previously. Here is an example of 1 model:
http://www.retherm.com/HowItWorks.htm

Dana1 has referenced a performance test of these units previously. I don't know how the unit I listed above performed, or if it was even tested. I do recall that they need to be installed vertically to be effective (at the time I previously read up on these units.)

We have a GFX waste water recovery unit
http://www.gfxtechnology.com/
It is actually our second one; we had one in our first house and liked it so much we installed one again in our second house. In our first house the well water was close to 40F all year round on our 650 foot deep well, so the unit did recover more heat. We are now on city water and it ranges from 40 in the dead of winter to 60 in the summer. I have it plumed so the cold water to the showers also runs through it. So our incoming water might be 50F, it gets warmed up to 60F to feed the pre-tank and run to the "cold" line in the shower. I see from about 15F increase in the coldest winter to about 10F in summer, still that’s 10F at 3 gallons a minute, quite a bit of heat recovered.

Honestly I think it is the single biggest bang for the buck in terms of cost to energy savings.

Going back to the original post though, if you did cool down the incoming water, then you would need even more hot water to make up for it when showering, so I don't think you would come out ahead or at least not by much.

Natural Resources Canada has a standardized test for rating the performance of drainwater heat exchangers, and maintains a comparative list by manufacturer & model here:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/drain.cfm?attr=4

In general, bigger diameter and longer length==higher performance from a heat-recovery point of view (both affect the interface surface area.) In my own home I have 4" x 48" version, which delivers enough performance that I can down-size the modulation level on the combi heating/hot water system I have kludged up and still take endless showers with the heating system on (wasting energy EVER so efficiently! :-) ) I would have gone with a taller version if I'd had the headroom, but it's still over 50% return on the NRCan test.

One thing to bear in mind when looking at the taller versions is that some models are more flow restrictive than others. (PowerPipe seems to have licked that problem by-design, but other approaches at paralellism work too.)

All heat exchangers need to be mounted as close to perfectly vertical as possible to get the full performance out of them, since they rely on the very low surface tension of soapy water to spread the flow in an even thin film over the interior of the drain. Five degrees of tilt would pretty much do it in, with the falling water covering less than 1/4 of the surface area.

Anyone know if there are any models availble that are effective in the horizontal? I have no vertical space in the basement and don't feel like cutting up walls. Thanks.

Posted By Dana1 on 02 Dec 2010 01:40 PM
Natural Resources Canada has a standardized test for rating the performance of drainwater heat exchangers, and maintains a comparative list by manufacturer & model here:

http://oee.nrcan.gc.ca/residential/personal/retrofit-homes/drain.cfm?attr=4

In general, bigger diameter and longer length==higher performance from a heat-recovery point of view (both affect the interface surface area.) In my own home I have 4" x 48" version, which delivers enough performance that I can down-size the modulation level on the combi heating/hot water system I have kludged up and still take endless showers with the heating system on (wasting energy EVER so efficiently! :-) ) I would have gone with a taller version if I'd had the headroom, but it's still over 50% return on the NRCan test.

One thing to bear in mind when looking at the taller versions is that some models are more flow restrictive than others. (PowerPipe seems to have licked that problem by-design, but other approaches at paralellism work too.)

All heat exchangers need to be mounted as close to perfectly vertical as possible to get the full performance out of them, since they rely on the very low surface tension of soapy water to spread the flow in an even thin film over the interior of the drain. Five degrees of tilt would pretty much do it in, with the falling water covering less than 1/4 of the surface area.

This statement has me a little baffled.  Thinking of the drain pipe coming from our shower, it's close to horizontal, probably pitched 15 degrees, for a few feet, then goes vertical.  In my mind it would make more sense to have the heat exchanger wrapping the horizontal(ish) section because  the water is going to be flowing more slowly across at that point, increasing the time for exchanging heat.  Am I just wrong on that?

The gallons-per-minute rate is going to be the same, but in a horizontal installation the water-copper mating surface area much smaller, and there will be laminar flows of the warmer water flowing over the cool-water in contact with the copper bypassing the heat exchange. When it's going down a vertical section the water clings to the side with a much thinner film with far greater contact area. That film of water is constantly rolling over itself in a turbulent mixing fashion, with no laminar flows to isolate the warmer water from the copper. (The "GFX" in some manufacturer & model names stands for "Gravity Film eXchange", a descriptor coined in the early US DOE-sponsored experiments with these devices some 25-30 years ago.)

I can't put my hands on it this second, but IIRC in an experimental setup a 3" x 60" exchanger delivered something like 12% heat recovery @ 2gpm in a horizontal installation, but beat 50% installed vertically. It wasn't a subtle difference at all.

They also make a version or versions that use a catch pan and pump and circulate the water back up for situations like a house on slab or with no other vertical access.

I did wonder about the possibility of a flat plate for a near horizontal installation, there is probably good reason they don't do that.

I happen to have the ECO-GFX S4-60

that’s 10F at 3 gallons a minute, quite a bit of heat recovered.

I'm considering one, so let's look at the numbers. Say 30 min a day for a specific shower drain. So 90 gallons and 7515 btu/day saved or 2.7Mbtu/year. Around $40/year with nat gas if I did that right. So a 20 year payback in this case?

Brock: There's one flat plate version (EcoDrain: see http://www.ecodrain.com/en/how-does-it-work ) for horizontal installation on the market, but it hasn't made the NRCan list yet, but they're claiming ~ 40%. The problem with flat plate versions is that they're not inherently self-cleaning, and would A: develop films of insulating crud on the drainwater side of the plates, reducing performance and B: require periodic cleaning to keep from clogging completely. EcoDrain claims to have solved those issues, but they don't have a very long real-world history on them. Gravity film heat exchangers have been around for a couple-three decades experimentally, and over a decade in numbers large enough to matter.

jonr: FWIW: I'm getting a not well calibrated ~17-19F delta-Ts (bigger deltas in mid-late winter when the incoming water is coldest) @ (also not well calibrated) ~2-2.5gpm with a 4x48" PowerPipe, if you want to run your utility cost numbers on that. Delta-Ts & efficiency go up as flow rates fall with any of these beasts. Head-to-head in the NRCan test the 4x48" PowerPipe only edges out Brock's EcoGFX S4-60 by 1.7%, so the measured performance difference in his (and my) measurements should have large error bars around them. The 4x48" PowerPipe is distributed by EFI from their Wisconsin branch (and will set up a commercial account for you over the phone with a cc. #. SFAIK they don't sell them "retail", but anybody can start an account with them. See: http://www.efi.org/wholesale/pdfs/power_pipe.pdf ) As a DIY it'll be well under a grand.

The 3" x 40" EcoInnovation ThermoDrain is somewhat less money and claiming 46.2% performance, (but isn't on the NRCan's third-party tested list yet): http://www.ecoinnovation.ca/images/docs/SavingsAnalysisTD340General2.pdf

From a present-value financial analysis, as a rule works out in favor of installing the fattest & tallest one that actually fits- the increased performance is almost always worth more than the increased cost.

Beyond mere utility cost savingsissues, what DWHR buys you is a HUGE increase in apparent water heating capacity in showering mode. A typical 40 gallon gas hot water heater can run more than an hour's worth of shower before running tepid, even in winter.

I would add a second to the apparent size or heating capacity of the hot water heater. In our last house we had a 40 gallon natural gas tank with our incoming water temp a constant 40F all year round. We had the water heater set to about 130F so we could get two showers back to back just running out of hot water at the end. Once we added the GFX I turned the hot water heater down to 120F and we never ran out of hot water. So my wife really liked it.

As far as the real numbers I would say ours in a minimum of 10F at 3 gpm, could be up to 15F at 6 gpm with both showers running. With our kids I think this will really save us from not having hot water. Right now we have an 8 year old boy, 7 girl, 5 boy and 3 girl. I can't image once they start really taking showers how much hot water they will use, our 8 year old is just starting to do that.

I am sure its minimal, but the amount of water in the exchanger always warms up to room temp after sitting a while which add a bit as well.

I'm going to tag another question to the end of this.. How hard is a system like this to retrofit? It's something I briefly looked into when we built, but didin't have the time to fully investigate so didn't do anything. We went with a pretty standard bradford white 40 gal water heater. I looked at the AO Smith....I forget the name - the yellow one that's supposed to be extremely efficient. It was also extremely expensive. It was hard to justify spending that much more to save maybe $50/year.

If the drainwater heat recovery system is relatively easy to retrofit, it could be a good option for me at some point in the future..

I would think it all depends on the location of the main drain water pipe and its proximity to the water heater. In our last house they were side by side, in this house they are about 6 feet apart. In our first house they were opposite ends on the basement.

Basically if you have access to the vertical section and it's relatively close to your water heater or incoming water line it shouldn't be too bad.

In thinking more about these, obviously the more hot water you use and the colder the incoming water the faster the payback and more worthwhile they become. I would guess in Florida they have pretty warm incoming water to start with so the recovery would be much less.

Nat gas vs electric water heater also makes quite a difference. A 2 gpm shower head vs 3 gpm has an incredible return on investment. I shower at 1 gpm and it doesn't bother me.

jon I agree. I have a low flow on the kids and started with a low flow on ours, but my wife made me change it out, she like tons of water and some things aren't worth arguing about When I shower I have the water just on enough to make water come out, if I turn it up all the way like my wife does I swear it stings when it hits me. My shower is about 12 gallons my wife is about 30 gallons...

Posted By jerkylips on 03 Dec 2010 11:41 AM
I'm going to tag another question to the end of this.. How hard is a system like this to retrofit? It's something I briefly looked into when we built, but didin't have the time to fully investigate so didn't do anything. We went with a pretty standard bradford white 40 gal water heater. I looked at the AO Smith....I forget the name - the yellow one that's supposed to be extremely efficient. It was also extremely expensive. It was hard to justify spending that much more to save maybe $50/year.

If the drainwater heat recovery system is relatively easy to retrofit, it could be a good option for me at some point in the future..

Depends on how accessible the drain plumbing is, and it's location relative to the water heater, but it's pretty straightforward:

http://www.youtube.com/watch?v=-jE0C7bsTv8

In some states (including WI) there are state & utility subsidies for them provide they meet some minimum test criteria, but sometimes the subsidy is restricted to only those heating with electricity.  (Minnesota Power customers can get $400 kicked in if they heat water with electricity.)  In VT, Vermont Gas customers can get $200 in rebate.  In OR it varies depending on the efficiency of the unit.  In WI it's local-utility dependent, not necessarily state-wide, but substantial, in some instances. You'd have to check locally to see if they have anything for you.

jonr: At 1gpm the fraction returned will be dramatically higher than at 2.5 or 3gpm.   A unit rated 52-53% at the NRCan standard (~2.5gpm) will deliver around 65-70% heat reecovery at 1gpm.  See: http://www.gfxtechnology.com/EFF.pdf  Of course, at lower flow the total amount of heat returned is also less.