Drain Water Heat Recovery

[sloeffle]

I follow a guy on Instagram that builds houses for folks with endless amounts of money. He recently featured this product, and I thought it was pretty interesting. They are priced around $450 via supply house. If it works as advertised I’d say it has a pretty good ROI.

residential drain water heat recovery

HOW DOES DRAIN WATER HEAT RECOVERY WORK?A simple, comfortable and maintenance free passive water heater booster that uses waste heat energy to preheat

ecoinnovation.ca

 

[EbS-P]

How much water can I heat for $450 + install with a heatpump water heater??? If I had endless amounts of money is just spend the $$ on more solar panels.

That said I would like to know easy it would be to replace/repair.

 

[JRP3]

Seems as if it would be easy to DIY, just a copper pipe with some copper tube wrapped around it.

 

[peakbagger]

They have been around for decades and in typical residential use the payback in probably decades. In special circumstance like home that use excessive amounts of hot water (lots shower, dishwashing and clothes washer or with hot water recirculation systems) its probably got a better payback.

 

[begreen]

This looks like a pretty simplistic heat recovery design. They sell them at Home Depots in Canada. They do work up to a point, though better for showers and not much value for bathtub water.

I installed a much larger and more sophisticated, high-volume, water-to-water heat exchanger in a big commercial photo lab back in he 1980s. It saved them so much money that it paid for itself in less than a year. But that was a situation where the flow of water coming off of the processors was continuous. I also put in a large volume air-to-air heat exchanger for that facility. It exchanged the air in the building 7 times an hour.

 

[ABMax24]

I've been kicking tires on one for about 5 years now. No matter how creative I am with the math I can't achieve a pay back in under 10 years.

Our natural gas is just too cheap to be worth capturing the heat from the drain water. Although because of our almost freezing cold supply water year round they'd make sense if we heated with electric.

 

[woodgeek]

I looked at these many years ago when my HW was oil fired and expensive. Now my (10 year old) HPWH costs me about $10/month I don't like the payback so much.

Note that for HPWH most 'preheaters' save very little since the COP is very high when heating cold ground water... and most of the energy is the final lift above 100°F. So even if a heat recovery unit recaptured half of the HW BTUs, it might save only 25% on energy, or about $30/year in my case.

 

[tlc1976]

Seems easy enough to DIY, especially $450 for a coil of pipe wrapped around a drain pipe.

It doesn’t seem like the drain water would stay long enough to do much. I think a better idea might be to drain into an intermediate tank, and have a large coil running through it for the water heater input water to absorb heat over time.

A trick for when you need heat in the house, is to leave bath water in the tub for awhile until it gets cold. That will at least draw the heat from the water before it goes down the drain. In the grand scheme of things I don’t know how much of a difference it really makes, but it’s free. That’s if you take baths. I believe it’s more efficient to just take a shower, even if the no heat is recovered.

 

[sloeffle]

I agree that you can’t make hot water cheaper than with a HPHW, or cheap NG. Personally I’m on my second HPHW in 5 years. The first one lasted 2 years and the heat pump died. The GE geospring heaters weren’t much better with their longevity. Mine was not a geospring.

With a new HPHW costing over 2k now ( thank you greedflation ) and a cheap conventional water heater costing a quarter the price I’m not so sure if the HPHW payback is there unless you have a very large house hold or use a ton of hot water. For my personal situation, when my HPHW dies again, I think it’s going to get replaced with a standard electric hot water heater.

My thought is, this non-mechanical device, which should have a pretty long lifespan ( unlike my experience so far with HPHW’s ) along with a conventional hot water heater might be a good combination for energy efficiency, simplicity and longevity.

Toronto Passive: Some Thoughts on Drainwater Heat Recovery - GreenBuildingAdvisor

Editor's Note: Lyndon Than is a professional engineer and Certified Passive House Consultant who took a year off from work to design and build a home with his wife Phi in North York, a district of Toronto, Ontario. A list of Lyndon's previous blogs at GBA appears in the sidebar below. For more...

www.greenbuildingadvisor.com

 

[JRP3]

I agree that you can’t make hot water cheaper than with a HPHW, or cheap NG. Personally I’m on my second HPHW in 5 years. The first one lasted 2 years and the heat pump died.

Replaced under warranty I hope?

 

[brenndatomu]

For my personal situation, when my HPHW dies again, I think it’s going to get replaced with a standard electric hot water heater.

Yeah with the OWB now you are probably on the right track there.
And as far as the drain HX goes...could maybe make it pay off in a commercial setting that uses lots of hot water, maybe? Probably cause maintenance headaches though, if I had to guess...

 

[sloeffle]

Replaced under warranty I hope?

Yes, but it definitely doesn’t give you the warm and fuzzies about the manufacturers ( AO Smith ) product.

 

[woodgeek]

Yes, but it definitely doesn’t give you the warm and fuzzies about the manufacturers ( AO Smith ) product.

My 10 yo unit is an AO Smith 80 gallon unit. Voltrex, I believe.

 

[mcdougy]

These drain water recovery pipes have been building code required here in our province for probably the last 10-15 years. Unless you are on a septic system, then they are not required. Like many of the "green" code requirements they are more of additional cost, recouping the cost is often longer than the appliance will last.

 

[myusername]

We have a powerpipe DHWR system and I've measured shower water heat recovery at ~40%. For our high cost oil (now a pellet boiler, but pellets are still expensive) in NH (not NG at our house), the ROI was <1yr. We've since added a HPWH (still crazy expensive electric) so our hot water costs are about 35-40% of what they were. Still a real ROI and quick at our house. We had reasonable access to the vertical portion of the drain line and our house is PEX so the install was pretty easy.
NH is a very high cost state for energy so savings add up quickly.

 

[woodgeek]

We have a powerpipe DHWR system and I've measured shower water heat recovery at ~40%. For our high cost oil (now a pellet boiler, but pellets are still expensive) in NH (not NG at our house), the ROI was <1yr. We've since added a HPWH (still crazy expensive electric) so our hot water costs are about 35-40% of what they were. Still a real ROI and quick at our house. We had reasonable access to the vertical portion of the drain line and our house is PEX so the install was pretty easy.
NH is a very high cost state for energy so savings add up quickly.

Cool. Are you using the DHWR with the HPWH? IF so, could you share some numbers? I would not assume that that the DHWR system would net you 40% savings on the HPWH, but more like 20%. COP falls as the water is heated, and is super high when the water is cold from the ground.

 

[myusername]

Cool. Are you using the DHWR with the HPWH? IF so, could you share some numbers? I would not assume that that the DHWR system would net you 40% savings on the HPWH, but more like 20%. COP falls as the water is heated, and is super high when the water is cold from the ground.

I am. However, the DWHR system was put in several years before the HPWH. I only have stats on the HPWH with the DWHR system in place. I haven't had enough curiosity to bypass it for a month and see the difference in electrical usage. Our HPWH is in the (heated) garage. I do see a large (~20%) difference in electrical usage on the HPWH between the cold season and hot season from the improved COP from the higher ambient temp in the garage. That's a long way of saying that I have no data to confirm or disagree with your contention. As the COP increases very rapidly on the HPWH with the ambient temp, I'd question my data if we just tried to compare 1 week with the next without very high confidence that the temp/humidity curve and our usage pattern are very similar.

My 40% number is measuring the temp rise on the pre-heated cold line. We also did some other energy efficiency updates around the time of the DWHR system (spray foam) so it's hard to be sure exactly what the savings in oil were from the DWHR.

Our ground water temp from our well is ~50F. I suspect that you are correct about the HPWH seeing minimal savings from the DWHR where a conventional water heater may see a stronger benefit.

 

[woodgeek]

Even 20% is some savings, esp if it had already paid for itself.

 

[DBoon]

Still a real ROI and quick at our house.

I'm guessing that you have a decent size family and/or teenagers using a lot of hot water (long showers) to get that kind of ROI. Roughly how much hot water do you think you heat in a year for showers?

 

[myusername]

A clarification - the 20% delta is the savings between winter and summer from our HPWH having a higher ambient temp in the summer. I don't know what the savings are on the HPWH for having the DWHR.

Yes, we use a decent amount of water and more with the HPWH. Because of how our HPWH is plumbed, we have a very large run to get from it to the shower - like 150-200 linear feet. So, we waste quite a few gallons of hot water just cooling in the plumbing lines. In fact, we lose so much that I can't leave the hot water recirc system on too long because the waste heat in the long run is about the same as the recovery of the HPWH.

I don't have any way to estimate our water usage. We're on septic so we have to be a bit careful. We're also on well so we don't have a water bill. My best guess is that we use 70-100 gallons a day of total (mixed) water through the shower on an average day for our family of 4.

 

[DBoon]

That would explain your fast payback - if you use a lot of shower hot water the drain back recovery is a pretty nice thing to have. Otherwise, it's tough to get it to pencil out.