jump to example.com

If we can take ReTherm’s claim at face value, by installing their device to recover waste heat that would otherwise go down the drain, we can save at least as much as we would with tankless water heaters, at a fraction of the install cost — which means the device might actually pay for itself in a reasonable period of time.

The ReTherm heat exchanger basically replaces a section of your sewer pipe.  As warm water flows through the heat exchanger, some of the heat in the water is absorbed by cold water flowing through the outer section.  This preheated water then travels to the water heater which reduces the amount of energy required to heat the water to the setpoint.

Their most expensive model is $760 Canadian or $620 US.  Quoting ReTherm’s 4-6 hours of install time for existing houses you can probably install one for $1,000, or less if you do it yourself, and they claim it’s much easier than installing a tankless water heater.  For a tankless water heater, you have to figure the cost of a good unit, rerouting intake and exhaust, a bigger gas line, man hours of labor, etc. — conservatively 3x the price of ReTherm’s solution.

ReTherm claims their technology can recycle up to 60% of the waste heat. Let’s be conservative and say 30%.  Here are some “back of the envelope” calculations using some figures from my own energy bill and some conservative assumptions:

  • 1 therm = approx. 10,000 BTU
  • 1 therm costs approximately $1
  • 8.34 BTU’s are required to raise 1Gal of water 1˚F
  • Cold water coming into the building is 60˚F
  • Your hot water heater is set to 120˚F
  • You shower at 90˚F
  • Your shower puts out 2.2 GPM
  • To get 90˚F water you use 1.1Gal of cold water + 1.1Gal of hot water
  • 60˚F * 8.34 BTU = 500 BTU to raise 1 gallon of water to 120˚F
  • 1.1 GPM of Hot Water * 10Min shower = 11GAL * 500 BTU = 5,500 BTU

One shower uses 1/2 therm, so 60 showers a month use 30 therms.  At my cost for gas this is $30 a month.  Conservatively recovering 30% of the waste heat, I should be able to cut my gas usage on showers to 21 therms a month, saving me $9 a month or $108 a year.  I could pay off the cost of the ReTherm in 10 years.

Yes, I know that I assume my water heater is 100% efficient, but I’m just trying to calculate the lower end of the savings.  If you have any experience with waste heat recovery, or if you just want to poke holes in my reasoning and math, give us a shout in the comments.

ReTherm [Corporate Site]


27 Responses to Grey Water Heat Recovery

  1. Justin says:

    What a rip-off. That “heat exchanger” is nothing but a piece of 1/4″ copper tubing wrapped around a 2″ drain line. Save yourself a grand (minus $30 in materials, tools you don’t have, and a beer) and you can do this yourself. You might save a few bucks, but it’ll all be lost because 20 gallons are sitting in the lines between your hot water heater and your shower getting cold all day/night long. Your money would be better spent on pipe insulation. $1000 dollars worth of that and you, my friend, can tell all your friends that your shower hot water supply is an adiabatic process.

  2. @Justin:

    1 US gallon 231 cu in.

    Assuming your using 3/4″ pipe: cross sectional area .44 sq in.
    Assuming 40ft shower run: 480 inches
    that gives us: .44 x 480 = 211.2 cu in.

    That’s still not even a gallon. Is your shower a block away from your hot water heater?

    Yeah I’ll give you that it does look like copper tubing wrapped around a copper pipe…They’re not really to specific about it.

    There’s got to be something to this though. They sell heat exchangers for removing the heat from indoor air and heating incoming fresh air. This process should only work better with water.

  3. George K. says:

    I think I would rather make my own version. Same concept, but wrap your incoming supply around a section close to the water heater output. The pros that I can see in doing it this way are that the pipe from a water heater is hotter than the water coming from the drain (mixed w/ cold at that point), and if done close to the heater, the pre heated water has less distance to travel and possibly cool before entering the tank. Wrap the assembly in insulation for a better transfer. Cons are that copper pipe is cheap, and you need to be good at bending pipe.

  4. George K. says:

    Last line should say copper pipe is NOT cheap!!!

  5. Chuck says:

    Totally with nays on this. For what amounts to some copper pipe wrapped around other copper pipe, it’s really, really expensive. As to returns, I can’t see the heat transfer being all that good, particularly after a couple of years of hair, grease and other drain skunge gets built up on the inside wall. Plus they blow a couple of PSI off your water pressure for the privilege, and if you have a well, you’re going to have to pay for the extra pump energy to cover that.

    Spend the money on a better water heater, better insulation or a solar DHW system, probably all a better investment.

  6. Barri says:

    I agree ith chuck. Money better sent elsewhere. I install bathrooms, central heating and solar heating and i cant tell you now that the heat transfer from that water running down that pipe in such small amounts so quick is hardly even going to affect the out going temp of the cold water. It will cool the waste pipe down a lot faster than it is heating the supply water up. i have seen some very nice solar heating roofing panels and even on a low light day with a little sun here and their we was putting ice cold water into the flow and with about a 300ft run the water was coming out the return so hot it would burn you 70-80c. i was amazed at how well they worked.

  7. Sean M says:

    Yup, it’s bunk IMO. Think about how fast that water is moving through the supply on it’s way to the water heater. There’s going to be negligible heat transfer from a tepid waste pipe.

  8. Brau says:

    Agree with Chuck, Barri, Sean, etc. This is a gizmo designed to sucker by appearing plausible. It’s not. It simply cannot transfer enough heat to live up to their claims. If it were designed to work well the grey water would have to flow through and around a radiator grill complete with fins; just like any fridge, A/C, HVAC, or car cooling systems.

  9. tim says:

    I think you are all too quick to dismiss this. If the supply is running thru that much 1/4 tubing, I think the heat transfer could be rather substantial. I think the bigger question is what the internal geometry of the drain pipe is like. In this application, you want to maximize the surface area of the pipe, but that goes against everything that would make a practical drain pipe.

    The whole concept is very smart: reclaim the energy that you are literally pouring down the drain. I think it would be better incorporated on the cold water line going to a shower though, so you use less hot water instead of preheating the hot water to your whole house.

    All in all, this thing is way too expensive. Make it or something similar out of an aluminum casting and the price would come way down.

  10. matt says:

    the “copper tubing” that is used has a square shape to it so one wall is in direct contact with the three inch center pipe (much better contact that with round tubing). Also there are four lines in parrallel wrapping around the center one to reduce the “lost PSI” Sounds like a good plan to me, also I’d mount it somewhere on the horizontal (seems to me you’d get slower waste water and better transfer…). I’d also like to point out that you’d have a much better recovery time for longer/more frequent showers, so maybe the fourth person in the morning would have a hot shower for once.

  11. rick says:

    I really cant imagine much of a benefit to this. However, you will see a benefit (is it measurable, thats a seperate concern). You could easily do other things to extend the volume of water between the cold water in and the hot water heater. In theory, if you ran the cold water pipe into a large tank, and then into the hot water heater, the ambient surroundings would help bring the water from ground temp to indoor air temp.

    I only point this out because after my hot water heater died, i still had “hot” water at the air temperature of my basement, still a brisk shower…. but noticably better than true cold.

    oh, and if the above copper coil is brazed to the waste pipe, it would work better.

  12. Joe says:

    Assuming that you buy the concept, the construction doesn’t make a lot of sense to me. What they’re selling is what a DIYer would make–wrap copper line around the outside of the drain pipe. If they are a manufacturer, and touting a radical new concept, why not build something that would maximize the heat transfer? A double-walled pipe (i.e., an effective water jacket with an inlet and outlet) should not be that hard to construct, would transfer heat more efficiently, use less material, and if made correctly, wouldn’t restrict wastewater flow.

    I’m still not buying that there’s enough heat available to recover, not after it has passed through the air, drained over a human body, then run along a fiberglass or cast iron tub to get to the drain.

  13. @Brau:
    When you talk about radiator fins, all of your examples are of air to liquid heat transfer rather than liquid to liquid heat transfer. I may be misunderstanding your point, but I think it’s two different technologies.

    I’d mount it somewhere on the horizontal (seems to me you’d get slower waste water and better transfer…)

    I’d have thought the same thing but they address this in their FAQ

    Does ReTherm have to be installed vertically? Can it be installed on the horizontal?

    Installing the ReTherm unit vertically is the only way to obtain the highest level of heat recovery. Having the unit installed horizontally will only deliver 1/5th of the heat recovery compared to the vertical install.

    I thought the same thing about aluminum too, but if aluminum is so much cheaper than copper why isn’t it used in plumbing?

    Also about going directly to the shower, instead of the hot water heater:

    I poo-pooed this idea when I first saw it (some other companies offering similar products do this), but the more I think about it the more I think it makes more sense. The only problem is you need the additional expense of a shower valve that regulates temperature

    Dealing with a few more issues:

    Right from the ReTherm page:

    The concept is simple. As warm/hot drain water flows down the vertical heat exchanger it clings to the surface of the inner section forming a very thin film of water. This thin water film allows for ultra-high heat transfer. Heat that was normally wasted now instead is rapidly absorbed by cold water flowing through the outer section.

    The last thing anyone needs is something new to keep clean. That’s why ReTherm is specifically designed to never need cleaning. Many types of heat exchangers require cleaning to maintain top performance, but not ReTherm. The fast moving water on the vertical center section continually scours the heat transfer surfaces. This prevents build-ups that would otherwise rob performance. With ReTherm it’s all about lifelong, hassle free heat recovery.

    Other than the clings to the surface of the inner section, they really don’t give any clue as to the how they can get efficient heat transfer. I’d like to see a cross section of the interior of their product.

  14. Sorry about the italics, I forgot an tag way up by the quote from matt.

  15. tim says:


    I think copper is the material of choice for plumbing over aluminum because of its ductility and the ease in working with it. A big problem when heating aluminum is that you do not get visual cues about how hot it is until it is almost melting, often leading to part damage.

    I bet it is also due to the fact that 150 years ago when plumbing started, copper was way cheaper than aluminum. Now, aluminum is way cheaper and easier to manufacture (though not cut and solder) than copper.

    As for the shower valve, whats nicer than a shower that warms up a little as you go? no need for a complicated valve.

  16. David Bryan says:

    I was pretty skeptical of this, so I did some looking around, and it turns out that it works. It’s called GFX technology, for gravity film exchange. That thin film that coats the inside of the drain pipe is why it needs to be mounted vertically, and the copper drain pipe allows for better heat exchange. It’s been tested thoroughly and proven to be effective. Pennsylvania Power and Light found the payback, based on electric water heating, to be from 2 to 5 years (I’m not sure when that dates from). Some electric companies are offering incentives for installing these. There’s a pretty good youtube video about it called “An Inconvient Truth About Water Heating” (that’s how they spelled it) that includes a Bob Vila clip from 1999 which made me want to cry when I thought about the price of copper back then. There are several companies making these in this country and they’re being used all over the world.

    • Andy Wahl says:

      Great comment. I will be installing one and doing some data collection.

      My home is already a net positive producing and very comfortable with clean air.

  17. Tim B. says:

    Hmmm – there actually ARE quite a few companies doing products similar to this, all with very similar claims / studies… In fact, here is one specifically for *horizontal* mounting, that has been generating some buzz on other sites: http://www.ecodrain.ca/en/how-does-it-work

  18. David Bryan says:

    That ecodrain site was good for a few chuckles.

  19. Chuck says:

    Nobody has addressed the condensation issue, either. You bring cold water into a relatively warm air space, and all that copper tubing is going to start condensing water like a fiend. That’s a mess I wouldn’t want to consider.

  20. Remarksman says:

    @Chuck: re: condensation:
    How different is this, really, than the cold water that runs down your shower drain when you first turn the shower on?
    This seems like a bogus “issue.”

  21. bob B says:

    I could see this being thing being used in a dishwasher system. To preheat the water going into a point of use hot water heater running the dishwasher.

    It could be used in new construction of an energy efficient home, but I don’t think they will sell many of these things to home owners trying to save money on the monthly bills.

  22. A classmate encoraged me to check out this site, nice post, fascinating read… keep up the good work!

  23. Mike says:

    I built a diy version, 8 meters of 15mm soft copper tube vertical spiral wound inside a 120mm drain pipe. The shower waste water runs down past the copper tube carrying the cold water to the shower mixer.

    Works very well, incoming cold water at approx 11 C is raised to 20 C before hitting the mixer. Thus the mixer position is now in a cooler position to get the same output water temp, whole thing stabilizers in approx 30 seconds after the water is first warmed after turning on.

    We used to get 4 consecutive 10 minute showers from our cylinder, now its > 6, ergo we are using a lot less hot water and saving money.

    Mike (NZ)

  24. Bigbru says:

    HEY! Common you guys! How many BTU’s in a THERM? 100,000 !!!!!

  25. Andy Wahl says:

    Wrap it with the right insulation on the outside and the condensation will not happen.

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