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]