We recently had a chance to install a new water heater, and with it, to install a Drain-Water Heat Recovery system. I wanted to share our experience and some real numbers on the cost savings.
A Drain-Water Heat Recovery system can save you money on your water heating bill by recovering some of the heat that you are pouring down your drain any time that you use hot water in your home. It takes that spare heat and pre-heats the water coming into your regular water heater, which then requires less energy to do its job.
In our system, we have a typical tankless hot water heater (but these systems work with tank systems too). The 3-inch drain pipe from the master bathroom runs in the wall just behind the water heater, so there was room to install a 48inch long, 3 inch diameter PowerPipe system.
Real World Performance Numbers
On a November evening in the Pacific Northwest, we got the following numbers.
|Inlet temperature from the city||57°F|
|Output of PowerPipe system||73°F|
|Temperature Rise||(73-16) = 16°F|
|Water heater set point||120°F|
We'd expect that the efficiency boost will be higher in the winter (colder water coming in will absorb more heat), and lower in the summer. Measuring on a typical Fall day seems like a good baseline.
The PowerPipe brand itself advertises around a 45% efficiency gain for this model, but it's likely they are estimating a much colder input water temperature, like you'd see in a typical Northern climate.
Overall, we use around 30 therms (a therm is 100k BTU) per month on hot water, so the savings will be around $10/mo in our area. With a ~$600 cost, that's a payback period of 5 years.