DWHR and HPWH vs Electric, Total Efficiency?
So it’s well known that a Heat Pump Water Heater (HPWH) is far more efficient at heating water than a standard electric water heater, but what happens when we look at the entire system top to bottom, including Drain Water Heat Recovery (DWHR) as well as the building’s main heat source?
A HPWH is most efficient with low water temperatures; the colder the incoming water and the lower the hot water temperature set point on the tank, the higher the overall Coefficient Of Performance (COP) will be. Using Rheem’s hybrid model as an example, they claim a COP of 3.5, or 350% efficiency, under standard test conditions.
A standard electric water heater is 100% efficient regardless of the water temperatures. Only standby losses are a factor.
So what happens to this situation when we throw DWHR into the mix? All of a sudden the incoming water temperature is much higher because DWHR captures 40-60% of the heat from the outgoing drain water. This, in theory, will raise the average water temperature in the tank and decrease the COP of the HPWH. A standard electric tank keeps on going at 100%.
Now consider the heat source for the building. A HPWH is taking heat from the building and putting it into the water. If the building has a 95% efficient gas furnace, the heat the HPWH uses is starting off with a 5% efficiency hit. If the building is heated with an Air Source Heat Pump that’s operating at a COP of 2.0 in cold weather what happens to system efficiency then?
A standard water heater might last 10-12 years. A HPWH will likely be the same as the tanks are built the same, assuming of course the heat pump is still working trouble free. So the cost to replace the HPWH is about twice as high every 10-12 years.
Now, just to make things extra confusing, throw in a standard water heater like the Rheem Marathon which has a lifetime guarantee against leaks (no replacement costs) as well as greatly reduced standby losses (3″ of foam instead of 2″). How does that measure up over time?
Maybe this has been studied before and I just haven’t found the paper? I’ve looked and all I seem to find are reviews of the water tank only, not the math to figure out the whole picture.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part