Our 10-year-old natural gas water heater had about reached its life expectancy, and it was the last thing responsible for us paying money to the gas utility every month. We replaced it this weekend with a heat-pump hybrid electric water heater! A Rheem Proterra 50 gallon tank, in our case. It has both standard resistive heating elements, and a heat pump (hence "hybrid"). The resistive elements draw 4500 watts when you need heat in a hurry, but the heat pump draws only 400 watts. Interestingly, at 400 watts, a heat-pump-only version of this tank could easily run on a standard 120 volt 15 amp electrical circuit.
We have been working towards this since we replaced our natural gas furnace with a ground-source heat pump, but supply chain issues and life in general meant we kept the natural gas water heater for an entire extra year.
First we drained the old water heater and removed the exhaust flue:
I had to be careful of the coolant lines for the ground-source heat pump. Pretty inconvenient that they run down the middle of the mechanical room like that.
Then I wrangled the new water heater into place and plumbed it up.
Extra stuff: there's a 2" foam pad between the concrete and the tank, both for thermal insulation and vibration isolation. Then a drain pan to catch any stray water and direct it away from the pad toward the floor drain. I have a "heat trap" loop on the hot water output, to prevent convection or thermosiphon heat losses. There's a thermostatic mixing valve to prevent scalding, and an expansion valve (rather than a bulky expansion tank) which routes to a plastic vinyl tube manifolded with the condensate drains from the ERV and the heat compressor on the water heater.
The Proterra tank is also an Internet of Things device, so I can schedule heating to run when our solar panels are likely to be producing extra electricity. I can also monitor how much hot water is in the tank, which has been my current pointless obessession since I installed the tank. The tank also reports how much electricity it consumes, but that hasn't really agreed well with the energy tracking I've done with my Emporia Energy monitor. I've seen other complaints about the energy reporting not being very accurate, so I'm going to give the benefit of the doubt to Emporia.
The old water heater used 12-14 therms of natural gas in a month, which is equivalent to about 350-400 kWh of electricity per month. We're on track to use about 60-70 kWh of electricity per month with the new hybrid water heater after the first week.
One tricky part is that we're planning to add a second "desuperheater" tank (already purchased, and taking up space in our finished basement) to scavenge heat from the ground-source heat pump and preheat the cold water before it goes into this tank. So the plumbing isn't final. But that will wait until the old natural gas piping is removed, to make room for the second tank.
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