I currently have 2x 50 gallon electric water heaters installed side by side in our garage to support 3.5 bathrooms 2 dishwashers and laundry. I’m considering switching to Rheems 120v heat pump water heater, my question is do I need to replace with 2x 50 gallon heat pump water heaters or 1x80 gallon? enter image description here

  • What region are you in? My understanding is that that 120V (standard outlet) hpwh takes a looong time to heat water, in part because the heat pump part is inherently slow in heating and in part because the small 120V heating element does not provide much boost. I was told this model was created so southern California would have a plug n play standard outlet hpwh option even if it doesn't work very well.
    – Armand
    Jul 27 at 19:44
  • 1
    Is there some reason you don't want to go with 240V for a hpwh? Don't you already have 240V for the existing water heaters?
    – Armand
    Jul 27 at 19:47
  • 2x50 = 100 gallons ready and waiting. 1x80 = 80 which is less than 100. More importantly, I suspect the heat pump water heaters in heat pump mode probably do a lot more work two of the 50 gallon running (recovering) than 1 x 80 gallon, and even more so if they (as I think many do) have an automatic resistance heat backup mode (for when you use too much at the same time). But the key is to look for specs in terms of gallons of hot water per hour. Jul 27 at 19:55
  • Thank you. We are in austin tx area. The only reason I was looking at the 120v is it was rated at 2250 watts and my back up generator could run them if we have an outage. I can do 2x 50 gallon heat pumps if needed I was just curious if what I have now is over kill and 1x 80 gallon would be sufficient
    – Martin
    Jul 27 at 20:42
  • 1
    Given what you're trying to replace, are you hard-copy dead-set on the Rheem units? I ask this because the first-hour recovery of unitary heat pump water heaters absolutely sucks relative to their tank size, and going to a split-system HPWH (such as a Sanden unit) would give you far better overall performance while not overtaxing your generator (the Sanden units top out at ~3kW but can deliver first-hour recovery stats comparable to gas heaters of a similar tank size) Jul 28 at 3:21

4 Answers 4


I seriously doubt you'll get enough performance out of a unitary HPWH

Given that you have a very high-draw house to the point where you have 2 fairly large electric water heaters in series, I seriously doubt you'll get anywhere near enough performance out of any single unitary HPWH. To drive home the point, the 50 gallon Rheem Professional Prestige is specified at a 63-gallon FHR, something that requires an 80 gallon 120V ProTerra to beat at 72 gallons of first hour recovery. Given that you have the two tanks plumbed likely in parallel, you'd be pushing it to get enough performance out of anything short of 2 80 gallon 120V ProTerras, or 2 50+ gallon 240V ProTerras at 67 gallons of first hour recovery apiece for that matter.

Enter the split-system HPWH, stage right

However, this doesn't mean you have to give up on having hot water available when on generator power, provided you're willing to spend some on a better hot water heater. As it turns out, unitary HPWH designs are severely limited in performance by the fact their heat pump unit has to be small enough to fit atop the water heater. Removing that limitation by moving the heat pump part outside gives you a split system HPWH architecture, which gives you several benefits:

The primary downsides, of course, are cost and avaiability: the SanCO2, while well-proven in its home market of Japan, is relatively new to the US market (and thus not carried by very many distributors), and the 83 gallon version (which'd be what it'd take to give you sufficient capacity) costs about the same as 2 of the 80 gallon Rheem heat pump hybrids. It also requires a bit of additional plumbing and wiring compared to a unitary design; all the refrigeration bits are in the outdoor unit, though, so it's still within the realm of what a qualified plumber can tackle.

  • All that fancy formatting, I thought this was a Harper answer! :D It's a standard TPE quality answer with a formatting upgrade. Very nice.
    – FreeMan
    Jul 28 at 13:07

Since what you have is more than what you are contemplating I would stick to these.

Look for a model of heat pump that is not integrated with the cylinder. There are many such brands and models.

Unless you buy a grunty one, it may well heat more slowly so you will want more storage.

We have two 300l cylinders, which is 80 gallons each. This is for 4 of us. And we have a 3kw heat pump on each. Part of the reason is that when all kids have left, we can turn one off. It works well for us in tetms of availability of water. We also have solar power so I use home automation to only heat when we are generating solar power.

  • what brand do you have? Jul 27 at 21:21
  • 1
    You probably have a lot more options in New Zealand. The US Marketing approach to heat pumps in general and heat pump water heating in specific is generally awful. Also overpriced .vs. the rest of the planet.
    – Ecnerwal
    Jul 27 at 22:44

This is perhaps not the answer you were looking for, but that number of bathrooms and dishwashers doesn't seem like it should require that amount of hot water. Are you sure that one 50-gallon heater would not suffice? If not, why not? If you have high-flow devices, like shower heads, consider replacing them with WaterSense models.


If those are in series you may consider replacing the first unit (the one that gets the coldest feed) with an 80GAL heatpump and keeping the second as-is.

HP are much more energy efficient bringing from cold (5°C) to lukewarm (40°C) than to bring from lukewarm (40°) to hot (60°) so doing as suggested you maximize the efficiency of the HP and in case of extra load the traditional heater will give the extra boost needed.

Also remember HP are slower compared to traditional boilers so allow for some extra capacity.

Not the answer you're looking for? Browse other questions tagged or ask your own question.