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Building Science

Introducing the 120-volt Plug-in Heat Pump Water Heater

The good news is the model can save thousands of dollars. The downside is limited backup heat, which could mean running out of hot water.

Four models of 120 volt plug-in heat pump water heaters

The 120-volt plug-in heat pump water heater is here! The effort to bring these things to market began in 2018, and now they’re available to buy. This technology has some significant advantages over the standard 240-volt heat pump water heater. But it also has a drawback. Let’s take a quick look.

The big 120 V plug-in advantage

The 120-volt plug-in heat pump water heater requires, well, half the voltage of the standard models, like the one I have in my basement. Any electrical device that uses 120 volts can be plugged into a standard outlet whereas 240-volt devices need special wiring and connections. You can plug in a 240-volt appliance to an outlet with a special plug and outlet. Think electric clothes dryers. Most 240 V appliances, however, are hard-wired.

The thing about 240 volt appliances, though, is that you need more than just a different kind of outlet and plug. You may need three changes to make a 240 V heat pump water heater work in your home. First, a 240 V appliance probably needs two breaker slots in your electrical panel. That could mean you need a bigger panel or to have an electrician rewire things within.

Second, it also uses more electricity. That could mean having to upgrade the electrical service to your house. If you’ve got, say, a 100 amp (A) service, you may need to upgrade to a 150 A service to install a 240 V heat pump water heater. (See my second article on electrification, though, to find out more about panel upgrades.)

Third, you’re going to have run a new circuit from the panel to the 240 V heat pump water heater. This will require paying an electrician to open the panel, running the wiring to the water heater location, and setting up the connection. When I electrified my 1961 home in 2019, I replaced the old fossil gas water heater with a 240 V heat pump water heater. The electrician charged me $600 to install the 240 V circuit.

So the big advantage of these new 120 V plug-in models is that they can save you thousands of dollars avoiding changes to the electrical system in your home.

The 120 V plug-in drawback

As you might expect, the lower voltage delivered to the plug-in heat pump water heater means less ability to heat water. That’s because it also gets less electrical power. The 240 V models are called hybrid heat pump water heaters because they have three ways of heating water.

They can use the heat pump. They can use electric resistance heating elements, the same kind you’d find in a standard electric water heater. Or they can use both the heat pump and the electric resistance.

With the 120 V plug-in models, you don’t get the big electric resistance heating elements that come with the 240 V models. Some don’t have the backup heating element at all. That’s great for efficiency, but it may limit your hot water.

I say “may limit your hot water” because the real difference between 120 V and 240 V heat pump water heaters is how much backup heat is installed. I have a 240 V model and keep it in heat-pump-only mode unless we have extra people staying with us and need faster water heating. That’s happened only once in four years though, but we do have the 80-gallon model.

Is a plug-in heat pump water heater right for you?

Heat pump water heaters use only a few hundred watts of power when they’re using only the heat pump. The highest I’ve seen mine go since I started monitoring my electricity last year is a bit above 400 W. Since I almost never turn on the electric resistance heat, I could have gone with an equivalent 120 V model.

What it comes down to is how much hot water your heat pump water heater can make and how much hot water you use. Well, I guess another factor would be how afraid you are of running out of hot water. We’ve had that problem only once with guests in the house.

If you have the electrical capacity already or are building new, you should be able to do it without adding much cost. In an older house that would require significant changes to your electrical system, the 120 V plug-in model can save you thousands of dollars.

Two ways to make 120 V plug-in heat pump water heaters go further is to get one with a larger tank or make the water hotter and then use a mixing valve to drop the temperature before use.

Heat pump water heater resources

There’s a lot of really good information available about both kinds of heat pump water heaters. Here are some good resources:

Have you installed or used any of these new water heaters? What are your thoughts?________________________________________________________________________

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.

26 Comments

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Allison,

    Can these be plugged into any 15 amp circuit, or do they need 20 amps?

    1. paul_wiedefeld | | #2

      It’s 15 for the rheems. One model is shared circuit, the other is dedicated.

      1. Expert Member
        MALCOLM TAYLOR | | #4

        Paul,

        Thanks. That's a big advantage.

      2. lance_p | | #14

        The 120v dedicated circuit Rheem model actually has a 12kBTU (1 ton) heat pump and recovers about 3x faster than the plug in models. It's not quite as efficient, but I imagine it would be FAR more practical to rely on than the plug in model.

  2. paul_wiedefeld | | #3

    These will help so many homes avoid a service upgrade. I wonder if a sunamp plug-in heat pump is on the way, that would let you get a larger buffer in the same footprint.

  3. rockies63 | | #5

    Matt Risinger just did a video on 2024's Top 7 Heat Pump water heaters - I think some were 110 volt.

    https://www.youtube.com/watch?v=9pBarDSaGKE&t=17s

    1. Chris_in_NC | | #9

      Interesting to see the new (for North America) LG unit, and the return of the Geospring line that isn't even on the market yet.

  4. charlie_sullivan | | #6

    In addition to the 120 V models there are also 240 V models that can go on a 15 A circuit instead of the traditional 30 A circuit. That allows them to be wired with the same 14/2 Romex you'd use for a 120 V circuit and they don't eat much of your available capacity. If you have a 120 V circuit that's available as a dedicated circuit for the water heater, you can convert it to 240 V at minimal cost without running new wire.

  5. D_Hallowell | | #7

    Does anyone know if the heat pump HWH noise issue has been improved with these 120v models?

    1. charles3 | | #12

      Precisely what I would like to know too.

  6. Ryan_SLC | | #8

    Here in Utah we are required (and areas in CA) to have Ulta Low NOx when using a gas heater. You'll see that notice on HD when you shop for any water heater.

    Is there any analysis on green benefit? I know in my area gas costs me less. That said, I am aware my state subsidize natural gas production and our electric is backwardly coal heavy.

    I'm unclear if my added electrical bill for going electric, in my area, is the green option or not.

    1. iainb | | #17

      From what I have read, even if you’re coal heavy a COP of 1.97 or higher means lower GHG emissions with a heat pump.

  7. [email protected] | | #10

    How much do you cut into the energy savings by keeping it at higher temperature and using a mixing valve, or by purchasing the larger tank (and having to keep that larger amount hot)? Seems like that might be a high price to pay to avoid once every few years people have to wait to shower.

    Any thoughts on instant electric water heaters? You avoid the tank loss (and they take up little space) but I've seen VERY mixed reviews on whether they work or not, and of course there you are trading of higher electric draws to get better operations.

    1. paul_wiedefeld | | #13

      If the room is 70F, a 120F tank would have about 30% fewer standby losses compared to a 140F tank. But 30% of a small number is probably insignificant.

      A tankless water heater doesn’t belong in a green home: the efficiency isn’t any better than a resistance tank and significantly worse than a heat pump. The same is true for gas. Besides efficiency, the performance is worse than a comparable tank too. They only save floor space. The marketing is strong though.

      1. Expert Member
        DCcontrarian | | #16

        The issue with efficiency and higher water temperature isn't just standby losses, it's also that a heat pump's COP declines as the temperature jump increases. Lifting water from say 50F to 140F is a bigger lift than from 50 to 120. It's going to take about 30% more energy.

        1. paul_wiedefeld | | #18

          Correct but 140F water goes further than 120F water, so you heat less of it.

          1. Expert Member
            DCcontrarian | | #21

            But you need to produce the same number of BTU's to end up with the same amount of hot water. Producing the same BTU's at 140 takes 30% more energy than at 120.

            That's the major difference between heat pumps and conventional heaters. With a conventional heater a BTU is a BTU.

  8. Expert Member
  9. LawrenceMartin | | #15

    The only concern I have with heat pump water heaters is that for a very small home in a cold climate (CL 5 and 6), where do you put it if you don't have a basement or some oversized room that can handle the cooling effect of the heat pump? I tried to incorporate one into my new home, but couldn't figure out how to do it. I wound up installing a tankless electric water heater (all electric house). One notable drawback is that our utility has since shifted to time-of-use charges. That means not showering or dishwashing from 4pm to 8pm. That is a minor inconvenience for us since we work from home and can schedule around it. Even when we do use it during that time, it only adds about $6 to our monthly electricity bill (no matter how many times we use it during the billing period). The electrical service was also not an issue and we still have a few open slots in our electrical panel. A big benefit is that we travel a lot. So, when we come home, we don't have to wait for the tank to heat 40 gallons of water to take a shower.

    1. paul_wiedefeld | | #19

      Other than space savings, really no reason to ever install a tankless electric water heater. It’s just a bad product. A tank either matches or greatly exceeds it’s efficiency (resistance vs heat pump) and a tank has none of the flow restrictions that tankless have.

      1. LawrenceMartin | | #22

        I think Tankless is great for the reasons I provided in my previous comment. Also, I don't have to worry about a tankless water heater failing and dumping water all over my basement as has happened to me in the past. I have a floor drain, but it is a pain in the neck. My house is all electric and we get all our power needs met by solar. I send about 60% of the electricity generated back to the grid. I would love to know what building science says about the efficiency differences. If it is a slight difference, it probably doesn't matter too much for the added convenience.

        1. Expert Member
          MALCOLM TAYLOR | | #23

          Lawrence,

          "Also, I don't have to worry about a tankless water heater failing and dumping water all over my basement as has happened to me in the past."

          Aren't the leaks heaters experience caused by joints and connections failing? Is there something different about tankless that precludes that?

          1. LawrenceMartin | | #24

            I'm not a plumber. and maybe my assumption is incorrect. But, I've not heard of this being a concern when looking into water heater options. Since they don't store a large amount of water it seems unlikely that the end-of-life water tank flooding failures seem unlikely. But, I guess, like anything with water, it can probably leak.

          2. paul_wiedefeld | | #25

            I mean a tank can leak for sure. I like this rationale much more than the “endless hot water” or “high efficiency” nonsense

      2. LawrenceMartin | | #26

        Oh yeah, I forgot to mention that we haven't had any real flow restrictions I have run two showers and the dishwasher at the same time with no problem. It might help that I have a power pipe waste water heat recovery drain that the showers and dishwasher drain through. So, that may help.

    2. iainb | | #20

      So there are solutions where space is a concern, but they are not in the north american market. https://sunamp.com/en-gb/hot-water-solutions-thermino-range/

      They use a phase change material t
      store heat, release as needed. I don't expect them to catch on when you have space to just install a bigger tank, but they exist for the space constrained market.

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