Our Heat Pump Water Heater – Part I: Choosing a Water Heater

tl;dr

  • We needed to replace our old gas water heater since it was old and it seemed likely to fail soon.
  • We are also electrifying and replacing our gas appliances with electric ones, primarily for environmental purposes.
  • We chose to go with a 120V plug-in heat pump-only water heater (i.e., not hybrid) for its simplicity to install despite apprehension that it’d suffice for a family of four.

In part two, I share our experience with it after living with it for about two months. Spoiler alert: it’s great!

Essential Concepts Crash Course

Hybrid vs Heat Pump-Only Water Heaters (≃ 240V vs 120V)

Most “Heat Pump Water Heaters” are actually hybrid, meaning they have both a heat pump and “traditional” resistive heating elements. The resistive heating elements are basically just coils of wire that get hot when electricity is sent through them. Using the heat pump to heat the water instead is 2-3 times more efficient, but it’s also slower.

Hybrid water heaters will use the heat pump to do most of the heating (efficiently), but then use the less-efficient resistive heating when water is actively being used so that more hot water will be available quickly. So, hybrid water heaters are — depending on usage and configuration — overall a bit less efficient to use than heat pump-only water heaters.

The resistive elements are also almost always 240V (so that they can heat faster; about twice as fast as if they were 120V), while heat pumps can operate at just 120V. So, while a heat pump-only water heater won’t be able to heat up as much water on demand, they can also be substantially less complex to install, and be less demanding on your electrical infrastructure (a single-pole 20A breaker vs double-pole 30A breaker) and supply (e.g., main panel capacity, battery capacity).

“HPWH,” “HPOWH,” and “HEWH”

  • Heat Pump Water Heater, HPWH: Unfortunately, “HPWH” is already widely used for both heat pump-only water heaters, and hybrid electric heat pump water heaters. I’ll use two additional terms when I need to be specific about the these two kinds here:
    • Heat Pump-Only Water Heater, “HPOWH”
    • Hybrid Electric Water Heater, “HEWH”: Uses both a heat pump and electric resistive elements.

Recovery Rate

Recovery Rate is essentially how quickly the water heater heats up water. This contributes to how much more hot water you can get than what the tank can hold.

Generally, gas water heaters can heat water the quickest (but also the least efficiently, and they emit greenhouse gasses in doing so), followed by HEWHs (by using the resistance elements), then HPOWHs.

Recovery Rate is stated in gallons per hour, but they can be hard to compare to each other since they vary in the “rise,” the number of degrees they’re heating the water (Rheem publishes 90ºF rise in some materials and 60ºF for others — these could be typos, or perhaps to obfuscate the poorer performance for the latter models?).

It takes 8.33 BTUs to heat one gallon of water one degree Fahrenheit. So, a model that has a “28 GPH @ 60°F rise” is doing 14k BTUs (28 × 8.33 × 60) per hour while a model that has a “12 GPH @ 90°F rise” is doing 9k BTUs (12 × 8.33 × 90) per hour.

First Hour Rating (FHR)

This is the number we care about most. It’s the number of gallons of hot water that a water heater can provide during the first hour of use when the tank is fully heated and ready to go.

This is basically the sum of the capacity of the water heater, plus the number of gallons it can heat in an hour (which correlates to the Recovery Rate). So, like Recovery Rate, gas water heaters will have the highest FHR, followed by HEWHs, then HPOWHs.

However, FHR doesn’t account for the fact that we can set the temperature of the water higher (e.g., 140°F), allowing us to get more of the temperature we want (e.g., 120°F)…

Water Temperatures and Practical Capacity

The difference between the temperature of the water in the tank and the temperature we want coming out of the faucet means that we’re able to get more of the desired hot water temperature than the tank capacity. For example, from a 65 gallon tank holding 140°F water, you can get about 108 gallons of 104°F water (a fairly hot shower) — with no recovery — when mixed with 50°F water from the cold tap (65 × 140°F / 104°F)) ÷ (104°F – 50°F) = 108). That’s more than six 17-gallon showers.

Our Situation

Gas Water Heater, and Urgency

Our water heater was about 10 years old and the pilot light went out once or twice every couple of months. We were afraid it would die completely at any moment, and if we didn’t replace it proactively then we could end up in a situation where we have to urgently replace it with another gas water heater in order to not be without hot water for days or weeks (as it may have required electrical work).

It had a 50 gallon capacity, and based on its label, I calculated that it had a 41.5 GPH recovery rating for a 90ºF rise (31.1k BTU).

Hot Water Usage

We are a family of four, and the 50 gal gas water heater was sufficient for our usage; that is, we never had a shortage of hot water even with four showers (one of which might have been a bath) while the laundry and dishwasher were also running.

😨 I did have concerns that a similar-sized heat pump-only water heater, which would have only had a 12 GPH recovery rate based on a 60°F rise (6k BTU, 1/5 our current gas water heater!), would not be sufficient for our usage. Another big unknown was how hot we were heating the water to in the gas water heater (and not knowing what practical capacity we were using).

Electrical

Wiring: Our gas water heater did not have any nearby outlet, and certainly not a dedicated 240V circuit that most HWPHs require.

Main Service: Our main service is 125A. Additional significant electric appliances, including a HEWH, may require an upgrade (a lot of work and expensive).

Battery and Backed-up load: We had just one Powerwall. We may not be able to add more significant electric appliances to the backed-up load.

Adding circuits to the non-backed-up load is non-trivial. A distant subpanel, “garage subpanel,” is wired to the non-backed-up connection of the Tesla Gateway. In order to add additional loads to the non-backed-up connection of the Tesla Gateway, another subpanel would have to be introduced in between the gateway and the garage subpanel (to be able to split the non-backed-up connection to the new appliances as well as the garage subpanel).

Climate

We live in northern California, which means that the HPWH would not be subject to operating in extreme cold temperatures.

ℹ However, many that live in colder climates may put their HPWH in a conditioned space or at least a space not subject to extreme cold (such as a garage or basement).

Our Decision: Plug-in (Shared Circuit), 65 Gallon Heat Pump-only Water Heater

We explored (and eventually decided to go with) a HPOWH that can be plugged into an existing (shared circuit) regular (120V) outlet.

Heat pump-only ⇒ 120V shared circuit plug in ⇒ Simplicity

There are heat pump-only models that only require 120V and can be plugged into a regular outlet (and even an outlet on a shared circuit) since they don’t have the resistive heating elements. This would avoid or reduce electrical work (cost and time) and complexities (logistics complexity / project management of coordinating electrical work as well as plumbing work):

  • The need to install a new 240V volt circuit, which would take up two slots in the electrical panel.
  • The potential need to upgrade the main service.
  • The potential need to add another battery or, alternatively, a new subpanel to add it to the non-backed-up load.

However, I was concerned with the HPOWH’s recovery rate…

Capacity: 50 → 65 gallons

Our 50 gallon gas water heater had an FHR of about 75 gallons. Rheem’s 50 gallon dedicated-circuit HPOWH has an FHR of only 51 gallons — too big of a difference to be risk I was comfortable trying!

By stepping up to a 65 gallon model, the FHR is 63, a less worrisome gap — a risk I was willing to take. 🎲🎲

The 65 gallon model we went with was a shared-circuit model, which we ended up plugging into the same outlet as our clothes washer and dryer (gas).

ℹ Rheem doesn’t appear to have a 65 gallon dedicated-circuit 120V plug-in model, 🤷‍♀️, simplifying choices that needed to be made here. It also avoided needing to install a new dedicated circuit.

I figured the risk of not having enough hot water could be further mitigated by heating up the water more (e.g., to 130-140°F instead of 120°F), which would result in needing to use/mix less hot water in order to receive, say, 102°F water. In fact, all of Rheem’s shared circuit HPOWHs that I looked at have HydroBoost, a built-in thermostatic mixing valve “that extends the hot water in the tank for maximum output performance and requires no electrical work.” Considering that the mixing valve is preset at the factory to 120°F, it would be logical to conclude — for the mixing valve to be of use in extending hot water — for the internal temperature to be set higher than that.

ℹ Additionally, >122°F is widely recommended to kill Legionella.

The CDC and WHO recommends storing hot water at 140°F.

Choosing a brand: Rheem

Some incentives only apply to specific models of appliances, unfortunately. For example, Golden State rebates (for California) specifies that the “heat pump water heater must be on the Qualified Products List at the time of purchase,” which only includes models from Rheem and A.O. Smith.

Also, when I started doing research in Q3 of 2023, Rheem was the only major manufacturer with a widespread offering of 120V plug-in HPWHs (available for more than a year) with plenty of articles, reviews, and discussions about them. A.O. Smith announced their plug-in models shortly after (and it would be some time before substantive reviews and discussions rolled in).

So, I focused primarily on Rheem models.

Incentives

Note that there are probably several incentives that you can take advantage of that will reduce the cost of purchasing and installing a heat pump water heater and electrical work (if necessary), including local (such as rebates from your utility), state (such as Golden State rebates for California), and federal (including the Energy Efficient Home Improvement Credit). Check out Rewiring America for more information.

Additional Rheem Details from my Research

Here are some additional details from my research that you might find helpful.

Or skip to part two of two (our experience with the water heater) if this is too much detail.

What’s the difference between the Shared Circuit and Dedicated Circuit models?

The dedicated circuit models have a more powerful heat pump compressor which allows them to produce hot water more quickly as it’s consumed (i.e., they have a higher recovery rate and FHR), resulting in a higher power draw that requires them to have their own dedicated circuit. This also lowers their UEF (3.0 vs 3.3).

“The dedicated circuit offers the largest compressor in the industry and delivers the fastest heat pump water heater (HPWH) recovery on the market, performing similarly to a standard gas water heater.”

Rheem

The dedicated circuit models have a recovery rate of 28 GPH @ 60°F rise vs the share circuit models’ 12 GPH @ 60°F rise.

RecoveryBTUskWhExpected Draw
in GPH,
60°F Rise
Recovery
× 8.33 BTUs/G/°F
× 60°F
BTUs
× 0.000293
@ 3× Efficiency1,
amps @ 120V
Shared Circuit
(e.g., PROPH65 T0 RH120-MSO)
125,9981.8586 watts2,
4.9 amps
Dedicated Circuit
(e.g., PROPH50 T0 RH120)
2914,4944.21,416 watts,
11.8 amps
1 Expected Draw is just the kWh divided by three, which comes from a common claim that HPWH can have an efficiency of up to 3× (or can do 3 watts of heating using just 1 watt of energy).
2 I only ever see my shared circuit model draw about 480 watts at most, FWIW.

HydroBoost (≃ Shared Circuit)

HydroBoost is extra hardware that Rheem puts on their shared circuit water heaters (it’s the black thing that runs up about ⅔ the height of the water heater) that mixes some cold water into the hot water as it leaves the water heater as a way to kinda stretch how much hot water it provides (since they have a lower recovery rate).

☝ I don’t think HydroBoost should be limited to the Shared Circuit models.

For the most part, what water temperature you choose at the sink or shower and how much of that you use will not take into account how much mixing is done at the water heater versus at the faucet (all that will vary is what faucet position you need to use to get the temperature you want). That is, HydroBoost doesn’t really change how much of the tank’s hot water you’ll want to consume, or how fast you need the recovery rate to be.

What makes HydroBoost useful is the scheduling feature. If you use scheduling to vary the temperature of the water heater throughout the day — which I do to make operating the water heater a tiny bit better for the environment (more on this in the following post) — then HydroBoost helps ensure that mixing at the faucet isn’t different throughout the day as temperature of the water in the heater varies. This can be useful regardless of whether the water heater is a dedicated or shared circuit model.

Is there a difference between Rheem’s Consumer (Performance Platinum) and Professional (ProTerra, Professional Prestige) models?

Some sites say that Rheem’s professional models generally have higher quality components, but I haven’t found any evidence of this — at least not for the HPWHs — and doubt that there’s actually any difference in the appliances themselves:

What is probably different (other than marketing):

  • Where they’re sold / Who they’re sold to (retail vs professional).
  • How warranties are handled.

Deciphering the model numbers of the consumer and professional plug-in models

Consumer models look like `XECCT10HM(S)00U0`.

Professional models look like `PROPHCC T0 RH120(-M(SO))`.

  • `CC` = capacity in gallons
  • `M` = HydroBoost / Shared Circuit. The `XE65T10HM00U0`, `XE65T10HMS00U0`, `PROPH65 T0 RH120-M`, and `PROPH65 T0 RH120-MSO` have HydroBoost; the `PROPH50 T0 RH120` does not.
  • `S` = LeakGuard. The `XE65T10HMS00U0` and `PROPH65 T0 RH120-MSO` have LeakGuard; the `XE65T10HM00U0`, `PROPH65 T0 RH120-M`, and `PROPH50 T0 RH120` do not (but it can be added)

Next: Our Experience with It

In part two of two, I share our experience with it (including installation and initial heating) after living with it for about two months. Spoiler alert: it’s great!


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