Our Heat Pump Water Heater – Part II: Our Experience with It

This is the second of two posts about our switch to a heat pump water heater (HPWH). In the first post, I share why we chose the HPWH that we did. In this one, I share our experience with it.

tl;dr

A HPWH has been working out great for our family of four.
- In particular, a plug-in, shared-circuit, heat pump-only (not hybrid) water heater (HPOWH) made for an easy install (and has been working out great for us).
HPWH are more efficient and better for the environment than gas water heaters, and the plug-in, shared-circuit HPOWH models are even more efficient.
- Scheduling further allows us to do most of the heating work when the energy is cleanest (and cheapest).
There are options for those who are worried that the model we got aren’t enough, including a larger tank, a dedicated-circuit model, or going with a hybrid.

Also, check out Our Electrification Journey for more about what we’ve electrified (which includes details about our PV+battery system and our range).

Installation & Initial Heating

Choosing a plug-in, shared-circuit model made installation relatively straight-forward for me as I didn’t have to get any electrical work done first (no need for a new dedicated outlet nor hard-wire junction box). I just plugged it into the same outlet as my clothes washer and dryer, which hasn’t been a problem. I was lucky that there was a nearby outlet where I wanted to install it.

After installation, it was plugged in around 2 pm and it finished initial heating to 120°F before 9 pm, but during that time it was unplugged a few times (maybe for 30-60 total minutes). Ambient temperature was about 65°F, and I’m guessing the cold water was about 50°F. I was surprised that it heat 65 gallons so quickly.

Energy & efficiency of initial heating

Theoretically, it should take about 11.1 kWh to heat 65 gallons from 50°F to 120°F. Two different ways to reach that number:

65 gallons × 8.33 lbm/gallon × 1 Btu/lbm×°F × 70°F = 37.9k BTU = 11.1 kWh
= 65 gallons × 3.79 liters/gallon × 1 kg/liter × 70°F × 5/9 °F/°C × 4184 J/kg×°C = 4.0 × 10^7 Joules = 11.1 kWh

My very rough estimate on actual energy consumption was about 3 kWH (= 450W × 6½ hours; more on power draw, below).

This would mean it had an efficiency of roughly 3.7× (11.1 kWh / 3 kWh). Again, this is the result with some very rough numbers in this equation (including the incoming water temperature, the amount of time the heating went on for, and the average power draw). That may be high, but maybe still within the ballpark of possible for heat pumps.

Power, Energy, and Cost after 5 Weeks

As you’ll see in the scheduling section below, heating starts daily with a starting temperature of 110°F and finishes when it reaches 140°F.

So far, I have about 5 weeks of (clean) data (mostly in May 2024).

Power

I’m seeing the unit draw between 330-480W; 330-380W when it starts, and it gradually climbs to about 480W over the course of hours. It seems that as the water gets hotter, it has to work harder to continue to heat the water.

The unit draws 2.7W while idle.

Energy & Cost

The HPOWH’s energy consumption has averaged 2.32 kWh per day.

If the cost of electricity is $0.12430/kWh (my utility’s Tier 1 rate), that’s about $0.29/day or $8.77/month. At the most expensive (Tier 3) rate, $0.29453/kWh, that’s about $0.68/day or $20.78/month.

With our solar and battery system, our cost of electricity is as low as $0.0290/kWh (see A look at the production and finances of my Tesla Energy system). If using $0.05/kWh instead (padding to account for increases in energy cost and increase in electricity usage), that would mean we’re paying about $0.12/day, $3.52/month, or $42/year to heat water for our family of four.

Cost Compared to Gas, Estimated

I don’t know how much it actually cost us to heat water with our gas water heater. So, here’s a calculated estimate.

Energy needed to heat the water we use

If we assume the HPOWH has an efficiency of 3×, then it’s putting about 6.96 kWh (3 × 2.32 kWh) of energy into heating water each day. That’s 0.2375 therms (or 23,749 BTUs).

Gas water heater energy and cost

Gas water heaters appear to be between 53-70% efficient. If we use 70%, then that means we needed 0.3393 therms of gas (0.2375 ÷ 0.70) to do the same amount of heating that our HPOWH is doing. Our current gas rate is $1.88593/therm. So, it would cost about $0.64/day, or $19.46/month. That’s about 2.2× using our electric utility rate, and 5.5× using our effective rate.

Just to see what the numbers look like, if a gas water heater is 100% efficient, then the cost would be $0.4479/day (0.2375 therms × $1.88593/therm), or $13.62/month; 1.6× using our electric utility rate, and 3.9× using our effective rate.

Cost Comparison Table

	Daily	Monthly	Annual
Electric HPOWH @ $0.05/kWh (172% our cost of electricity with solar and battery)	`$0.12`	`$ 3.52`	`$ 42`
Electric HPOWH @ $0.12430/kWh (my utility’s Tier 1 rate)	`$0.29`	`$ 8.77`	`$105`
Electric HPOWH @ $0.29453/kWh (my utility’s Tier 3 rate)	`$0.68`	`$20.78`	`$249`
Gas @ $1.88593/therm, 100% efficiency (my utility’s Tier 1 rate)	`$0.44`	`$13.62`	`$163`
Gas @ $1.88593/therm, 70% efficiency (my utility’s Tier 1 rate)	`$0.64`	`$19.46`	`$234`
Gas @ $2.32938/therm, 70% efficiency (my utility’s Tier 2 rate)	`$0.79`	`$24.04`	`$288`

So, it appears that operating the HPOWH at our electric utility’s most expensive rate is slightly more expensive than operating a gas water heater at our gas utility’s cheapest rate (a possibility that increases with electrification).

Scheduling to heat with our solar

I used Rheem’s scheduling feature to make it so that most water heating would happen during the day.

During the summer, our solar produces more than we use and our batteries can hold (and pushes the excess to the grid). By heating our water during the day, we use that excess rather than heating off of the battery or grid.

This is ever so slightly cheaper than if it heated using grid energy (we aren’t on net metering, so using energy we’d push to the grid is cheaper than energy we’d buy from the utility).
More importantly, using energy generated during the day (whether from our solar or from the grid) is generally cleaner and ever so slightly better for the environment (and the grid).
This also reduces demand from the batteries: reducing cycles, reducing the chance the batteries run out before the next morning’s solar production, leaving more energy in the case of outage, etc.

Water Temperatures and Practical Capacity

Reiterating the first post, 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 when mixed with 50°F water (with no recovery). That’s more than six 17-gallon showers, or more than enough to handle four showers, dishes, and laundry.

While the schedule will allow the temperature of the water in the tank to vary throughout the day, the HydroBoost feature of our water heater (more on this in the first post), which automatically mixes water at its outlet so that it’s 120°F (this can be changed), reduces the variation we would experience at the tap. That is, the spot on our faucet that would give us, say, 100°F doesn’t change with the temperature of the water in the heater because HydroBoost ensures that outgoing temperature is always 120°F at most.

The Schedule

I have been and will continue to tweak the schedule, seeing how much I can reduce how much heating it does (by shortening how long the hotter time windows are, and reducing the temperatures within them). Perhaps weather changes with the seasons might also change how we consume and heat the water. This is what the schedule looked like in May of 2024.

The schedule, shown by the red line (its y-axis is on the left):

1 AM – 11 AM: 110°F

We don’t need much hot water in the middle of the night and the early morning. So we let the water stay relatively cool and save all the work of heating it for later when the sun has been shining. In all likeliness, the temperature of the water probably actually remains near 120°F during this time; a holdover from the previous slot of the schedule.

11 AM – 4 PM: 140°F

After the sun has been shining for a while, shown by the yellow line (not to scale), and given ample chance to begin replenishing the battery, the water heater starts heating to 140°F. This is most of the heating that we’ll need for the rest of the day.

4 PM – 7 PM: 130°F

The water heater is set to maintain enough hot water to handle the upcoming evening load despite possible heavy usage during this time.

7 PM – 1 AM: 120°F

Like the preceding window, the water heater is set to maintain hot water for the evening load, including possible late showers.

☝ I actually tried to have this window end at about 12:15 AM, but the schedule didn’t work properly. More on this in the ‘Rheem Smart Features and EcoNet Android app’ section, below.

ℹ How hot are 104°F, 120°F, and 140°F?

Here are some water temperatures for reference:

40-65°F — cold tap water (actually varies much more than this; water coming out of the “cold” tap can be warm in some places!)
78-82°F — not-cold pool
92°F — warm pool
98.6°F — average normal body temperature
102°F — hot tub
100-108-°F — hot shower
120°F —
- Widely recommended minimum hot water temperature (or even required; e.g., California Health and Safety Code § 114192) to kill Legionella (which causes Legionnaires’ disease)
- Widely recommended as the highest temperature for hot water at the tap to prevent scalds (e.g., U.S. Consumer Product Safety Commission).
- First-degree burn in about 8 minutes, and third-degree burn in about 10 minutes.
- Typical default setting for new water heaters.
140°F —
- First-degree burn in about 2 seconds, and third-degree burn in about 6 seconds.
- Maximum temperature of Rheem HPWHs.

Actual Energy Usage

The blue line (its y-axis is on the right) shows the average energy consumption of the water heater by hour (from about 5 weeks of data). Per the schedule, most of its energy usage is during solar energy production. It does kick in during evening water usage, but at least it’s minimal (and easy for the battery to handle), especially during peak usage (and when solar generation is dropping off; see the duck curve).

Recirculation + Scheduling and/or HydroBoost ⇒ 😬

If you use recirculation that’s based on temperature, such as a system that uses a cross-flow/crossover valve (where hot water is pushed into the cold water supply), that could experience issues if the hot water temperature can be below the valve’s trigger temperature. This could happen if your scheduling includes temperatures below the trigger temperature, and/or if your HydroBoost mixes to a temperature below the trigger temperature.

What happens then is the hot water reaching the valve may never result in the valve closing, and the recirculation will run continuously (while the recirc pump is scheduled to run).

A couple options to mitigate this issue:

Reduce the trigger temperature of recirc valve, and/or
Ensure the recirc pump is not scheduled to run when the water temperature might below the valve’s trigger temperature.

Rheem Smart Features and EcoNet Android app

The Android app is really bad, but at least it works just well enough to be useful. I’d give it 2/5 stars.

ℹ Rheem has two EcoNet Android apps. From what I can tell, they’re identical except for a few cosmetic differences (i.e., their functionality, information architecture, and version numbers are the same, and the UI/UX is mostly the same). I used the “EcoNet” one more than the “Rheem EcoNet” one.

The Good: Essential Functionality

The app can be successfully used for essential functions, such as changing the water temperature, setting the water heater to vacation mode, and verifying status. The scheduling features are awesome when they work.

The Bad: Any Functionality Beyond the Essentials

Temperature Status: There have been a few times where the temperature shown in the app is different from what’s shown on the water heater.
Wi-Fi setup during the initial setup wasn’t smooth. It failed the first couple times I tried — or at least appeared to — and when I came back to it later, it just worked.
Scheduling: For the first couple weeks, the water heater wouldn’t follow the schedule I set. At various times throughout the day I would check the target temperature of the water heater (both in the app and at the heater), and it would often be different from where it was supposed to be according to the schedule. After several tweaks to the schedule, I got it to follow the schedule, I think by avoiding having a time slot that falls between midnight and 1:00 AM.
Scheduled Away/Vacation: I tried to use the “Scheduled Away/Vacation” feature once, but it didn’t actually automatically set it to Away (at the beginning) nor back to Home at all.

The Ugly: The UX

The app requires you to input things like name and phone number to use it.
If you haven’t set your “Home,” then trying to set a Scheduled Away/Vacation fails (with an unhelpful message). I’m not interested in the automatic (geofencing) Home/Away feature and I’d rather not have to put my address in (so I put a fake address in).
When set to Away, you can’t click into the water heater (to see its schedule, status, health, etc.).

Conclusion

I would recommend considering a 120V plug-in, heat pump-only water heater! I was worried that the Recovery Rate (and First Hour Rating) might not be enough to support our family of four, but we’ve had no issues. It heats faster than I expected, and going with 65 gallons probably helps (but maybe 50 would have been enough).

If we start experiencing shortages (if the kids start taking longer showers), then maintaining 140°F longer in the schedule should help handle the increased demand.

For those who have greater water needs (more people in the household, more baths, or whatever), there are also 80 gallon models.

If that’s still not enough (or if, say, you don’t have space for the 80 gallon model), then a hybrid HPWH is still a great replacement for gas water heaters (comparable performance, but likely cheaper to operate, and better for the environment).

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