My Experience with a Hybrid Heat Pump Water Heater

In October 2023, my wife emerged from the bathroom one morning to report that we had no hot water. I found that our gas-fired, on-demand water heater had failed and was leaking a small amount. I called our plumber, who confirmed we needed a new water heater.

We immediately faced the following decision: Should we replace our water heater with a similar gas-fired on-demand unit or change to an electric heat pump water heater? We have been hearing a great deal about the efficiency and ecological benefits of heat pump water heaters, and on-demand gas units like the one we had are said to emit considerably more carbon than even gas-fired tank units.

Placement for a heat pump water heater would be a challenge. The water heater had been located in the attic, and the heat pump water heater was somewhat taller than even the gas-fired tank unit that was installed at the time we bought the house. That meant it could not fit in the attic and would need to be installed in the garage. We found a reasonable location, although we would lose storage space in the garage. It also meant that the loop for our hot water recirculating system needed to be extended down to the garage, making the loop longer.

The plumber said that he could replace the on-demand gas unit with a similar one in a couple of days, but it would take a week or so to obtain a heat pump unit, which would also cost considerably more. We had to make a snap decision because we didn’t want to be without hot water any longer than necessary. Wanting to be good ecological citizens, we opted for the heat pump unit despite the cost, complexity, and delay in installation.

As promised, the plumber obtained the heat pump water heater, an American Standard 50-gallon unit (model ASHPWH-50). Installation took about two days and involved running a considerable amount of pipe from the old location in the attic to the new location in the garage, installing a new 30-ampere 220-volt electrical circuit at the heater, and bracing against earthquakes. The installation went well, except a defective water leak sensor had to be bypassed.

First Impressions

The first thing I noticed about the new water heater was its significant noise. The specification sheet listed it as emitting 40.1 dBA, which is about the same as a refrigerator’s (and also an implausibly specific value). However, the water heater was much louder than this.

The on-demand water heater spoiled us: we never ran out of hot water. We soon found that not to be the case with the new water heater, and I turned the water volume for my showers much lower than before. Perhaps scarcity is a good thing.

The new water heater is a “hybrid” unit with a heat pump and also a conventional heating element. It has several modes:

• Green (heat pump only)
• Comfort (“rational use of the heat pump and, if necessary, the heating element”)
• Fast (heat pump + heating element)
• i-Memory (“learns your habits and replicates them autonomously”)
• Electric (heating element only)

There’s also a Boost mode, which is like Fast, but reverts to the previous mode when it gets up to temperature. Initially, it was set for Comfort, but I couldn’t understand when it was and wasn’t running the heating element. I asked product support and the installer, but I still couldn’t make sense of Comfort mode, so we changed to Green. I manually turn on the Boost feature when we need extra hot water.

The noise issue

The noise generated by the water heater was bothersome, especially since it was noticeable in my office adjacent to the garage. I downloaded a sound level measurement app on my phone. I sent a video to American Standard (actually Ariston) product support. They told me I had not measured it under the right conditions. Of course, my garage is not an anechoic chamber.

I consulted “the internet” and found conflicting answers about the noise generated by heat pump water heaters. Some comments described them as very noisy, and others found them quite acceptable. I tried to find others locally with similar water heaters for comparison and failed. In late 2024, the behavior of the water heater changed: sometimes it had a loud “whirr,” and sometimes it didn’t. This prompted me to make several recordings of the sound (and analyze it with a spectrum analyzer app) and determine that much of the excess noise was coming from the fan in the heat pump. I again contacted product support, and they agreed to ship me a replacement fan. My plumber replaced it, and the noise is much better now.

The app

Like many new devices, the water heater connects to the internet through my WiFi and, through a cloud service, allows an app on my phone to control and monitor it. The app, called Comfort Link, is acceptable but less than excellent.

Good:

• Allows me to remotely control the water heater, including turning on Boost mode when needed and turning off the water heater when we’re on vacation
• Displays the water heater temperature so I can judge whether to take a shower
• Shows the history of power usage by the water heater by month, day, and hour

Not so good:

• The water heater has an icon that appears when it judges when it’s OK to shower. This isn’t on the app, but it would be very useful.
• The water heater temperature shown on the app doesn’t agree with that shown on the water heater. The support people explained that the water heater has several temperature sensors, and the app displays (I think) the average between the tank top and tank bottom temperature. But that doesn’t explain why the front panel displays something different.
• Limited history of power usage data (e.g., daily data for only the current and previous month)
• Poor resolution of power usage data. Measurements seem only to be taken only once per hour, so the heating element usage is reported in 4.5 kWh increments and the heat pump in 0.5 kWh increments.
• There seems to be no API to download data other than through the app.

Efficiency

The heat pump is indeed more efficient than the electric heating element. This is suggested by the usage graph, where the orange bars represent energy usage from the heating element:

But of course, the days we used the heating element are also days where I turned on Boost mode because we were using more hot water, which exaggerates the difference. I measured the tank temperature over time as it heated up:

• Heating element (4.5 kW): temperature rise 68.9°F/hr = 15.3°F/kWh
• Heat pump (0.45 kW): temperature rise 17.3°F/hr = 38.7°F/kWh (77°F ambient)
• Heat pump (0.45 kW): temperature rise 11.9°F/hr = 26.5°F/kWh (61°F ambient)
• Heat pump (0.45 kW): temperature rise 11.4°F/hr = 25.3°F/kWh (57°F ambient)

As expected, the heat pump’s efficiency depends on the ambient temperature, which determines the amount of heat the heat pump can “harvest” from the air. Compared with the heating element, the heat pump’s overall efficiency ranges from a factor of 1.65 to 2.53.

Operating cost

The heat pump water heater is efficient, but is it more cost-effective than our previous gas water heater? As noted above, it isn’t possible to make an exact comparison because we don’t know how our usage may have changed. If we make the worst-case assumption that our usage hasn’t changed, we are now using an average of about 5.3 kWh per day (153 kWh/month divided by 29 days). At an off-peak rate of $0.44/kWh, that works out to $2.33/day. Counting only those days where we used only the heat pump, that’s about 4 kWh/day or $1.76/day. Before getting the new water heater, our gas usage was about 8 therms/month, virtually all of which was for hot water. At our current rate of about $2.50/therm, that works out to $20/month or $0.67/day.

As our plumber had hinted when we installed the new water heater, gas is relatively inexpensive compared to electricity at current prices (unless you’re generating your own electricity). While an electric heat pump water heater is efficient and ecologically responsible, don’t expect it to save you money at current rates.