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As we look to replace fossil fuel water heaters with greener options, delivering enough hot water is an important concern. Heat pump water heaters (HPWHs), the leading option for electric water heating, are ultra-efficient. But the heat pump mechanism, which extracts heat from the ambient air and transfers it to the tank, is slow. As a result, HPWHs need larger tanks than gas water heaters to supply a comparable amount of hot water.
As with heat pumps and air conditioners, right-sizing water heaters is important. Larger units come with significantly higher costs. They’re also taller and wider, which can cause problems in low basements and small mechanical rooms. Here, I’ll explain three measures of HPWH size—volume, recovery, and first hour rating (FHR)—and then discuss how to combine them with data on occupants and fixtures to choose a HPWH that meets your needs.
Where to find water heater data
The first place I look for data on HPWHs is usually a manufacturer’s product data sheet like this one from State. The data sheet includes efficiency and output ratings, electrical requirements, and other key product features. It also includes a drawing showing overall dimensions and locations of piping and electrical connections. One data sheet typically covers multiple sizes within a product line.
Another useful resource is the Energy Star HPWH Product Finder, a searchable database of over 300 models that can be filtered by voltage (120V vs. 240V), brand, rated volume, and FHR. The database also provides tank height and diameter and allows sorting by Uniform Energy Factor (UEF), a measure of efficiency.
The U.S. models listed are eligible for the 30% Federal Tax Credit, up to a maximum credit of $2000 (users can also filter for Canadian models). In addition, the site also shows rebates available from…
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6 Comments
Great article. The tricky part might be estimating shower lengths. I know mine vary, and are probably shorter when I have a stopwatch running.
Side question. Any news about low GWP refrigerants in HPWHs?
Thanks, Charlie. Regarding low GWP refrigerants, most of the current integrated HPWH are using R-134a, an HFC (GWP=1430). Apparently, substitution is hard for this application because of the high condensing temperatures needed. There is the SanCO2 split HPWH, which uses CO2 (GWP=1) but has limited application in single family homes due to high cost and freeze protection requirements. I recently stumbled across an AO Smith 50-gallon plug-in that model that uses R-513A, an HFO/HFC blend (GWP=573), so there are some changes on the horizon. I am going to add this to my list of possible topics for future articles.
Interesting to hear about the R-513A model. I'm also watching for dehumidifers with lower GWP refrigerants--there are a few R-32 models that have come out but I think they are all supposed to switch by January. That doesn't have the same high-temperature challenge.
Anyway, that would be a great topic for a future article!
Do HPWH's of different volumes from a given manufacturer use the same capacity heat pumps or do the larger capacity ones also have higher capacity heat pumps. Are there differences among manufacturers?
My impression is that, within a model line, the 50, 66, and 80-gallon models generally have the same heat pumps and just differ in tank volume and geometry.
Rheem lists heat pump Btu/hr separate from recovery and FHR in their spec sheets. The 240V models I looked at all had 4200 Btu/hr heat pump capacity. The same is true of their 120V shared-circuit models. The 120V dedicated-circuit model has a larger, louder compressor (12,000 Btu/hr).
If the manufacturer doesn't list compressor output, you may be able to estimate it based on the recovery rating and wattage of the resistance elements.
LG has an inverter HPWH with a "turbo mode" that can speed up the compressor during periods of high demand. I wasn't able to get detailed performance specs on this.
There's another option for increasing capacity, if you have enough horizontal installation space and electric panel space: buy a cheap electric resistance water heater and plumb it in series, downstream of the HPWH. I found this to be cheaper than buying an 80 gallon HPWH (which wouldn't fit in my basement anyway) and gives me additional backup if the HPWH ever fails. There was an article about this option here, but I can't find it now. Anyway, I spent months deciding what storage volume I'd need for my triplex (4000 sq ft., 5 bedrooms, 3.5 bathrooms, 2 washing machines) and eventually landed on a 50 gallon HPWH set to 130F with another 50 gallon ERWH tank in series, set to 125 F, with a temperature mixing valve at the end decreasing the output down to 115 F or so . This configuration has worked out great for me so far. The HPWH does the vast majority of the water heating. The ERWH goes days without running at all, the longest it's ever run so far was about 20 minutes, and so far it's used only about 7 kWh/month. Based on the ERWH runtimes, I can tell that we've never run out of hot water. I was coming from a very inefficient tankless coil setup with a vastly oversized oil boiler, so my savings in oil have been significant ($100+ per month) already, even with $0.20/kWh electric rates and falling oil prices.
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