Heat pump · hybrid
Heat pump water heater sizing
A heat pump water heater is a tank, so size it by First Hour Rating against your busiest hour, exactly like any other tank. Then consider going a size up: ENERGY STAR recommends upsizing heat pump tanks, because in efficient heat-pump-only operation they reheat slowly.
Updated
Step 1: size by First Hour Rating
Work out your peak-hour demand in the calculator and look for a model whose FHR is at least that. Our worked family of four needs about 76 gallons at 50°F incoming water; a family of five with a bath in the busy hour, 103.
| Model | Tank | UEF | FHR |
|---|---|---|---|
| Rheem PROPH50 T2 RH400-SO | 50 gal | 3.88 | 67 gal |
| A. O. Smith HPTS-50 | 50 gal | 3.80 | 65 gal |
| Bradford White RE2HP5010 | 50 gal | 3.89 | 65 gal |
| Rheem PROPH65 T2 RH400-SO | 65 gal | 4.05 | 76.6 gal |
| A. O. Smith HPTS-66 | 66 gal | 3.70 | 82 gal |
| Bradford White RE2HP6510 | 65 gal | 4.20 | 82 gal |
ENERGY STAR’s criteria require heat pump water heaters to have a First Hour Rating of at least 45 gallons per hour, and a UEF of at least 3.30 for integrated models.
Step 2: decide whether to upsize
ENERGY STAR’s design guidance for heat pump water heaters says: “For maximum efficiency, upsize the tank over the standard practice used for electric resistance or fossil fuel-fired water heaters,” and to follow the local plumbing code’s minimum first-hour rating requirements and manufacturer recommendations.
The reasoning is recovery. The compressor adds heat slowly, so the more hot water is stored, the longer the unit can stay in its efficient mode during a busy hour instead of switching on its resistance elements. Upsize when:
- you want to run in heat-pump-only mode most of the time;
- your busiest hour is close to the model’s First Hour Rating;
- your incoming water is cold, since FHR is rated at 58°F.
Other sizing factors
- Space and air: the heat pump draws heat from the surrounding air, so the manufacturer specifies a minimum room volume or ducting. Check it before choosing a larger tank.
- Electrical: many models need a 240 V circuit; some newer models plug into 120 V and have lower ENERGY STAR UEF thresholds.
- Code minimums: in UPC jurisdictions, the bathroom and bedroom table applies to heat pump tanks too.
Heat pump sizing questions
What size heat pump water heater do I need?
Size it by First Hour Rating like any tank: at least your peak-hour demand. The 50-gallon heat pump models we checked rate 65–67 gallons and the 65–66-gallon models 77–82. ENERGY STAR advises upsizing the tank compared with standard practice for resistance or fuel-fired heaters.
Do heat pump water heaters recover slowly?
In heat-pump-only mode, yes: the compressor adds heat much more slowly than a gas burner. Hybrid models also have resistance elements that switch on when demand is high, which is why their First Hour Ratings are comparable to or higher than standard electric tanks. Using an efficiency-only mode reduces how much hot water you get in a busy hour.
Should I replace a 50 gallon electric tank with a 50 or 65 gallon heat pump?
Compare First Hour Ratings with your busiest hour. The 50-gallon electric tanks we checked rate 60–63 gallons; 50-gallon heat pump models 65–67. If you plan to run in heat-pump-only mode to save energy, the larger tank gives more stored buffer, which is the reasoning behind ENERGY STAR's upsizing advice.
Sources
Water Heater Sizing Calculator for iPhone
Take the calculator to the job site
The app runs the same sizing engine as this site, offline. Basic tank and tankless sizing is free. Lifetime Pro, a one-time $4.99 purchase, adds tank vs. tankless comparison, custom fixtures, saved jobs with notes and PDF reports you can send to a customer.
