Definition
What is First Hour Rating on a water heater?
First Hour Rating (FHR) is how many gallons of hot water a tank water heater can deliver in one hour, starting fully heated. It counts both the stored water and what the heater reheats during that hour, so it is the number to compare with your household’s busiest hour, not the tank size.
Updated
The official definition
FHR comes from the U.S. Department of Energy test procedure for water heaters. The regulation defines it as:
“an estimate of the maximum volume of ‘hot’ water that a non-flow activated water heater can supply within an hour that begins with the water heater fully heated (i.e., with all thermostats satisfied).” — 10 CFR 430, Subpart B, Appendix E
In the test, water is drawn from the tank in a set pattern while the burner or element runs, and the hot water delivered over the hour is totaled. Two conditions matter for how you use the number:
- Supply water is 58°F (±2°F). If your incoming water is colder, a tank will deliver less than its rating.
- The outlet is set to deliver about 125°F (±5°F). A lower thermostat setting means less usable hot water.
Where to find it
- EnergyGuide label: the yellow label on a new tank shows the first hour rating.
- Spec sheet: search the model number from the rating plate. FHR is usually listed with capacity, input and recovery.
- Directories: the AHRI certified-product directory and the ENERGY STAR product finder list FHR by model.
How to use FHR to size a tank
Work out your peak-hour demand, the hot water your busiest hour uses, and choose a tank with an FHR at least that large. For example, three showers, the kitchen tap and a dishwasher load in one hour draw 55.9 gallons of hot water at 50°F incoming water, so look for an FHR of 56 or more. Try it in the calculator.
Why tank gallons are the wrong number
A tank never delivers all of its stored water at full temperature; cold water entering at the bottom mixes in as it drains. Meanwhile the burner or element is adding heat. How much hot water you get in an hour depends on both, and gas burners typically put in heat several times faster than an electric element. That is why a 40-gallon gas tank can have a higher FHR than a 50-gallon electric one. See real spec-sheet examples.
About “FHR = 70% of tank + recovery”: you will see this rule of thumb on many sites. It is an approximation of what the test measures, not how FHR is determined. Use the published rating.
FHR thresholds you will see
The federal test groups tank heaters into draw patterns by first hour rating, and the energy efficiency standards differ by pattern:
| First Hour Rating | Draw pattern |
|---|---|
| under 18 gal | Very small |
| 18 to under 51 gal | Low |
| 51 to under 75 gal | Medium |
| 75 gal and up | High |
ENERGY STAR also sets FHR floors in its criteria: at least 45 gallons per hour for heat pump water heaters and at least 51 gallons per hour for gas storage models.
Recovery and FHR
Recovery rate is the gallons per hour a heater can bring up to temperature while running continuously. It is one ingredient of FHR. The recovery rate calculator shows why a 40,000 BTU/hr gas burner reheats far faster than a 4,500 W element.
First Hour Rating questions
What is a good First Hour Rating?
One that is at least your peak-hour demand. There is no universal good number: a couple might need 40 gallons and a large family 80 or more. Work out your busiest hour, then compare.
Is First Hour Rating the same as tank size?
No. Tank size is how much water is stored. First Hour Rating is how much hot water the heater can deliver in an hour starting full, which includes water reheated during that hour. A fast-recovering tank can have an FHR well above its tank size; a slow one can rate below it.
How is First Hour Rating calculated?
It isn't calculated from a formula; it is measured. The federal test in 10 CFR 430 Appendix E draws hot water from a fully heated tank in a set pattern with 58°F supply water and the outlet set to deliver about 125°F, and totals what it delivers in the hour. Rules of thumb such as 70% of tank capacity plus recovery are rough approximations of that result.
Do tankless water heaters have a First Hour Rating?
No. Flow-activated heaters are rated by maximum GPM instead: the most hot water they can supply per minute in the federal test, over a nominal 67°F rise.
Sources
- 10 CFR Part 430, Subpart B, Appendix E: Uniform test method for measuring the energy consumption of water heaters, eCFR.
- Residential water heaters key product criteria, ENERGY STAR.
- Water Heating: Energy-efficient strategies for supplying hot water in the home (DOE/GO-102001-0785, 2001), U.S. Department of Energy.
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