Water Heater First-Hour Rating
First-hour rating, or FHR, estimates how much hot water a storage heater can deliver during an hour that starts with a fully heated tank. It combines the contribution of stored water and recovery during the test. It is not tank volume, continuous hourly output, or a guaranteed number of showers.
Use FHR to compare candidate storage heaters with your busiest-hour hot-water demand. Also check what happens after that hour and how the proposed operating mode affects recovery.
How the number differs from gallons
| Measure | What it answers |
|---|---|
| Rated storage volume | How much water does the tank hold? |
| First-hour rating | What is tested delivery from a fully heated starting condition? |
| Recovery information | How does the appliance rebuild hot-water capacity? |
| Tankless GPM at rise | What flow can instantaneous equipment heat? |
ENERGY STAR's definition states that FHR is measured under the UEF test method, with a 125°F outlet temperature. That standardized measurement is different from every household's fixture temperatures and operating pattern. ENERGY STAR first-hour definition.
A.O. Smith's 40-gallon direct-vent gas model lists an 84-gallon first-hour delivery. The example demonstrates why nominal tank gallons and FHR cannot be treated as interchangeable. It does not establish that every 40-gallon tank performs that way. A.O. Smith model specifications.
Compare it with a realistic hour
List the household's hot draws during the busiest hour. Account for actual fixture flow, duration and cold-water mixing. For an illustrative schedule, hot-side draws of 18 gallons, 20 gallons and 10 gallons total 48 gallons. These are assumed heater-side quantities, not generic allowances for showers or appliances.
Then compare specific FHR values with that demand and discuss a margin for uncertainty with the installer. If the estimate is near a candidate's limit, refine the inputs rather than declaring the model suitable from a rough calculation.
Our water heater gallons guide explains mixing, and what size water heater explains how to build the schedule. The size calculator provides a planning starting point.
Understand the limitations
FHR begins with a fully heated tank. It does not prove that the same quantity can be delivered every following hour, or that a large short draw can be satisfied at any point within that hour. Draw timing and recovery matter.
For heat-pump water heaters, ask which operating conditions and heating mode support the published rating. Do not assume a rating that uses backup elements describes compressor-only recovery in a cold room. Read heat-pump efficiency alongside delivery data.
Use FHR before price comparisons
Have qualified installers verify the demand profile, exact model and installation constraints. Compare installed cost among models that meet the same duty, check exact-model rebates, and use best water heaters criteria to assess efficiency, warranty and local service after capacity is established.