First Hour Rating (FHR) Explained for Tank Water Heaters
The First Hour Rating (FHR) is a standardized performance metric applied to storage tank water heaters that quantifies how much hot water a unit can deliver in the first hour of operation starting from a full tank. Regulated under U.S. Department of Energy appliance standards and required on the federal EnergyGuide label, FHR is the primary sizing variable consumers and licensed plumbers use to match tank capacity to household demand. Understanding how this metric is calculated, what affects it, and where its limits apply is essential for correct equipment selection and code-compliant installation. The Water Heating Listings index references FHR data across listed equipment categories.
Definition and Scope
First Hour Rating is defined by the U.S. Department of Energy (DOE) under test procedures codified in 10 CFR Part 430, Subpart B, Appendix E for storage water heaters. The test establishes a standardized draw pattern designed to simulate peak morning household demand. FHR appears on the federally mandated EnergyGuide label alongside the unit's Uniform Energy Factor (UEF), making it a regulatory disclosure requirement — not an optional marketing figure.
FHR applies exclusively to storage tank water heaters. It does not apply to:
- Tankless (instantaneous) water heaters — rated by gallons-per-minute (GPM) flow rate at a specified temperature rise.
- Heat pump (hybrid) water heaters — carry an FHR value when they include a storage tank, but their standby heat-loss profile differs from conventional units.
- Solar water heaters with storage tanks — auxiliary backup elements may have an FHR, but solar contribution is measured separately.
The DOE test procedure uses a standardized inlet water temperature of 58°F and a delivery temperature of 125°F. Any FHR published by a manufacturer must be derived from this controlled test protocol, not from proprietary laboratory conditions.
How It Works
FHR is not simply tank storage capacity. The rating combines two components: the usable stored hot water volume at the start of the draw period, plus additional water heated by the burner or element during that hour as the tank recovers.
The standardized test procedure follows this structure:
- Tank preheating — The unit is brought to thermostat setpoint (typically 120–125°F) before testing begins.
- Initial draw — 64% of the rated tank volume is drawn off at the regulated flow rate.
- Recovery phase — The heating system activates; the test continues drawing water at intervals calibrated to the DOE draw profile.
- One-hour cutoff — The total volume of water delivered at or above 110°F (the DOE threshold for "hot") is tallied.
- FHR reported — The total gallons delivered meeting the temperature threshold is the unit's First Hour Rating.
A 50-gallon gas tank water heater typically achieves an FHR of 70–90 gallons per hour, because gas burners recover quickly enough to supplement the stored volume within the test window. A 50-gallon electric unit — limited by lower wattage and slower recovery — typically returns an FHR of 55–70 gallons per hour. This distinction makes FHR a more operationally meaningful metric than storage capacity alone for professional sizing purposes.
The recovery rate — measured in gallons per hour at a 90°F temperature rise — is the primary variable separating a high-FHR unit from a low-FHR unit of identical storage volume. Fuel type, burner BTU input (for gas), and element wattage (for electric) drive recovery rate directly.
Common Scenarios
FHR becomes a practical sizing tool when matched against household peak-demand profiles. The DOE's Water Heater Sizing guidance recommends estimating peak-hour demand based on simultaneous fixture use, then selecting a unit whose FHR meets or exceeds that demand figure.
Typical residential demand benchmarks (based on DOE fixture estimates):
| Activity | Approximate Gallons Used |
|---|---|
| Shower (8–10 min) | 10–12 gallons |
| Showerhead at 2.5 GPM (10 min) | 25 gallons |
| Dishwasher cycle | 6–10 gallons |
| Clothes washer (warm/cold) | 7–10 gallons |
| Handwashing (single use) | 1–2 gallons |
A household where two showers, one dishwasher cycle, and two handwashing events occur simultaneously in the morning peak hour requires an FHR of approximately 55–65 gallons minimum. A licensed plumber sizing for that load would select a unit with an FHR at or above that threshold — not simply a 40- or 50-gallon tank without checking the FHR label.
Permit and inspection requirements for water heater replacement — administered through local building or plumbing departments under adoptions of the International Plumbing Code (IPC) or Uniform Plumbing Code (UPC) — do not mandate a specific FHR. However, inspectors may flag an undersized replacement unit as a code deviation if the installation departs significantly from original equipment specifications on record. The Water Heating Directory Purpose and Scope page identifies the jurisdictional frameworks under which this equipment is regulated.
Decision Boundaries
FHR functions as a necessary but not sufficient sizing criterion. Three boundaries define where FHR alone is insufficient for equipment selection:
1. FHR vs. continuous demand
FHR measures peak delivery within one hour. A household with sustained high demand across multiple hours — commercial-adjacent residential use, short-term rental properties, or large multi-bathroom homes — requires evaluation of recovery rate as a continuous metric, not just the one-hour FHR figure.
2. Gas vs. electric FHR comparisons
Gas units consistently achieve higher FHR values per gallon of storage than electric units of equivalent tank size. A 40-gallon natural gas unit may carry an FHR of 67–72 gallons, while a 40-gallon standard electric unit may rate at 50–58 gallons. Comparing FHR figures across fuel types without accounting for this structural difference leads to undersizing when switching from gas to electric.
3. FHR and UEF as paired metrics
A high FHR does not indicate high efficiency. A unit with a large burner input will recover faster — raising its FHR — but may lose more standby heat through the flue, lowering its Uniform Energy Factor (UEF). The DOE's EnergyGuide label displays both figures side by side for this reason. The How to Use This Water Heating Resource page describes how efficiency ratings and performance metrics are applied within this reference network.
Safety standards applicable to storage tank water heaters — including pressure relief valve requirements and temperature controls — are governed by ANSI Z21.10.1 (gas storage water heaters) and UL 174 (electric storage water heaters), administered through IAPMO and ANSI-accredited certification bodies. FHR test procedures feed into certification testing but do not themselves constitute a safety standard.
References
- U.S. Department of Energy — Water Heating
- 10 CFR Part 430 — Energy Conservation Program for Consumer Products (eCFR)
- ANSI Z21.10.1 — Gas Water Heaters, Volume I (ANSI)
- International Association of Plumbing and Mechanical Officials (IAPMO)
- Consortium for Energy Efficiency (CEE) — Residential Water Heating Specification
- International Plumbing Code (IPC) — International Code Council