What Is the Water Heater First Hour Rating (FHR) Calculator?
First Hour Rating (FHR) is the standardized performance rating reflecting the number of gallons of hot water a residential water heater can deliver in an hour.
How Does the Water Heater First Hour Rating (FHR) Calculator Work?
The initial usable draw is modeled at 70% of tank volume before cold incoming supply water stratifies and cools the delivery stream.
Hourly recovery GPH is calculated by dividing thermal heat input by the energy needed to raise one gallon of water by ΔT degrees.
Water Heater First Hour Rating (FHR) Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Accounts for 70% usable initial draw before temperature drops plus continuous hot water recovery generated during the 60-minute window.
How to Use the Water Heater First Hour Rating (FHR) Calculator
- Enter tank capacity in gallons.
- Select fuel source and enter burner BTU or element wattage.
- Enter incoming cold water temperature and desired hot setpoint.
Step-by-Step Example Calculation
50-Gallon 40,000 BTU Gas Heater with 70°F Rise
Input Values:
Understanding Your Result
Higher FHR accommodates back-to-back morning showers, dishwasher runs, and laundry cycles.
Factors That Affect the Result
- Winter cold water supply temperatures severely reduce recovery speed compared to summer.
When Should You Use This Calculator?
- Selecting a new water heater to meet peak hour family hot water demand.
Assumptions & Limitations
- Assumes steady-state thermal transfer with 80% efficiency for atmospheric gas and 98% for electric resistance.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Conforms to US Department of Energy 10 CFR Part 430 test procedures.