The First Hour Rating (FHR) measures how much hot water a storage water heater can deliver in the first hour of use, starting with a full tank. It combines tank capacity and recovery rate, making it a key sizing metric for household peak demand. Tankless units do not use FHR; they are rated by flow rate at a specific temperature rise.
/fɜrst aʊər ˈreɪtɪŋ (FHR)/
The First Hour Rating (FHR) measures how much hot water a storage water heater can deliver in the first hour of use, starting with a full tank. It combines tank capacity and recovery rate, making it a key sizing metric for household peak demand. Tankless units do not use FHR; they are rated by flow rate at a specific temperature rise.
What does First Hour Rating tell you?
It tells you how many gallons of hot water a tank water heater can supply in the first hour of use.
Source: The Cooling Company
FHR combines tank size and recovery speed.
A family of four chooses a water heater with an FHR that supports back-to-back showers and morning laundry without running out of hot water.
Estimate your peak hour hot water demand and choose an FHR that meets or slightly exceeds it.
“Estimate your peak hour hot water demand and choose an FHR that meets or slightly exceeds it.”
FHR provides a more practical sizing guide than tank size alone because it includes recovery rate.
How do I estimate my peak hour demand?
Add up hot water use for showers, laundry, and dishes during your busiest hour.
Is a higher FHR always better?
Only if it matches your demand. Oversizing can increase standby losses.
Do heat pump water heaters have an FHR?
Yes. Storage-type heat pump models list FHR values on the label.
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