How it’s calculated
- Peak demand = Σ uses in the busiest hour × hot gallons per use
Worked example
2 showers, 1 dishwasher load, 1 hot washer load and 1 sink use in the busiest hour.
Inputs
- Showers in the busiest hour
- 2
- Hot water per shower
- 10 gal
- Dishwasher loads
- 1
- Hot water per dishwasher load
- 6 gal
- Clothes washer hot loads
- 1
- Hot water per washer load
- 7 gal
- Kitchen / bathroom sink uses
- 1
- Hot water per sink use
- 4 gal
Results
- Peak-hour demand (target First Hour Rating)
- 37 gal
Assumptions and limits
- Per-use gallons are editable typical values. A shower using 2 gpm for 8 minutes at about 60% hot water uses roughly 10 gallons of hot water.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
What is First Hour Rating?
The gallons of hot water a tank heater can supply in one hour starting full. It combines tank size and recovery rate.
Is a 50 gallon tank enough for a family of four?
Often, but check the First Hour Rating against your peak-hour demand rather than tank volume alone.