What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- 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.
What happens, step by step
- The clock runs through your busiest hour; each use adds to the bar.
- Dishwasher: 1 × 6 gal = 6 gal of the hour’s total.
- Clothes washer: 1 × 7 gal = 7 gal of the hour’s total.
- Showers: 2 × 10 gal = 20 gal of the hour’s total.
- 37 gal in one hour: choose a First Hour Rating at least that high.
The answer: peak-hour demand (target first hour rating) is 37 gal.
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.