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FHR = Sum Fixture Demands
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The First Hour Rating (FHR) is a measure of how much hot water a water heater can supply during a busy hour. It's calculated by summing the hot water demands of all fixtures that might be used simultaneously during peak usage.
The calculator uses the FHR equation:
Where:
Explanation: The equation calculates the total hot water demand by adding up the individual demands of all fixtures that might be used during the peak hour.
Details: Proper water heater sizing is crucial for ensuring adequate hot water supply during peak usage times while maximizing energy efficiency. An undersized water heater will run out of hot water quickly, while an oversized unit will waste energy and increase operating costs.
Tips: Enter the hot water demand for each fixture in gallons per hour (gal/hr). Common fixture demands: shower (2.0-2.5 gal/hr), dishwasher (1.0-1.5 gal/hr), washing machine (1.5-2.0 gal/hr), sink (0.75-1.5 gal/hr).
Q1: What is a good FHR for a typical household?
A: For a typical 2-3 bedroom home with 2-3 bathrooms, an FHR of 50-70 gallons per hour is usually sufficient.
Q2: How does FHR differ from tank capacity?
A: Tank capacity is the total amount of hot water stored, while FHR measures how much hot water the system can deliver in the first hour of use, taking into account recovery rate.
Q3: Should I consider peak usage times when calculating FHR?
A: Yes, FHR should be calculated based on the maximum expected simultaneous usage during the busiest hour of the day (typically mornings or evenings).
Q4: How does water temperature affect FHR calculations?
A: The calculations assume standard temperature settings (typically 120°F). If you use higher temperatures, you may need to adjust the fixture demand values.
Q5: Are there other factors to consider beyond FHR?
A: Yes, you should also consider energy efficiency, fuel type, space constraints, and local climate when selecting a water heater.