Water Heater Sizing Calculator
Size tankless and storage tank water heaters by calculating peak simultaneous flow rates (GPM) and groundwater temperature rise (ΔT).
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How Water Heater Capacity is Sized
Water heaters are sized based on two factors: peak simultaneous flow rate (gallons per minute, GPM) and temperature rise (ΔT).
Temperature Rise (ΔT)
ΔT = Target Temp (120°F) - Ground Temp
BTU Heat Requirement
BTU/hr = GPM × 500 × ΔT / Thermal Efficiency
First Hour Rating (FHR) for Storage Tanks
Storage tanks are evaluated using the First Hour Rating (FHR), which is the total volume of hot water a fully heated tank can deliver in 60 minutes. It accounts for stored volume plus 1 hour of recovery heating.
Frequently Asked Questions
Why do tankless water heaters deliver less hot water in winter?
Cold winter groundwater (e.g. 40°F) requires an 80°F temperature rise to reach 120°F, compared to only 50°F rise in summer. Because the gas burner output has a fixed BTU limit, the unit restricts water flow to guarantee the output temperature.
What size gas line does a whole-home tankless water heater need?
Most 199,000 BTU whole-home tankless units require a dedicated 3/4-inch gas supply line. Older 1/2-inch lines typically cannot supply enough gas volume, causing flame failure under peak demand.
How many gallons for a family of 4 with a storage tank?
A family of 4 typically requires a 50-gallon gas tank or a 65-to-80-gallon electric tank (with a First Hour Rating between 60 and 70 gallons) to accommodate consecutive morning showers.