Water Heater Sizing Calculator
Size tankless and storage tank water heaters by calculating peak simultaneous flow rates (GPM) and groundwater temperature rise (ΔT).
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.