Field calculator
Water heating BTU calculator (GPM x 500 x rise)
Sizing a water heater, a booster, or a recirculation load comes down to the heat needed to raise a flow of water a certain number of degrees. The rate in BTU per hour equals the flow in gallons per minute times 500 times the temperature rise in degrees Fahrenheit. Enter the flow and the rise from incoming cold to delivered hot. The 500 constant is specific to water: 8.33 pounds per gallon, times 60 minutes per hour, times 1 BTU per pound per degree. The result is the energy to heat a continuous flow, which is exactly how you size an instantaneous or tankless heater, an electric booster (shown here in kW), or a recirculation system. A storage tank heater is different: it can briefly deliver more hot water than its burner or element can heat by drawing the tank down, so a tank type is sized to the peak demand and the recovery rate together, not to the instantaneous rate alone. Use the real incoming water temperature for the season, and confirm the selection against the manufacturer and the fixture load.
Result
Water heating rate: BTU/hr = GPM × 500 × the temperature rise (delta-T in degrees F). Enter the flow and the rise from incoming to delivered temperature. The 500 constant is for water (8.33 pounds per gallon times 60 minutes per hour times 1 BTU per pound per degree). This is the energy needed to heat a continuous flow, useful for sizing an instantaneous or tankless heater, a booster, or a recirculation load. A storage water heater can briefly deliver more than its input by drawing down the tank, so size a tank type to the peak demand and the recovery rate together, and confirm against the manufacturer and the fixture load.
anvilfield.com/calculators/water-heating-btu-rate-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Water heating FAQ
How do you size a water heater?
Find the building's peak-hour hot water demand in gallons per hour, then match recovery plus usable storage to it on the coldest-month inlet rise. Apply a diversity factor for the occupancy. Storage covers the spike; recovery refills it. The stamped design, manufacturer tables, and adopted code control the final selection.
What is recovery rate on a water heater?
Recovery rate is the gallons per hour a heater raises from the cold inlet to the setpoint. It equals the input in BTU per hour times efficiency, divided by 8.33 and the temperature rise. Recovery drops as the rise grows, so size it on the cold winter inlet, not the summer one.
Tank or tankless water heater for a commercial building?
A storage tank rides through spiky peaks on stored gallons and is forgiving; tankless makes hot water on demand with no standby loss but must be sized by flow at the cold-inlet rise and goes lukewarm if undersized. Semi-instantaneous or a staged bank splits the difference. The demand curve decides.
What temperature should a water heater be set to?
Set storage at 140 degrees F to keep the tank out of the Legionella growth band, and temper delivery to 120 degrees F or below with a thermostatic mixing valve so it does not scald. At 140 degrees a burn happens in about five seconds. Confirm the delivery limit against the adopted code.
What is first-hour rating?
First-hour rating is the gallons of hot water a storage heater delivers in one hour from a full tank, shown on the residential EnergyGuide label. It equals the storage times about 0.7 plus the recovery in gallons per hour. Match it to your peak-hour demand instead of buying on tank size alone.