EngineerHub.orgDrawing No. EH–TH–044 // Thermal Engineering & HVAC
Water Heater / Hot-Water Sizing Calculator
Reviewed August 2026
Build a peak hot-water hour, estimate tank storage and recovery, and calculate the power required for tankless or high-flow service.
01 // Hot-water demand
Peak-hour tasks
Planning allowances used by the task counters: shower 45 L, bath 75 L, dishwasher 23 L, clothes washer 45 L, and sink/handwashing task 6 L of mixed hot water. These are editable only through the “Other mixed hot water” adjustment and should be replaced by measured or fixture-specific demand for design work.
02 // Sizing result
Background & method
Residential water-heater sizing should consider peak demand, storage and recovery together. ENERGY STAR defines FHR as the amount a storage heater can supply in the first hour, while ASHRAE notes that shorter peak draws can be governed strongly by tank volume.
FAQ
A manufacturer first-hour rating (FHR) is a tested measure of how much hot water a storage heater can deliver in the first hour starting fully heated. This calculator shows an equivalent mixed-water first-hour delivery for comparison; it is not a substitute for the manufacturer’s rated FHR.
A colder inlet increases the temperature rise and therefore the energy or burner/electric power needed for the same hot-water flow.
Many residential peak draws occur over 10–20 minutes. During a short peak, there is less time for recovery, so tank storage can govern delivery.
Thermal power is based on mass flow × water heat capacity × temperature rise, divided by the selected efficiency/COP factor.