Size a water heater and storage tank from the fixtures in a building, with the ASHRAE per-fixture method: peak hourly demand, recovery rate, heater power in kW and Btu/h, and tank volume.
| Building | Demand factor | Storage capacity factor |
|---|---|---|
| Apartment house | 0.30 | 1.25 |
| Hotel | 0.25 | 0.80 |
| Hospital | 0.25 | 0.60 |
| Office building | 0.30 | 2.00 |
| School | 0.40 | 1.00 |
| Gymnasium | 0.40 | 1.00 |
| Private residence | 0.30 | 0.70 |
| Industrial plant | 0.40 | 1.00 |
A large tank lets a smaller heater cover a short peak, which suits offices and schools where most hot water is used in one short period (hence the high storage factor). Hotels, hospitals and apartments use hot water more evenly through the day, so they rely more on recovery. The trade-off can be shifted: more storage with less recovery, or the reverse, as long as the first-hour delivery meets the peak.
Store hot water at 60 °C (140 °F) or above to control Legionella, and use thermostatic mixing valves to deliver it at a safe temperature, usually about 43–49 °C (110–120 °F) at fixtures. For peak hot water flow to size pipes, use the hot WSFU in the Fixture Unit Calculator; for the expansion vessel use the Expansion Tank Calculator.
Multiply the number of each fixture by its hourly hot water demand from the ASHRAE table, add them up, then multiply by the building's demand factor. The result is the peak hourly demand the heater must recover.
Multiply the probable maximum hourly demand by the storage capacity factor for the building type, then divide by the usable fraction of the tank, typically 0.7.
Heating 100 litres per hour from 10 °C to 60 °C takes 100 × 4.186 × 50 ÷ 3,600 = 5.8 kW, divided by the heater efficiency.
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