Hot Water Demand: engineer's guide
Hot water demand is estimated by occupancy type and peak-hour multiplier. Storage capacity and recovery rate together must meet the peak plus continuous demand.
What this calculator does
The calculator applies ASHRAE hourly demand profiles for the selected building type and returns storage volume plus recovery kW.
Formulas used
- Daily demand:
Vday = Npersons × L/person/day - Storage:
Vstorage = Vpeak-hour × 0.7 (roughly) - Recovery power:
Pheat = m × cp × ΔT / trecovery
Variables
- cp = 4.186 kJ/kg°C for water
- ΔT = hot − cold supply temperature
- trecovery = recovery time in seconds
Worked example
Scenario: 100-key hotel, 200 L/day/room hot water.
- Daily total: 100 × 200 = 20,000 L/day
- Peak hour: 15% of daily ≈ 3000 L in one hour
- Storage: 0.7 × 3000 ≈ 2100 L usable
- Recovery for remaining 900 L in the hour at 45°C rise:
- P = 900 × 4.186 × 45 / 3600 ≈ 47 kW → select 50 kW electric or 60 kW gas boiler
Common mistakes to avoid
- Sizing storage for daily total (huge over-size)
- Missing recirculation pipe standing loss
- Ignoring cold-inlet seasonal variation
- Under-sizing recovery for sequential peak (gym after breakfast)
Frequently asked questions
Legionella temperature?
Store at ≥ 60°C, distribute ≥ 55°C to control Legionella; provide thermostatic mixing at outlets for scald prevention.
Heat pump water heaters?
Higher efficiency (COP 3+) but longer recovery — oversize storage accordingly.
Standards and references
ASHRAE Applications Ch. 50, BS EN 806
Related tools on this site
Open the calculator that pairs with this guide: Hot Water Demand.
Verification note
This guide is planning support only. Final design must be verified against the applicable project specification, authority requirements, manufacturer data, and the standards cited above. Consult a licensed engineer for construction-issue calculations.