Hot Water System Sizing — Heater Capacity and Recirculation

Hot water sizing balances two quantities that trade against each other — stored volume and heating power. More of one lets you use less of the other.

Estimate hot water demand

Work from fixture hot-water flow rates and a usage pattern, or from a per-person daily figure (typically 40–60 L/person/day at 60 °C for residential). Identify the peak hour demand — morning in homes, meal times in food service.

The energy balance

where ṁ is the hot-water mass flow (kg/h ≈ L/h), c_p = 4.19 kJ/kg·K, and ΔT is the rise from cold inlet to storage temperature (e.g. 25 °C → 60 °C = 35 K).

Storage vs continuous-flow

• Storage heater: a tank rides through the peak while a modest element or burner reheats it. Size = usable storage covering the peak hour minus what the heater recovers during that hour.
• Continuous-flow (instantaneous): no tank; the heater must deliver the full peak flow × ΔT instantly, which is a large power draw. Best for low or intermittent demand.

Recirculation loop

On runs where the distance from heater to the farthest fixture exceeds the code’s dead-leg limit (often ~7.5 m of pipe, or 1 L of stored water), a recirculation loop keeps hot water at the fixture. Size the return pump for the loop’s heat loss:

Insulate both flow and return; add a check valve, balancing valve, and a timer or aquastat so the pump is not running 24/7.