Boiler Size Calculator (Preliminary)

How is boiler capacity decided?

A boiler has two jobs: heating the building and producing domestic hot water. Capacity is chosen to suit both.

Heating power equals the heat the building loses on the coldest day. The right figure comes from a heat loss calculation. Without one, a rough value is area times a unit load: Q = A × q. An allowance for distribution loss and reheat is added on top.

Instantaneous hot water power is what it takes to bring flowing water up to temperature: Q_hw = ρ × c × V × Δt. In practical form, kW equals flow (L/min) times temperature rise divided by 14.33.

Why does hot water govern a combi?

Heating 10 litres per minute from 10 °C to 40 °C takes 10 × 30 / 14.33 = 20.9 kW. The heating load of a 120 m² flat is usually below that. Because a combi stops heating while hot water is drawn, the two are not added, the larger is taken. That is why combis of 24 kW and above are common in homes.

Example

A flat with a heat loss of 12 kW needs 13.8 kW of heating power with a 15 percent allowance. Ten litres per minute of hot water needs 20.9 kW. For a combi with hot water priority the required capacity is 20.9 kW, and the next standard size up is chosen.

About load per unit area

Heat load per square metre varies severalfold with climate, insulation, glazing ratio and the position of the flat (mid floor, top floor, corner). The tool therefore suggests no value. Use the figure from the room heating load tool or from a heat loss calculation.

Limits

This is a preliminary estimate. Storage systems, cascade boilers, commercial buildings and process heat need their own calculations. Final selection uses a heat loss calculation, a hot water demand calculation and manufacturer data.