Approach: Area-based heat-loss estimate adjusted for climate, insulation, and ceiling height. Domestic hot water (DHW) needs are included for combi boilers using simple tiers (1 bath ≈ 21 kW, 2 ≈ 28 kW, 3+ ≈ 37 kW). System/heat-only adds ~2 kW to recover a cylinder. The recommended size rounds up to the next standard model.

Metric path: Baseline demand (Good 60 / Avg 70 / Poor 100 W/m²), multiplied by climate (Warm 0.8 / Temperate 1.0 / Cold 1.3 / Very Cold 1.5) and a height factor vs 2.4 m (clamped 0.6–1.6). Space heat (kW) = Area × Effective W/m² / 1000.

Imperial path: Base BTU/ft² (Warm 30 / Temperate 40 / Cold 50 / Very Cold 60) × insulation factor (Good 0.85 / Avg 1.0 / Poor 1.2) × height factor vs 8 ft (clamped 0.6–1.6).

Selection: Required output = max(space heat, DHW). Never choose smaller; select the nearest larger standard size. Oversizing well beyond this harms efficiency and comfort.

Units: 1 kW = 3,412 BTU/hr. Outputs shown are heat output. If comparing to input-rated appliances, allow for efficiency (e.g., 90% input ≈ output ÷ 0.9).