Size a boiler or furnace from heat loss and appliance efficiency.
Output must cover heat loss and hot water, and input is that divided by efficiency. Condensing appliances can exceed 100 per cent on lower heating value, which is why efficiency figures above 100 appear in specifications. Confusing input and output ratings is the classic sizing error and typically results in an appliance around 10 per cent undersized.
Boiler/Furnace Sizing
Input rating = (heat loss + hot water) × (1 + margin) ÷ efficiency
Input rating = (heat loss + hot water) × (1 + margin) ÷ efficiency Output must cover heat loss and hot water, and input is that divided by efficiency. Condensing appliances can exceed 100 per cent on lower heating value, which is why efficiency figures above 100 appear in specifications.
Confusing input and output ratings is the classic sizing error and typically results in an appliance around 10 per cent undersized.
This calculator takes 5 inputs: Design heat loss, Appliance efficiency, Hot water allowance, Sizing margin, Fuel price per kWh. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.