Boiler/Furnace Sizing Calculator
Size a boiler or furnace from heat loss and appliance efficiency.
About Boiler/Furnace Sizing Calculator
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.
Formula
Boiler/Furnace Sizing
Input rating = (heat loss + hot water) × (1 + margin) ÷ efficiency
- AFUE
- Annual fuel utilisation efficiency, the seasonal output over input
- input versus output
- Appliances are often marketed on input but sized on output
Worked example
With the default values below, Boiler/Furnace Sizing Calculator returns Input Rating Required of 14.348. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Design heat lossminimum 0.1 kW
- 9 kW
- Appliance efficiencyaccepts 30–110 %
- 92 %
- Hot water allowanceminimum 0 kW
- 3 kW
- Sizing marginaccepts 0–60 %
- 10 %
- Fuel price per kWhminimum 0
- 0.07
Results
- Input Rating Required
- 14.348 kW
- Output Rating Required
- 13.200 kW
- Input Rating
- 48,957 BTU/h
- Fuel Cost at Full Load
- $1.004
- Indicative Annual Fuel Cost
- $958.70
Step by step
- 1Values used
- 2Boiler/Furnace Sizing
- 3Input Rating Required= 14.348 kW
- 4Output Rating Required= 13.200 kW
- 5Input Rating= 48,957 BTU/h
- 6Fuel Cost at Full Load= 1.004
- 7Indicative Annual Fuel Cost= 958.70
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Boiler/Furnace Sizing calculated?
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.
Why does Boiler/Furnace Sizing matter?
Confusing input and output ratings is the classic sizing error and typically results in an appliance around 10 per cent undersized.
What values do I need to enter?
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.