Combustion & Heating Calculators

Calculate furnace heat output in BTU/h and kW from input rating and efficiency.

Calculate heating appliance efficiency from output and input.

Calculate electric furnace current and BTU/h output from kW and voltage, single or three-phase.

Calculate furnace temperature rise from supply and return temperatures and estimate airflow from output.

Estimate natural gas use in ft³, m³ and therms plus cost from appliance input, run hours and heating value.

Calculate appliance firing rate by timing the gas meter test dial.

Educational combustion air volume check: compare room volume with 50 ft³ per 1,000 BTU/h of appliance input.

Calculate boiler thermal efficiency using the direct (input–output) method.

Flue-Gas Combustion Efficiency — coming soon to Welcome to HVAC once verified reference data is in place.