Work out heating load instantly with clear inputs, formula shown and shareable results.
Design heat loss splits into two independent paths: conduction through the building fabric, given by U x A x ΔT, and the energy needed to warm outdoor air entering by ventilation or infiltration, given by 1.08 x CFM x ΔT. Adding them yields the boiler or furnace output the building needs on the coldest design day.
Heat loss
Q = U x A x ΔT + 1.08 x CFM x ΔT
Kilowatt conversion
kW = BTU/h / 3412.142
Preliminary estimate only. Verify against a full room-by-room heat loss calculation and local energy code before sizing equipment.
Q = U x A x ΔT + 1.08 x CFM x ΔT. Design heat loss splits into two independent paths: conduction through the building fabric, given by U x A x ΔT, and the energy needed to warm outdoor air entering by ventilation or infiltration, given by 1.08 x CFM x ΔT.
It is the product of air density, specific heat and 60 minutes per hour at sea level standard conditions, so 1.08 x CFM x ΔT gives sensible BTU/h directly.
This calculator takes 4 inputs: Envelope area (walls, roof, glazing), Average U-value, Design temperature difference, Ventilation / infiltration airflow. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.