Furnace BTU Calculator

Inputs

Heated floor area (A)

Conditioned floor area served by the furnace or zone being screened.

m²

Heating load factor (q/A)

Rough BTU/h per square foot at a 70°F design temperature difference. Lower for tight/efficient buildings; higher for cold or leaky buildings.

Btu/h·ft² @70°F

Design temperature difference (ΔT)

Indoor design temperature minus outdoor winter design temperature. Use the absolute difference.

°C

Furnace AFUE (η)

Annual Fuel Utilization Efficiency. Output capacity = input capacity × AFUE.

Sizing margin (M)

Capacity margin applied after the heat-loss estimate. Keep modest unless a real load calculation supports it.

Supply-air temperature rise (ΔT_air)

Expected furnace air temperature rise used to estimate required supply airflow.

°C

Default result

Design heat loss(q_loss)

18.46kW

Pass

typical rough screening range for many homes

Also computed

Required output capacity(q_out)20.31kW

heat-loss estimate plus sizing margin

Furnace input rating(q_in)21.38kW

nominal input capacity before selecting the nearest equipment size

Supply airflow(Q)2,180m³/h

from output BTU/h and selected supply-air temperature rise

Specific heat loss(q/A)35Btu/h·ft²

normalized to the entered floor area and design ΔT

Method notes 3 notes

This is an area-based screening estimate, not a room-by-room heat-loss calculation.
Formula: heat loss = floor area × load factor × design ΔT / 70°F. Output capacity then applies your margin; input BTU/h divides by AFUE.
Final furnace sizing should follow ACCA Manual J / Manual S or the applicable local method, equipment data, duct airflow limits, combustion air, venting and local code.