Furnace BTU Calculator

Inputs

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Conditioned floor area served by the furnace or zone being screened.
ft²
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
Indoor design temperature minus outdoor winter design temperature. Use the absolute difference.
°F
Annual Fuel Utilization Efficiency. Output capacity = input capacity × AFUE.
%
Capacity margin applied after the heat-loss estimate. Keep modest unless a real load calculation supports it.
%
Expected furnace air temperature rise used to estimate required supply airflow.
°F

Results

Default result
Edit inputs
Design heat loss (q_loss)
63,000 Btu/h
Pass

typical rough screening range for many homes

Also computed

Required output capacity (q_out) 69,300 Btu/h

heat-loss estimate plus sizing margin

Furnace input rating (q_in) Pass 72,950 Btu/h

nominal input capacity before selecting the nearest equipment size

Supply airflow (Q) 1,283 cfm

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

Specific heat loss (q/A) 35 Btu/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.

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