Furnace Sizing Calculator for Montana Winters
Determine the minimum furnace output (BTU/h) needed to keep your Montana home comfortable during extreme cold snaps. Enter your home details below.
Formulas Used
1. Base Envelope Heat Loss
Qbase = HTF × (Floor Area × Ceiling Height) × (ΔT ÷ 65)
Where HTF (Heat Transfer Factor, BTU/h·ft³) = 4–8 depending on insulation quality, and 65 °F is the Manual J normalisation delta.
2. Window Heat Loss
Qwindow = Window Area (ft²) × U-value (0.30 BTU/h·ft²·°F) × ΔT
Window Area = Floor Area × Window %
3. Infiltration Heat Loss
Qinf = 0.018 × ACH × Volume (ft³) × ΔT
0.018 = specific heat of air (BTU/ft³·°F); ACH ranges from 0.25 (excellent) to 0.80 (poor).
4. Gross Furnace Output
Qgross = (Qbase + Qwindow + Qinf) ÷ (AFUE ÷ 100)
5. Recommended Size (with safety buffer)
Qrecommended = Qgross × 1.25, rounded up to the nearest 5,000 BTU/h
6. Temperature Difference
ΔT = Tindoor − Toutdoor design
Montana design outdoor temps: Zone 6A = −10 °F, Zone 6B = −15 °F, Zone 7 = −25 °F