Furnace Sizing Calculator for Utah Altitude Adjustment

Determine the properly sized furnace (BTU/hr) for your Utah home, with automatic altitude derating per ASHRAE and manufacturer standards. Higher elevations reduce combustion efficiency, requiring a larger furnace to deliver the same usable heat.

Salt Lake City ≈ 4,226 ft | Park City ≈ 6,900 ft | St. George ≈ 2,880 ft | Provo ≈ 4,551 ft
80% = standard; 90–96% = high-efficiency condensing; 97–98% = ultra-high

Formulas Used

1. Envelope Heat Loss:
Qenvelope = Uavg × Afloor × ΔT

2. Window Heat Loss:
Qwindow = Uwindow × Awindow × ΔT  (Awindow = 15% of floor area)

3. Infiltration Heat Loss (ASHRAE):
Qinf = 0.018 × ACH × Volume × ΔT

4. Total Heat Loss:
Qtotal = Qenvelope + Qwindow + Qinf

5. Altitude Derate Factor (AGA/ASHRAE):
D = 1 − 0.04 × max(0, (Altitude − 2,000) ÷ 1,000)
4% capacity reduction per 1,000 ft above 2,000 ft elevation

6. Required Furnace Input BTU:
BTUinput = Qtotal ÷ AFUE ÷ D

7. Effective Output BTU at Altitude:
BTUoutput = BTUinput,selected × AFUE × D

Assumptions & References

  • Altitude derating follows the American Gas Association (AGA) and ASHRAE Fundamentals Handbook standard: 4% capacity reduction per 1,000 ft above 2,000 ft elevation.
  • Design temperature differences (ΔT) are based on ASHRAE 99% heating design temperatures for Utah climate zones: Zone 3B (St. George) ΔT=45°F, Zone 5B (SLC/Provo) ΔT=75°F, Zone 6B (Logan/Vernal) ΔT=85°F, Zone 7 (Park City/mountains) ΔT=95°F.
  • Simplified envelope U-values per insulation quality are derived from IECC 2021 Table R402.1.2 and Manual J residential load calculation guidelines.
  • Window area assumed at 15% of conditioned floor area — a common Manual J default for residential construction.
  • Infiltration calculated using the air change method: Q = 0.018 × ACH × Volume × ΔT, where 0.018 BTU/ft³/°F is the volumetric heat capacity of air.
  • Standard furnace sizes (40,000–200,000 BTU/hr in 20,000 BTU increments) reflect common residential gas furnace product lines.
  • AFUE (Annual Fuel Utilization Efficiency) per DOE 10 CFR Part 430; minimum federal standard is 80% AFUE for non-weatherized gas furnaces.
  • This calculator provides an estimate only. A full ACCA Manual J load calculation by a licensed Utah HVAC contractor is required for permit and equipment selection.
  • Utah requires HVAC permits for furnace replacement in most jurisdictions; contact your local building department.

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