Gas-fired equipment is rated at sea level. At altitude, it delivers less heat. The reason is air density.
The derating rule
Capacity at altitude = Rated capacity × (1 − 0.04 × altitude in thousands of feet)
Or equivalently:
Derating factor = 1 − (0.04 × h/1000)
Where h is elevation in feet above sea level.
Example: 100,000 BTU furnace
Altitude Factor Actual Output
──────────────────────────────────────
Sea level 1.00 100,000 BTU/hr
2,000 ft 0.92 92,000 BTU/hr
3,000 ft 0.88 88,000 BTU/hr
5,000 ft 0.80 80,000 BTU/hr
7,500 ft 0.70 70,000 BTU/hr
A 100,000 BTU furnace in Denver (5,280 ft) delivers ~80,000 BTU.
Why it happens
Air at altitude has the same oxygen percentage (21%) but lower density:
Sea level: ρ = 1.225 kg/m³
5,000 ft: ρ = 1.005 kg/m³ (−18%)
Each cubic foot of air contains 18% fewer oxygen molecules. The burner gets less oxygen per cycle. Less oxygen = less combustion = less heat.
The density ratio:
ρ = P / (R × T)
P at 5,000 ft ≈ 84,556 Pa (vs 101,325 at sea level)
Ratio = 84,556 / 101,325 = 0.834
The 4% per 1,000 ft rule approximates this as 0.80 at 5,000 ft — slightly conservative, which is appropriate for safety-critical combustion equipment.
The sizing mistake
Heating load calculation shows 90,000 BTU/hr needed.
At sea level:
Select 100,000 BTU furnace
Margin: 100,000 − 90,000 = 10,000 BTU (11%)
Result: adequate
At 5,000 ft (same furnace):
Actual output: 100,000 × 0.80 = 80,000 BTU
Deficit: 90,000 − 80,000 = 10,000 BTU short
Result: house cannot hold setpoint on coldest nights
Correct sizing at 5,000 ft:
Required input = 90,000 / 0.80 = 112,500 BTU
Select: 120,000 BTU furnace
Actual output: 120,000 × 0.80 = 96,000 BTU
Margin: 96,000 − 90,000 = 6,000 BTU (7%)
Combustion air also needs correction
The burner needs more volume of air to get the same oxygen:
Sea level: 15 ft³ of air per ft³ of natural gas
5,000 ft: 15 / 0.80 = 18.75 ft³ of air per ft³ of gas
Combustion air openings must be 20% larger at 5,000 ft. Undersized openings cause:
Incomplete combustion
Carbon monoxide production
Sooting and heat exchanger fouling
Premature equipment failure
Not all equipment derates the same
Atmospheric burners: full 4%/1000 ft derating applies
Induced draft: full derating applies
Power burners: may partially compensate
Sealed combustion: may auto-adjust fuel-air ratio
Condensing (modulating): check manufacturer specs
Always check the installation manual for altitude-specific requirements. Some manufacturers offer high-altitude orifice kits instead of applying blanket derating.
For quick air density checks at any altitude and temperature, use the air density calculator on CalcEngineer.
