DEV Community

Cover image for Why Gas Furnaces Lose 4 Percent Capacity Per 1,000 Feet of Elevation
Evgenii Konkin
Evgenii Konkin

Posted on

Why Gas Furnaces Lose 4 Percent Capacity Per 1,000 Feet of Elevation

#hvac #engineering #combustion #altitude

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)
Enter fullscreen mode Exit fullscreen mode

Or equivalently:

Derating factor = 1 − (0.04 × h/1000)
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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%)
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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%)
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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
Enter fullscreen mode Exit fullscreen mode

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.

Top comments (0)