CSLB C-20 HVAC ContractorPlanning and LayoutHard
A gas furnace is rated at 100,000 Btu/hr input at sea level. It will be installed at an elevation of 6,000 feet. Applying the standard derating guideline of 4% reduction in input capacity per 1,000 feet of elevation above 2,000 feet, what is the furnace's derated input capacity at this altitude?
- A100,000 Btu/hr
- B84,000 Btu/hr
- C76,000 Btu/hr
- D92,000 Btu/hr
Show answer & explanationAnswer & explanation
Correct answer: B. 84,000 Btu/hr
Elevation above the 2,000-ft threshold = 6,000 − 2,000 = 4,000 ft. Derate = 4,000 ÷ 1,000 × 4% = 16%. Derated capacity = 100,000 × (1 − 0.16) = 100,000 × 0.84 = 84,000 Btu/hr.
Why the other options are wrong
- A. Ignores altitude derating entirely.
- C. Overstates the derate by using 24% instead of the correct 16%.
- D. Uses only an 8% derate, calculated from the wrong elevation baseline.
Altitude Derating of Gas Furnaces
Combustion equipment input capacity must be reduced above 2,000 ft elevation because thinner air reduces oxygen available for combustion, typically at a rate of about 4% per 1,000 ft above 2,000 ft.
- Threshold begins at 2,000 ft elevation
- Standard derate ~4% per 1,000 ft above threshold
- Affects heating input capacity and combustion efficiency
Memory trick: Above two grand, cut four percent per thousand you stand.