CSLB C-20 HVAC ContractorHeating SystemsHard
A furnace rated at 80,000 Btu/h input is being installed at an elevation of 6,500 feet. Per manufacturer instructions following standard altitude derate practice, input must be reduced by 4% for each 1,000 feet of elevation above 2,000 feet. What is the derated input rating the technician should use when sizing the orifice?
- A76,800 Btu/h
- B65,600 Btu/h
- C60,000 Btu/h
- D72,800 Btu/h
Show answer & explanationAnswer & explanation
Correct answer: B. 65,600 Btu/h
Elevation above the 2,000 ft baseline = 6,500 − 2,000 = 4,500 ft, or 4.5 increments of 1,000 ft. Reduction = 4.5 × 4% = 18%. Derated input = 80,000 × (1 − 0.18) = 80,000 × 0.82 = 65,600 Btu/h.
Why the other options are wrong
- A. 76,800 Btu/h reflects only a 4% reduction, ignoring the 4.5 increments above 2,000 ft.
- C. 60,000 Btu/h corresponds to a 25% reduction, more than required for this elevation.
- D. 72,800 Btu/h would result from only a 9% reduction, half the correct derate percentage.
Altitude Derate for Gas Appliances
A code-based adjustment reducing gas furnace input rating by approximately 4% per 1,000 ft of elevation above 2,000 ft to compensate for reduced oxygen density.
- Baseline elevation for standard orifice sizing is sea level to 2,000 ft
- Reduction typically applied via smaller orifice size, not just derated rating on paper
- Manufacturer's high-altitude kit or orifice tables should be consulted for exact values
Memory trick: Every thousand feet past two thousand, shave four percent.