How does the load become an equipment selection at altitude?
The Manual J load is the demand. Equipment is selected against it under ACCA Manual S, Residential Equipment Selection (ANSI/ACCA 3 Manual S-2023), which applies manufacturer performance data at the local design conditions and sets size limits so that installed equipment neither short-cycles nor runs short. Between the load and the selection sits altitude.
Gas-fired equipment. Air density falls with elevation, and with it the oxygen available per cubic foot for combustion. The fuel-gas code and manufacturer instructions reduce a gas appliance's input rating by 4 percent for each 1,000 feet above 2,000 feet. Steamboat Springs sits near 6,700 feet in town and higher on the mountain, which places a furnace or boiler at roughly 75 to 80 percent of its sea-level rating. The adjustment is made with high-altitude burner orifices or manifold pressure per the manufacturer's kit, and the installer is responsible for it. An 80,000 BTU per hour furnace at sea level is therefore a 60,000 to 65,000 BTU per hour furnace here, and the load must be met by the derated number.
Heat pumps and air conditioners. Thinner air also reduces the heat the outdoor coil can move and the heat each cubic foot of supply air carries. Manufacturers publish altitude correction factors for capacity, and Manual S carries an altitude appendix; selection uses the corrected capacity, not the rated one. On the air side, the same heat requires more airflow at elevation: one published example puts 1,500 cubic feet per minute at sea level at about 1,900 at 5,000 feet, and a utility installation standard notes roughly 10 percent more volumetric flow at 5,000 feet for the same mass flow. That is where the airflow measurement enters the sequence.
Order of operations. Load first, at the design conditions for the site. Derate the candidate equipment for elevation from the manufacturer's data. Select within Manual S limits against the derated capacity. Then confirm the duct system can carry the airflow the selected equipment needs.
Last reviewed 2026-09-06. Accurate to the best of our knowledge as of that date. Rules change — that's why the date is there.