At Salt Lake City’s 4,200-foot elevation, air density is about 15% lower than at sea level. Gas appliances must be derated approximately 4% per 1,000 feet — a 100,000 BTU furnace delivers roughly 83,000 BTU in Salt Lake City. Air-moving equipment (fans and air handlers) also moves less air mass per CFM at altitude, slightly reducing effective cooling capacity. In Utah mountain communities at 6,000–8,000 feet, deration becomes even more significant. HVAC equipment must be sized based on altitude-adjusted performance, not nameplate ratings. RainFire Builders incorporates altitude correction into every Manual J calculation and equipment specification.