Does Utah's altitude affect my furnace sizing?

Yes — gas combustion is less efficient at altitude because there's less oxygen per cubic foot. A furnace rated at 80,000 BTU at sea level delivers only ~70,000 BTU at Wasatch Front elevations (4,200-5,000 ft). Standard derate is 4% per 1,000 feet above 2,000 ft.

This matters for sizing two ways: your furnace needs to be sized against actual delivered BTU, not nameplate, to heat the house adequately on design days. And the orifice on the gas burner needs to be changed to a smaller size so the burner doesn't run rich and create soot or carbon monoxide.

A good installer does both automatically in Utah. If you've ever moved in from a lower-elevation state and find your furnace struggles on cold nights, altitude derate may have been ignored at install.