Furnace Altitude Derate Calculator

More detail in How Altitude Affects Furnaces, ACs and Heat Pumps and How to Choose the Right Furnace Size.

Questions

Why does a furnace lose output at altitude?

Air is thinner at Colorado elevations, so there is less oxygen to burn the gas. Manufacturers require the furnace’s gas input to be reduced (derated) — commonly about 4% per 1,000 feet — with orifice or gas-pressure changes, and sometimes a high-altitude pressure switch.

Do all brands use 4% per 1,000 feet?

No. Methods vary: some manuals apply 4% per 1,000 ft of total elevation once above 2,000 ft, others start counting at 2,000 ft, and some premium models need only about 2%. Always follow the installation manual for your model.

What happens if a furnace is not set up for altitude?

It can over-fire, run hot, trip safety switches, make more carbon monoxide and wear out early. Correct setup is part of a proper install.

Sources
1. ACHR News — furnaces at high altitude
2. Town elevations: our city pages (sources listed on each)