Heating-First, Sized for Altitude
Fort Collins is climate zone 5B: 5,797 heating degree days against 624 cooling degree days (NOAA 1991–2020). At 5,003 ft, gas furnaces derate roughly 10–20% and air conditioners lose roughly 10–20% of their sea-level capacity, so a system sized from sea-level ratings is undersized here. We size from a load calculation, not a rule of thumb.