Denver's Altitude and Your Furnace: What 5,280 Feet Actually Changes

Most HVAC equipment is designed and rated at sea level. Denver is not at sea level. That gap is the single most common reason we find systems that were “installed correctly” but never worked quite right.

Less oxygen, same gas

A furnace burns a mix of gas and air. At a mile up, each cubic foot of air carries about 17% less oxygen. If the gas pressure isn’t reduced to match, the flame runs rich: yellow tips, soot on the burners, carbon monoxide in the flue, and a heat exchanger working hotter than designed.

Manufacturers publish derating tables for this, typically about 4% of input per 1,000 feet of elevation. In Denver that means a furnace rated at 100,000 BTU should be set up to burn roughly 80,000. Some models need a different orifice size; most need the manifold gas pressure adjusted with a manometer. Either way, it takes a technician who measures instead of assumes.

What happens when it’s skipped

• Short cycling, because the heat exchanger hits its limit temperature too fast
• Sooted burners and a flame sensor that needs cleaning every few months
• Higher CO readings in the flue, which is a safety issue in a tight home
• A heat exchanger that cracks years before it should

We see all four constantly on furnaces installed by out-of-state chains and by well-meaning DIYers.

Air conditioners lose capacity too

Thin air carries less heat away from the condenser coil, so an AC unit in Denver delivers a little less cooling than its nameplate says. That matters for sizing. A unit that was “right on the edge” at sea level will be undersized here, and an undersized AC runs all afternoon without ever catching up. The fix is a proper Manual J load calculation that accounts for elevation, not a rule of thumb like “one ton per 500 square feet.”

Heat pumps are actually a great fit

Cold-climate heat pumps hold their capacity well in Denver’s dry cold and don’t have the combustion derating problem at all. Paired with a gas furnace in a dual-fuel setup, the heat pump handles the majority of the heating season and the furnace takes over below a set outdoor temperature. Denver’s typical winter, with cold nights but a lot of sunny 40-degree days, is close to ideal for this.

What to ask any installer

1. Will you derate the furnace for our elevation, and how?
2. Will you measure manifold pressure with a manometer after install?
3. Did your load calculation include elevation?
4. Will you do a combustion analysis before you leave?

If the answer to any of those is a blank look, keep shopping. Every Duality installation includes all four, and we’ll show you the numbers on your invoice.