If your furnace was installed straight out of the box with no altitude adjustment, it is not producing the heat its sticker promises — and it may be slowly damaging itself trying. Here’s the plain-English version of why.
Thinner air, weaker fire
Gas furnaces burn a precise mixture of fuel and oxygen. At Aurora’s elevation — roughly 5,400 feet — every cubic foot of air carries less oxygen than at sea level. A furnace tuned for sea level runs fuel-rich here: a softer, dirtier flame that produces less heat and more soot.
That’s not a subtle effect. Manufacturers publish specific derating procedures — changing orifices, adjusting gas pressure, verifying combustion — for exactly this reason.
What "derating" actually means
Derating is the process of adjusting a furnace’s gas input for elevation so the burn stays clean and the output is honest. Done right, it means correct manifold pressure, the right orifice size, and a combustion check against the rating plate — not just firing it up and walking away.
A furnace that never got this treatment often shows it: soot around the burners, a yellow lazy flame, a heat exchanger running hotter than designed.
The heat exchanger problem
The heat exchanger is the metal wall between the fire and your breathing air. Chronic over-firing — common on non-derated units at altitude — stresses that metal year after year. Cracks follow. And a cracked heat exchanger is the one furnace problem with no repair path: it’s replacement, full stop.
This is why we check combustion on every furnace call in Aurora, not just installs. A ten-minute test tells us whether your system was set up for this elevation.
What you can do
- Ask the question: when was your furnace last combustion-tested? If the answer is "never," it’s worth a visit.
- Watch the flame: through the sight glass, a healthy gas flame is steady and mostly blue. Yellow, lifting, or dancing flames deserve a call.
- Take CO alarms seriously: at altitude, marginal combustion is more common — working detectors on every floor are non-negotiable.
