How High Altitude Affects Furnace Sizing, Output & Efficiency in Colorado Springs

Natural gas burners need oxygen to combust. At 6,035 feet, atmospheric pressure is roughly 80% of sea level, which means every cubic foot of air drawn through a furnace carries about 20% less oxygen. An untreated furnace simply can't burn its full rating — output drops and combustion byproducts climb.

That's why manufacturers publish altitude derating tables. Most equipment must be re-orificed (smaller gas burner orifices) or otherwise adjusted to restore correct combustion at 6,000–7,000 feet. A furnace installed out of the box without altitude correction will run inefficiently, cycle poorly, and in some cases vent unsafe combustion products.

A typical uncorrected furnace loses roughly 4% of input capacity per 1,000 feet of elevation. At Colorado Springs' altitude that's around a 20% output reduction — the difference between a furnace that holds 70°F on the coldest morning and one that falls behind during a cold snap.

This is also why 'my neighbor has the same size house with a smaller furnace' comparisons fail here. A properly installed, altitude-corrected 70,000 BTU furnace can comfortably heat a home that an uncorrected 80,000 BTU unit struggles with. Manual J load calculations done for our climate — plus altitude-corrected equipment — are what actually determine comfort.

The AFUE percentage on a furnace label is measured at sea level. At altitude, real-world efficiency can drop a few points if combustion isn't properly tuned. Two furnaces with the same 96% AFUE sticker can behave very differently in a Colorado Springs basement depending on whether the installer corrected the gas input and verified combustion with a analyzer at startup.

We commission every installation with a combustion analysis: measuring oxygen, carbon monoxide, and stack temperature at the burner, then adjusting gas input until the numbers match the manufacturer's altitude specification. That's the difference between a furnace that performs at its rated 96%–98% and one that quietly costs you extra every gas bill.

Thinner air also changes how exhaust behaves. High-efficiency condensing furnaces use PVC venting with precise length and diameter limits, and at altitude those allowances tighten. Undersized combustion air openings — common in finished basements — cause the pressure switch faults that shut furnaces down mid-winter.

El Paso County permitting (through Pikes Peak Regional Building Department) also requires combustion air and venting to meet code for our elevation. We pull permits and size venting for 6,000+ feet on every replacement, so your warranty stays valid and your family stays safe.

Proper sizing here is a four-step process, not a guess from square footage:

Heat pumps lose heating capacity as temperatures drop, which is why many Colorado Springs homes pair one with a gas furnace in a dual-fuel setup: the heat pump carries the shoulder seasons efficiently, and the furnace takes over during cold snaps. Sizing both pieces for our elevation — and programming the switchover temperature correctly — is what makes dual-fuel worth the investment here.

If you're weighing a furnace against a heat pump or a combined system, our team lays out the real installed costs and operating tradeoffs for your specific home. Complete system packages start at $10,400, and every option is quoted with upfront, no-surprise pricing.