Sooting Glass and Lazy Flames at 5,000 Feet: Altitude, Orifices, and Your Fort Collins Fireplace

September 4, 2026

If your Fort Collins gas fireplace burns with tall, lazy yellow flames and the glass goes black within a week of cleaning it, the odds are good that nothing is broken. The fireplace is running rich, because it is burning sea-level gas in 4,997-foot air, and the fix is an orifice kit and a pressure setting, usually covered by the $150 to $300 service call plus the kit, not a new fireplace. This post explains the physics, the code, the symptoms, and what a proper altitude setup looks like from Fort Collins up to Estes Park.

If you smell gas at any point, leave the house and call Xcel Energy or 911 from outside.

The physics, briefly

A gas burner mixes gas with air in a fixed ratio set by two things: the size of the orifice (the tiny brass jet the gas comes through) and the manifold pressure pushing the gas through it. The gas jet pulls primary air in through the venturi as it passes. At sea level the manufacturer sets orifice and pressure so the mix burns clean and blue with yellow tips on the logs.

Climb to Fort Collins at 4,997 feet and the air has roughly 15 to 17 percent less oxygen per cubic foot than sea level, because it is thinner. The orifice does not know that. It delivers the same volume of gas into less oxygen, and the mix goes rich. Rich mixtures burn slower and cooler, so the flame lifts and stretches, turns yellow, and does not finish combustion before it hits the cold glass or the logs. What is left over is soot, which is unburned carbon, and carbon monoxide, which is unburned carbon’s gas.

Go up to Estes Park at 7,743 feet and it is worse by nearly the same amount again.

The code

The National Fuel Gas Code (NFPA 54 / ANSI Z223.1) requires that gas appliances installed above 2,000 feet have their input derated by about 4 percent per 1,000 feet, and manufacturers’ installation instructions carry the same rule and make the installer responsible for the adjustment. Manufacturers implement it with high-altitude orifice kits and conversion kits published by elevation band, and natural-gas kits typically start at 4,500 feet, which means essentially every fireplace in Fort Collins, Wellington at 5,194 feet, Loveland at 4,997, and Timnath at 4,869 is supposed to have one. Windsor at 4,797 and Greeley at 4,676 sit right around the threshold, and the derate applies to them regardless of which kit the manufacturer specifies. At Estes Park’s 7,743 feet the reduction is well over 20 percent and the kit is a different one again.

The rule is not new. It comes from the same aerodynamics that make a furnace and a water heater need derating here, and every HVAC contractor in Larimer County knows it for furnaces. Fireplaces get missed because they are installed by whoever the builder sent, they “work” when lit, and nobody looks at the flame closely until the glass is black.

How it happens

• The unit arrived set for sea level and nobody opened the kit. The most common case. Production builders installing dozens of fireplaces in a phase on the Harmony corridor in the 1990s, or in Timnath and Windsor in the 2010s, did not always fit the kit that shipped in the box.
• A replacement part was installed at sea-level settings. A new valve or burner from the parts counter arrives at the factory default. The tech who fitted it did not reset the pressure.
• A fuel conversion was done without the altitude step. Natural gas to propane or back, common in Estes Park and rural Wellington when a house moves from a tank to a new main. The conversion kit changes the orifices and the regulator; the altitude kit changes them again.
• The fireplace was moved. A used insert from a Denver-area house at 5,280 feet is close; one from Nebraska is not.

Reading the flame

A healthy gas fireplace flame is mostly blue at the base, with yellow-orange tips dancing on the logs. Trouble looks like this:

Soot is the one to take seriously. On a sealed direct-vent unit the exhaust goes outside, so the soot is mostly a cleaning and efficiency problem. On a vented gas log set in an Old Town masonry fireplace, or a B-vent stove, or any vent-free set, the combustion products are in your house, and an un-derated unit produces more carbon monoxide than a properly set one. A CO reading is part of every service visit for exactly this reason.

What the fix actually is

1. Confirm the cause. Logs and embers off the burner ports, air shutter checked, pilot flame checked, orifice pulled and read against the manufacturer’s chart for the fuel and the elevation. A closed air shutter and a misplaced log get fixed first, because they cause the same symptom for free.
2. Fit the correct orifice. The high-altitude orifice from the manufacturer’s kit for the unit’s elevation band. Not a drilled-out orifice, not a “close enough” size from the bin. The kit for a Fort Collins address and the kit for an Estes Park address are different parts.
3. Set manifold pressure. Measured with a manometer in inches of water column and set to the manufacturer’s figure for the elevation and fuel. Natural gas and propane run at different pressures; propane through the wrong orifice is dangerous, not just sooty.
4. Reset the air shutter for the new mix, if the unit has one.
5. Clean the glass and logs, put the logs back on the manufacturer’s diagram, and test-fire. The flame should be blue-based with yellow tips within a minute.
6. Record the readings. Orifice size, manifold pressure, CO. On paper, for the next tech.

Most of this is done inside the $150 to $300 service call, with the kit as the only added part. Where the unit has been sooting for years and the burner is clogged, or the thermopile has been fouled by soot, the repair page ranges apply.

Propane doubles the problem

Propane carries more energy per cubic foot than natural gas and runs at higher pressure through smaller orifices. Get the orifice wrong on a propane fireplace at altitude and the mixture is not just rich but far rich, which is the recipe for heavy soot and real carbon monoxide. This is the Estes Park special: a cabin on tank propane at 7,743 feet with a unit that shipped set for sea-level natural gas. The Estes Park page covers who supplies gas up there; the short version is Xcel Energy on the mains through the Big Thompson Canyon and propane suppliers off them, and the parts on the truck have to match.

What it costs, and what it does not

What it does not cost is a new fireplace. A 20-year-old direct-vent unit in south Fort Collins that has sooted since the day it was installed is usually a fine fireplace with the wrong orifice, and once it is set for 4,997 feet it burns the way the brochure promised. The pricing page has the rest of the numbers, and the annual service visit checks the orifice and pressure against the elevation every year, because the fireplace that was right last year is still right, but the replacement valve someone fitted in March might not be.