Commerce City Furnace Repair and Combustion Diagnosis

Commerce City is at 5,157 feet, and that places a 12.6% derate on every gas furnace in town: 100,000 BTU/hr of rated input becomes 87,400 BTU/hr of real input. That figure is a combustion safety issue before it is a comfort issue. A furnace never adjusted for 5,157 feet does not simply make less heat - it makes heat badly, and the byproduct of burning too much fuel in thin air is carbon monoxide. MoJo Home Services diagnoses with a combustion analyzer, not by feel.

How does elevation turn into a carbon monoxide risk in Commerce City?

A gas orifice is a fixed hole. It meters the same volume of fuel whether the air around it is sea-level dense or Commerce City dense at 5,157 feet, 3,157 feet above the point where the derate begins. The oxygen available to burn that fuel drops but the fuel does not, so the mixture goes rich. Rich combustion means unburned carbon: soot on the burners, yellow tips rolling off the flame, higher stack temperature and rising CO in the flue. That is why the 12.6% is stated as a required derate rather than a recommendation.

What does a correct combustion test look like at 5,157 feet?

An analyzer probe in the flue, upstream of any dilution, with the furnace at steady state for several minutes. We record air-free carbon monoxide against the 400 ppm ANSI ceiling and want it dramatically lower, plus oxygen and carbon dioxide percentages, stack temperature and draft in inches w.c. Manifold pressure gets read at the same time - typically 3.5 inches w.c. on natural gas before any derate adjustment - and compared to what the orifice size should be for a Commerce City installation. Numbers on a slip, not an opinion.

What are the warning signs a Commerce City furnace is burning wrong?

Soot or black streaking around the burner compartment. Flame that is yellow and lazy rather than mostly blue and firm. A vent connector showing rust or condensation stains where the flue gas should have stayed hot enough to rise. Repeated CO alarm activations, or headaches and drowsiness that ease when you leave the house. A furnace that runs longer than it used to for the same result is also a signal, because the 12.6% derate combined with poor combustion compounds the capacity loss.

Why do draft and vent problems show up more at this elevation?

Draft pressure switches close on a stamped negative pressure, often a few tenths of an inch w.c. At 5,157 feet the inducer is moving thinner air, so a switch already near its margin - or an inducer wheel with dust loading, or a condensate-blocked pressure port on a 90% unit - fails to make on a cold start and the sequence never reaches ignition. Common-vented furnace and water heater arrangements deserve extra attention here, because vent sizing tables assume flue gas will carry enough buoyancy to clear the run.

What gets adjusted versus what gets replaced?

Adjusted: orifices and manifold pressure to match 5,157 feet, blower speed to bring temperature rise inside the nameplate range, and burner cleaning to restore a firm flame. Replaced: pressure switches, inducer assemblies, igniters, flame sensors, gas valves and control boards. Not repairable: a cracked heat exchanger. If combustion testing shows CO climbing the moment the blower starts, that is a heat exchanger symptom and the unit gets shut down rather than patched.

How often should a furnace here be tested rather than just filtered?

Once per heating season, with instruments. A filter change is maintenance; a combustion test is diagnosis, and at 5,157 feet it is the only way to confirm the appliance is still fueling correctly. Orifices do not change on their own, but burners foul, inducer wheels load with dust, vent connectors shift, and heat exchangers develop cracks that first show up as CO rising the instant the blower energizes. An annual analyzer reading also gives you a year-over-year trend for the same furnace at the same elevation, which is far more useful than one pass-fail verdict.

Who permits furnace work in Commerce City?

Gas appliance permitting for Commerce City runs through the Adams County jurisdiction. Repairs at the component level normally do not require a permit; a furnace changeout or heat exchanger replacement does, and we file it. We also leave the combustion readings with you so the next technician has a baseline for 5,157 feet rather than starting blind.

Which neighboring areas do you serve?

The derate changes as you move around the north metro, so a setup does not transfer between towns. Brighton at 4,984 feet runs 11.9%, Northglenn at 5,377 feet runs 13.5%, and Henderson sits between them. A furnace moved or bought secondhand from one of those towns needs its orifices verified for Commerce City. Call 720-807-4050 for heating service across this 53,696-resident city.

Where else we work

The same trucks run furnace repair in Henderson, furnace repair in Brighton, furnace repair in Northglenn, furnace repair in Bow Mar on the same rotation.

For the other half of the system, AC repair in Commerce City.

Wider view: furnace repair, or everything in Commerce City.