DENVER GUIDE

When a Denver furnace calls for heat and does not light

The thermostat clicks into a call, a draft motor spins up, and the burner never catches. Or the closet stays quiet after the setpoint is raised. A furnace that will not ignite is a sequence you can describe before it is a parts order. These notes cover the sounds, the gas-smell stop, and the single elevation question worth putting to a technician.

By Mike Maziarz · Sources checked October 6, 2026 · How we write these guides

The sequence you can hear

Stand in the hall, not in the burner compartment. On a call for heat many furnaces start a small motor first, the inducer that proves draft, then attempt ignition, then open the gas valve, then wait before the blower moves air. If you hear the motor and then silence, say that. If you hear nothing at all, say that too. Those are different stops.

Cold air from the registers with the blower already running is not the same as a dead closet. The blower can move basement air while the burners stay dark. A click and a retry, over and over, is another pattern. Write how many attempts you actually heard. Do not sit and cycle the thermostat for an hour to build a longer count.

Hot-surface igniters and intermittent pilots both exist in Denver houses. You cannot tell which one you have from the brick on the front of the building. Do not remove the burner door to find out. The model plate on the outside of the cabinet is enough for the person who will open it.

Smell gas and the checklist ends

A gas odor at the furnace closet ends every other step on this page. Get people out of the building. Do not flip wall switches, the furnace disconnect, or the thermostat on the way out. From outside, call the emergency number printed on the gas bill. Xcel Energy is the name many of those bills carry. This page does not publish a replacement number.

Do not hunt for the leak with the door open and the lights clicking. Do not decide the odor is weak enough to ignore because the burners have failed before. A failed ignition and a gas smell can arrive together. The utility call comes first. A heating company visit comes after the building is cleared.

If there is no gas odor and no alarm, you can keep observing from the hallway. The moment either one appears, the observation is over. Neighbors in a shared wall situation, a Capitol Hill stair closet included, still leave their own unit. You are not the person who verifies the pipe.

Checks that stay outside the burner

With no gas smell, confirm the thermostat is in heat and the setpoint is above the room temperature it is reading. A schedule can hold the system in a setback you forgot about after a mild afternoon. Replace a dirty filter only if it slides out without tools. A filter collapsed across the blower is a note, not a reason to unbolt the blower door.

A switch that looks like a light switch on the side of the cabinet is often the service disconnect. If it is obviously off, and you are sure that is what it is, turning it on once is a fair note. If you are not sure, leave it. One breaker reset is enough. A second trip means the breaker stays off.

Filter slides and thermostat batteries are ordinary house care. Gas valves, burners, and control boards are not. The line between those jobs is the point of the do-it-yourself or call a pro page. Nothing in that split asks you to prove draft with the blower door off.

One elevation question, then stop

Denver sits about a mile above sea level. Combustion air at that height is not the same density as air at a coastal bench, and a furnace can be set up with that fact in mind. Your question to the technician is only this: is this furnace set up with the elevation in mind, and how can you tell from what you see on this visit?

Listen for a plain answer. You are not collecting an orifice size, a chart value, or a code section to repeat later. This guide will not supply those figures. If the answer is a shrug or a slogan, ask once more for the setup on this cabinet. Then write the words they actually used.

A boiler in another house is a different appliance with the same class of question. On this visit the subject is the furnace that would not light. Do not let a general lecture about altitude replace a look at the flame, the vent, and the intake on the unit in front of you.

When a flame starts and then dies

Ignition that catches and then drops out is a different story from ignition that never happens. The inducer may keep running. A high-efficiency furnace may refuse to continue when the condensate drain is backed up or a float switch is open. You can look at a plastic trap outside the burner only if it is visible without removing a safety cover. Do not bypass that switch with a jumper or a piece of tape.

Short cycling, a flame that rolls, or soot where you have not seen soot before belongs in the notes, then in a technician's visit. If a carbon monoxide alarm sounds, or anyone has a headache, dizziness, or flu-like symptoms that ease when they step outside, get everyone out and call emergency services. Do not keep the furnace in a test cycle while you search the flue. The carbon monoxide page is the longer version of that exit.

Vent pipe that has come apart in a basement, intake pipe buried under the first snow, or a nest in a sidewall cap are things you can see from a safe stance. Photograph them. Do not climb an icy roof to reseat a cap, and do not tape a cracked plastic vent as a lasting repair.

After heat is back, what the visit was

When a technician restores ignition, ask what failed in the sequence: draft proof, igniter, flame sense, condensate switch, gas valve, or something else they can name. Ask what they changed and what they only cleaned. Those two answers keep the next no-heat morning from starting at zero.

The bill for that work depends on access, parts, and whether the call was after dark on the first hard night. This page does not price any of it. The factors that move a heating invoice, still without dollar figures, are on the repair cost factors page.

A furnace that lights does not answer a summer complaint. The coil above or beside it, and the outdoor unit in the side yard, are a separate path when the house will not cool. Use the air conditioner not cooling notes for that season instead of another round of ignition tests in July.