INDIANAPOLIS GUIDE

When the furnace will not light

The first cold night often announces itself in the basement. You turn the thermostat up, hear a fan try to start, then a click, and the vents stay cold. Sometimes there is no sound at all, only the clock on the thermostat and a house that will not catch up. Gas furnaces are common here, including in older houses with the unit beside the water heater. Heat pumps and dual-fuel setups exist too, and they fail differently. Before you touch a panel, sort out whether this is a no-heat note for a technician or a leave-the-house moment.

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

What a failed light-off sounds like

A modern gas furnace follows a sequence you can hear from the stairs. An inducer fan starts, there is a pause, then a click or a glow as the burner tries to light. If the flame proves, the blower comes on after the heat exchanger warms. If the flame does not prove, the unit tries again or locks out and goes quiet. Write which of those you heard, and how many tries, before you reset anything.

A standing-pilot furnace, more likely in an older house, is quieter. Look only through a sight glass, without removing the door. If you cannot see a pilot safely, stop and write that down. Do not open the burner compartment to hunt for it.

No sound at all points somewhere else: a thermostat that is not calling, a switch that is off, or a power problem. A click and then silence points at the lighting attempt. Those two notes send a technician down different paths, and they are both things you can record without tools.

If you smell gas, stop and leave

A gas smell is not a troubleshooting step. Leave the building. Do not flip light switches, do not use the thermostat, and do not start a car in an attached garage. From outside, call the gas utility emergency number printed on your bill. Many Indianapolis addresses are served by Citizens Energy Group. Use the number on the bill. Do not use a number you found in a search result or on a magnet that might be out of date.

Do not go back in to open windows as a first move if the smell is strong. Get people and pets out, then make the call. Tell the utility whether the smell is at the furnace, at the meter, or through the whole house. After they clear the building, a heating company can look at why the burners did not light.

A faint smell that shows up only for a second when a furnace finally lights can be different from a smell that stays in the room. If you are unsure, treat it as real and leave. You can describe the difference later. You cannot undo staying inside to decide.

Checks that stay on your side of the panel

With no gas smell, three checks are reasonable. Confirm the thermostat is set to heat, the setpoint is above the room temperature, and the fan is on auto rather than a setting you did not choose. Replace a filter only if you can slide it out without tools and you already own the right size. A filter packed solid can make a furnace overheat and shut down before the burners stay lit.

Look for a switch that looks like a light switch on or near the furnace. People hit it while carrying laundry. If it is off, turning it on is fair. If the furnace still will not light, leave the switch on and stop. Do not press a reset button over and over. A single reset is something to mention. A string of resets can hide a lockout.

Do not jump out a safety switch, tape a door switch down, or try to light a pilot with a match if you smell gas or you are unsure. That work is not a homeowner repair. The do-it-yourself or call a pro page draws that line in more detail, including what belongs to a licensed technician on a fuel-burning appliance.

What the technician is sorting out

Ignition problems have a short list of ordinary causes, and you do not have to name the winner. A dirty flame sensor, a failed igniter, a closed gas valve, a pressure switch that never proves the vent, or a condensate drain that is backed up on a high-efficiency furnace can all end in the same quiet cabinet. Your job is the history, not the part name.

High-efficiency furnaces in a basement often have a plastic vent and a drain. If you see water around the unit or a hose that has come loose, say so. Do not pull hoses apart. In a closet or attic air handler that is actually a furnace, the same drain issue can shut the burners down, and the location changes how the water shows up, in a pan or on a ceiling.

If the outdoor unit is part of a heat pump and the gas furnace is the backup, say that too. A cold house can be a heat pump that never handed off to the furnace. Mention both pieces of equipment so the visit is not scoped as a furnace-only call.

What to put in the service notes

Write the sequence, the time of day, and the outdoor weather in a sentence. Ice on the vents outside, snow drifted against a sidewall vent, or a cold snap after a mild week all belong in the note. A blocked intake or exhaust can keep a furnace from proving flame, and it is something you can see from the yard without climbing.

Photograph the data plate and any blinking light on the control board only if you can see the light through a sight glass or a window in the door. Do not remove the door to chase a code. If a code chart is printed on the panel, a photo of that chart is useful. The technician still has to confirm it.

Ask, when you call, whether the visit includes diagnosis and what changes the scope. Parts, after-hours timing, and access all affect the bill. The cost factors page sticks to what moves a quote, without dollar figures. Have the model number ready so the call is about your furnace.

After it lights, and when to get out

When heat returns, stand at a supply vent and at the thermostat for one full cycle. You want steady warmth, a blower that starts and stops on its own, and no new smell that stays in the hallway. If the burners light and then the furnace shuts off again within a few minutes, write the timing. Short cycling is a finding, not a finished repair.

A carbon monoxide alarm during or after a heating attempt changes the plan. Get everyone outside and call emergency services. Do not hunt for the source and do not keep the furnace running to see if the alarm clears. Headache, dizziness, or flu-like symptoms that ease outdoors get the same response. The carbon monoxide guide covers alarms and fuel-burning heat without turning you into an investigator.

Cooling season is a different machine, even when it shares the blower. If the furnace lights now and the house will not cool in July, that is a separate note. The cooling guide starts with humidity, a frozen coil, the filter, and the outdoor unit.