Why Is My Furnace Not Igniting? 7 Causes and What to Do

When the Furnace Runs but Never Lights

A furnace that will not ignite usually tells you why if you know where to look. The thermostat calls for heat, the inducer fan may hum, and then nothing lights, or the burners flicker and shut off. In Burnaby and across Metro Vancouver this call comes in most often on the first cold week of fall, when a furnace that sat idle all summer is asked to work again.

The checks below are the ones a homeowner can do safely in a few minutes. Anything that involves the gas valve, the ignitor, the flame sensor, or the control board is work for a licensed gas technician, and each step says so.

7 Reasons a Furnace Won't Ignite

Work through these in order. The first three are quick homeowner checks. The last four point to parts that need a technician.

1. Thermostat settings or dead batteries. Confirm the thermostat is set to Heat, not Cool or Off, and set it a few degrees above room temperature. A blank or dim screen usually means dead batteries. Replace them and wait five minutes for the furnace to respond.
2. Power switch or tripped breaker. Most furnaces have a light-switch style power switch on or near the unit, and it gets bumped off more often than you would think. Check it, then check the furnace breaker in your panel. Reset a tripped breaker once. If it trips again, call us.
3. Blower door not seated. A safety switch behind the lower panel cuts power when the door is loose. If someone recently changed the filter, press the panel firmly back into place.
4. Clogged air filter. A filter packed with dust starves the furnace of air, the heat exchanger overheats, and the high-limit switch shuts the burners down. Replace the filter and try again. If you cannot remember the last change, it is probably overdue.
5. Cracked or worn hot-surface ignitor. Modern furnaces light the burners with a hot-surface ignitor that glows orange for a few seconds. Ignitors crack with age, and you may hear the inducer run with no glow or no light-off. Replacing the ignitor is a technician job. Call a licensed tech rather than handling the part yourself.
6. Dirty flame sensor. If the burners light for a few seconds and then shut off, the flame sensor probably cannot confirm the flame. A film of residue builds up on the rod over a season. Cleaning and testing it is part of every annual furnace tune-up we do.
7. Venting, pressure switch, or condensate problems. High-efficiency furnaces vent through plastic pipe and drain water through a condensate line. A blocked vent, a clogged drain trap, or a failed pressure switch will stop ignition as a safety measure. These are diagnosis jobs for a technician.

Many furnaces also flash a code on a small light inside the viewing window of the lower panel. Write down the flash pattern before you call. It helps the technician bring the right part.

How to Reset Your Furnace Safely

A single reset clears many lockouts. Do it once, not repeatedly.

• Set the thermostat to Off.
• Turn the furnace power switch off and wait about 30 seconds.
• Turn the power back on, then set the thermostat to Heat a few degrees above room temperature.
• Give it five minutes. The inducer starts, the ignitor glows, and the burners should light.

If it locks out again, the furnace is protecting itself from a real fault. Leave it off and book a furnace repair visit.

When to Call a Licensed Gas Technician in BC

Gas appliance work in British Columbia is regulated by Technical Safety BC, and ignition, gas valve, and venting repairs belong to a licensed gas fitter. Call a technician when:

• the ignitor does not glow, or the burners light and drop out within seconds
• the breaker trips again after one reset
• the furnace flashes the same error code after a reset
• you see water pooling around a high-efficiency furnace, or soot around the unit

Maple Furnace Heating & Air Conditioning repairs American Standard, Trane, Lennox, and most other furnace makes from our shop on Merritt Ave in Burnaby. If the furnace is near the end of its life, we will say so and lay out furnace replacement options with a free written quote.