You set the thermostat to heat, hear the system try to start, and then — nothing. The blower might run, but there is no heat. Or the furnace tries to light, fires briefly, and shuts back down. This is one of the most common furnace problems we see in Calgary homes, and the cause is usually one of a few things.
Check the thermostat first
Before assuming the furnace is broken, confirm the thermostat is set to heat mode and the setpoint is above the current room temperature. If the thermostat uses batteries, check that they are not dead — a blank or flickering screen is a giveaway. Try setting the temperature five degrees above the current reading to make sure the system gets a clear call for heat.
Dirty flame sensor
This is the most common cause of a furnace that lights briefly and then shuts down. The flame sensor is a small metal rod that sits in the burner flame. Its job is to confirm that the burner actually lit after the gas valve opened. When the sensor gets coated with oxidation or residue, it cannot detect the flame properly, so the control board shuts the gas valve as a safety measure.
A technician can clean the flame sensor in a few minutes and it usually solves the problem immediately. This is a very common maintenance item on furnaces that have not been serviced recently.
Failed ignitor
Most modern furnaces use a hot-surface ignitor — a small ceramic element that glows red-hot to light the gas. These ignitors are fragile and have a limited lifespan. When they crack or burn out, the furnace cannot light the burner at all. You might hear the inducer motor start and run, but the furnace never fires.
Ignitor replacement is a straightforward repair. If your furnace is more than 8 to 10 years old and the ignitor has never been replaced, it is a reasonable item to carry as a spare.
Gas supply issues
If the gas valve to the furnace is closed or the gas meter was shut off, the furnace will go through its startup sequence but never light. Check that the gas valve on the supply line to the furnace is in the open position — the handle should be parallel to the pipe. Also check that other gas appliances in the home are working, which confirms gas is flowing to the house.
Tripped safety or lockout
Furnaces have multiple safety circuits — high-limit switches, pressure switches, and rollout switches. If any of these trip, the furnace will not ignite. Some furnaces will show a flashing LED code on the control board that indicates which safety tripped. If you can see a flashing light pattern on the board, write it down before calling — it helps the technician diagnose the issue faster.
Some lockout conditions can be reset by turning the furnace off at the switch or breaker for 30 seconds and turning it back on. If the furnace locks out again after resetting, the underlying problem needs professional diagnosis.
When to call for help
If the thermostat checks out and the gas supply is on but the furnace still will not light, it is time to call a technician. Repeated failed ignition attempts can flood the heat exchanger with unburned gas, which is a safety concern. Do not keep resetting the furnace if it locks out more than once.
YYC Mechanical provides residential furnace repair across Calgary. If your furnace is not lighting and you need help, give us a call.