Why is my furnace not turning on? Usually it has lost power, the thermostat isn't calling for heat, or a safety control has shut it down and is holding it off. The first two you can often sort out yourself. The third is the furnace protecting the house, and repeated resets won't get you past it. So the first job is telling those three apart.
No power, or no ignition? Watch what it does
Turn the thermostat up a few degrees, then watch and listen at the furnace.
- Nothing at all. No hum, no small fan, no light through the little window in the door. That usually means no power or no call for heat.
- The small fan runs, but the burners never light. That fan is the inducer motor. Carrier says when it runs and the burners don't light, "it is likely an ignition or pressure switch issue."
- The burners light, then go out within seconds. See the guide to short-cycling furnaces.
- Air moves from the vents, but it's cold. Power is fine; the problem is heat. See our guide to cold air from the vents.
Smell gas or hear the CO alarm? Leave first
If you smell gas, the City of Calgary's gas leak guidance is to leave the building right away, and "do not turn light switches on or off, use a phone, or create a spark." Once you're somewhere safe, phone ATCO Gas's emergency number, 1-800-511-3447, or 911. More on that in what to do about a gas smell near the furnace.
When a carbon monoxide alarm sounds, the City's carbon monoxide safety page says to "leave your house and call 911," and not to go back in "until the Fire Department tells you it is safe to do so."
Start at the thermostat
- Mode. On Heat, not Cool or Off.
- Setpoint. Carrier and Lennox both say it has to be higher than the current room temperature. Raise it a few degrees.
- Batteries. If the screen is dim or blank, Lennox says: "Replace the batteries with fresh ones to rule this out."
If a wired thermostat with no batteries is blank, Lennox says to check your electrical panel for a tripped breaker. A dead wired thermostat and a silent furnace often mean the same thing: no power at the furnace.
The furnace switch and breaker
Most furnaces have their own on/off switch. Carrier says it's "usually found on the side of the unit or on a nearby wall" and often looks like a light switch, so it's easy to flip off by mistake. The City of Calgary adds, on its home maintenance page, that the switch "will be located between the furnace and the entrance and will be marked."
Then check the furnace breaker in your panel. Bryant is clear: "reset it once. If it trips again, leave it off and call a professional."
Last, the door. Trane notes a furnace may have a blower door switch that "cuts power and gas to the furnace when the door is removed." If you've just changed a filter behind that door, a door that isn't fully seated can leave the furnace dead. Check it's closed and latched. Never tape, hold or jumper that switch.
Check the filter
Carrier says a clogged filter restricts airflow, and "This can cause the system to overheat and shut down automatically."
Slide the filter out. If it's caked with dust, fit a replacement of the same size; the dimensions are marked on its edge. Change intervals vary by filter type and furnace, so follow your owner's manual.
Read the status light without opening the furnace
Many modern furnaces have a control board light visible through a window in the door. Carrier's furnace repair guide says: "Match the flash sequence to the code chart on the panel and tell your technician."
Trane says a slow green blink "may mean there is simply no call for heat," while red blinking generally signals a problem, and the meaning depends on make and model. Bryant says to check the owner's manual rather than assume a code means the same thing on every model. Write the flashes down for when you call.
Trane says not to take the front panel off while the furnace is running, so read it through the window. No light at all usually means going back to the switch and breaker.
Calgary winters: vents and the condensate drain
Snow and ice on the outside pipes
A high-efficiency furnace has a plastic exhaust pipe, and often a plastic intake pipe too, running out a side wall or up through the roof. The City of Calgary says some ice at the exhaust is normal, but "If the furnace will not start up when it is cold, ice build up would be one of the first things to check for." Its winter storm advice is short: "Clear snow away from furnace and exhaust vents." Check after a big snowfall.
Sweep off loose snow with a gloved hand or a soft brush and leave any ice where it is. Don't fit a cap or screen or alter the pipes; the City says the only screens that belong there are ones the manufacturer provides. If the pipes look open and the furnace still won't start, that's a technician call.
A frozen or blocked condensate drain
These furnaces also make water, which leaves through a small drain line. Bryant explains that if the line blocks, "a float switch can prevent the furnace from operating to help prevent water damage." Its warning is plain: "Do not bypass a condensate safety switch or disassemble internal components." Where that line passes through an unheated garage or crawl space, it can ice up when the temperature drops hard. Water around the furnace base is one sign; see a furnace leaking water.
When it tries to light and gives up
Inducer running, no flame: Carrier points to the ignition or the pressure switch. Bryant notes that if electronic igniters "crack or fail, gas will not ignite." Carrier's guide adds that a furnace can lock out after a fault such as a tripped limit switch.
None of this is homeowner work. Trane says, "You should not attempt to clean or replace the flame sensor yourself," and Bryant says, "Never attempt to repair a gas valve yourself."
The flame rollout switch shuts the burners down if flame escapes where it shouldn't. Lennox is blunt: "One trip deserves attention. Multiple trips in a season mean you need a technician, not a reset button."
If the gas stove or water heater is out too, Carrier says to contact your utility.
One reset, then stop
Bryant's advice is to turn off power to the furnace, restore it, and follow the manufacturer's reset instructions in the owner's manual. If it doesn't run normally after one reset:
- Don't keep cycling the power. A furnace that keeps locking out is reporting the same problem.
- Don't jumper, tape or bypass the door, pressure, rollout, limit or float switches.
- Don't open the burner compartment.
Bryant says repeated safety shutdowns need professional diagnosis.
Checks to run before calling
- What it does on a call for heat: nothing, inducer only, lights then quits, or cold air
- Thermostat in Heat mode, set a few degrees above the room, new batteries if it takes them
- Furnace switch on, breaker reset once at most
- Blower door closed and latched
- Filter checked
- Status light flashes written down
- Snow brushed off the outside pipes
If it still won't turn on
If the furnace still sits there after that list, call 587-834-3668 with two details: what it does on a call for heat, and what the light shows. Ignition and gas-side faults need one of our Alberta-licensed gas fitters. We price the repair before any work starts. Call and we'll tell you honestly when we can get there. We dispatch from both Calgary and Airdrie. For a whole house gone cold, see no heat issues. The furnace repair page walks through a repair call, and Airdrie homeowners can start at furnace repair in Airdrie.
