Furnace Won't Turn On? 9 Things to Check Before Calling

Why Your Furnace Goes Completely Silent (And What It Means)

A furnace that refuses to start is one of the most alarming things a Canadian homeowner can face, especially when outdoor temperatures are sitting at -20°C in Winnipeg or -15°C in Calgary. The good news is that modern gas furnaces are engineered with multiple layers of safety interlock switches, and the most common reason a furnace won't turn on is that one of those interlocks has tripped — not that the heat exchanger has cracked or the inducer motor has failed. In many cases, the furnace is protecting itself, and the fix is something a careful homeowner can resolve without tools.

That said, 'furnace won't turn on' can mean several very different things mechanically. It could mean the thermostat is sending a call for heat that the control board never receives. It could mean the board receives the signal but the inducer motor doesn't spin because a pressure switch is stuck open. It could mean the ignitor glows but the gas valve never opens because a flame sensor is coated in oxidation. Each scenario has a distinct diagnostic path. The nine checks below follow the actual ignition sequence of a modern two-stage or modulating gas furnace, from the very first electrical signal all the way to a sustained flame, so you're always troubleshooting in the right order.

• Modern furnaces have 5–7 safety interlocks that can each prevent startup
• Most no-heat calls involve something in the first three checks below
• Working in sequence saves time and avoids misdiagnosis

Check 1: Thermostat Settings and Power

The thermostat is the brain of the whole system, so start here. First, confirm the system switch is set to HEAT, not COOL or OFF — it sounds obvious, but seasonal switches get bumped more often than you'd think, especially in homes with young children or where the thermostat is near a busy hallway. Next, set the temperature setpoint at least 3–5°C above the current room temperature. A call for heat only goes out when the setpoint exceeds the measured ambient temperature by a meaningful margin; some thermostats have a built-in 1–2°C deadband to prevent short cycling, so a setpoint that's just barely above room temperature may not trigger anything. Then set the fan to AUTO rather than ON — if the fan is set to ON, air will circulate even without a call for heat, which can make it seem like the furnace is running when it isn't heating.

If you have a smart thermostat like an ecobee or a Google Nest, check the app to confirm the device is online and that no scheduled hold or vacation mode is overriding the heat setting. These thermostats communicate over Wi-Fi, and a router reboot or a firmware update overnight can sometimes leave the thermostat in an offline state where it stops sending signals to the furnace. Also inspect the thermostat's wiring connections if you're comfortable removing the faceplate — the R (power), W (heat), G (fan), and C (common) wires should all be seated firmly in their terminals. A loose W wire is a surprisingly common cause of a furnace that has power but never starts. Finally, replace the batteries if your thermostat uses them; a low-battery condition causes erratic or absent signaling on many models even when the display still appears lit.

Check 2: The Furnace Power Switch and Circuit Breaker

Every forced-air furnace in Canada is required to have a dedicated disconnect switch within sight of the unit — it looks exactly like a standard wall light switch and is almost always mounted on the furnace itself or on the wall beside it. This switch gets turned off by plumbers, HVAC technicians, and homeowners doing seasonal maintenance, and then forgotten. Before you do anything else at the furnace itself, verify this switch is in the ON position. While you're at the unit, also check that the furnace door panels are fully seated. Nearly every furnace manufactured in the last 20 years includes a door interlock switch — a small push-button switch that depresses when the blower door is closed — and if that door is slightly ajar, the interlock cuts power to the entire control circuit. Even a 2–3mm gap is enough to defeat the switch.

At your electrical panel, locate the furnace breaker — it is typically a 15-amp single-pole breaker labelled 'furnace' or 'air handler.' A tripped breaker sits in a middle position rather than fully on or fully off. Reset it by pushing it firmly to OFF and then back to ON. If the breaker trips again within a few seconds of startup, stop — that indicates a real electrical fault (likely a shorted inducer motor or control board) that requires a licensed electrician or HVAC technician. Do not keep resetting a repeatedly-tripping breaker. Also check the panel for a separate breaker dedicated to the humidifier or air handler; in some installations these are separate circuits, and a tripped humidifier breaker can confuse diagnostics because the furnace will run but the air will be extremely dry, leading some homeowners to believe the furnace isn't working properly.

Check 3: The Furnace Filter — The Silent Killer of Heat Exchangers

A clogged air filter is the single most common cause of furnace shutdown across all Canadian provinces. When the filter becomes so blocked with dust, pet hair, and debris that airflow drops below design minimums, the heat exchanger overheats and the high-limit safety switch trips, cutting power to the burners. The blower may continue to run for a few minutes after a limit trip in an attempt to cool the heat exchanger down, and then the furnace tries to restart — and trips again on the same overtemperature condition. This repeating cycle is called short cycling on high limit, and it can damage the heat exchanger over time through repeated thermal stress. Natural Resources Canada recommends checking your filter monthly during peak heating season and replacing it every 1–3 months depending on filter type and household conditions.

To check the filter, locate the filter slot — usually at the base of the furnace where the large return air duct connects, or inside the blower compartment itself. Slide the filter out and hold it up to a light source. If you cannot see light through it, or if the filter has visible grey or brown buildup, replace it immediately. Standard 1-inch fibreglass filters cost $2–$8 CAD each at any hardware store; thicker 4-inch media filters run $25–$50 but last 6–12 months. After replacing the filter, reset the furnace by turning the power switch off, waiting 30 seconds, and turning it back on. If the limit switch tripped from overheating, it may need 15–20 minutes to cool before the furnace will restart normally. Some high-limit switches are automatic reset and some are manual reset, requiring you to press a small button on the switch itself, which is visible inside the furnace cabinet after removing the blower door.

Check 4: The Ignitor — The Most Commonly Failed Component

The hot surface ignitor (HSI) is a fragile silicon carbide or silicon nitride element that heats to roughly 900–1,100°C to ignite the gas burners. It is the most frequently replaced component on Canadian gas furnaces. When an ignitor fails, the ignition sequence typically plays out like this: the inducer motor starts, the pressure switch closes, the control board sends 120V to the ignitor for 15–30 seconds, but the ignitor doesn't glow, so the gas valve never opens, the flame sensor never detects a flame, and the control board locks out after 2–3 failed ignition attempts. The diagnostic clue is a furnace that runs the inducer for 30–60 seconds, goes quiet, waits a few minutes, and then tries again — typically 2–3 times before locking out with a fault code.

You can visually inspect the ignitor by removing the burner access panel (the upper door on most furnaces) and looking into the burner cavity with a flashlight. A good ignitor will glow bright orange-red within 15–20 seconds of a startup attempt; a failed one stays dark. Do not touch the ignitor element with bare fingers — even the oils from skin can create a hot spot that accelerates failure. Replacement ignitors cost $35–$90 CAD for the part and are model-specific; installation typically takes an HVAC technician 30–60 minutes. If your furnace is under 10 years old and the ignitor has already failed once, ask your technician about whether the control board may be delivering incorrect voltage — some boards deliver low voltage that causes ignitors to fail prematurely. For older furnaces approaching the end of their service life, a failed ignitor can be a good prompt to explore your upgrade options using our furnace comparison tool.

Check 5: The Flame Sensor — A $10 Part That Disables the Whole Furnace

The flame sensor is a thin metal rod, typically stainless steel with a porcelain insulator, positioned so that its tip sits directly in the burner flame. Its job is to verify that ignition has actually occurred — it does this by passing a tiny DC microamp current through the flame itself (ionized combustion gases conduct electricity) and back to the control board. When the sensor is coated in oxidation, carbon, or silica deposits, it can no longer pass sufficient current, so the control board sees no flame and shuts the gas valve within 2–5 seconds of ignition — even when the burners are clearly lit. The result is a furnace that lights, runs for 2–5 seconds, shuts off, tries again 2–3 times, and then locks out with a fault code that typically indicates failed to sense flame or flame rollout.

Cleaning a flame sensor is one of the few furnace maintenance tasks that a mechanically capable homeowner can perform safely with the furnace powered off. Locate the sensor rod — it's a single wire running to a small L-shaped bracket at the front of the burner assembly. Remove the single mounting screw, slide the sensor out, and gently rub the tip with fine steel wool or a Scotch-Brite pad until the metal is bright and clean. Reinstall it, ensuring the tip is positioned in the flame path, reconnect the wire, and test. This cleaning costs nothing and fixes a substantial percentage of short-cycling complaints. If the furnace still short-cycles after cleaning, the sensor itself may have a hairline crack in the porcelain insulator — replacement sensors cost $15–$40 CAD. Technicians performing annual maintenance should clean this component as a standard step; if yours doesn't, it's worth asking about as part of your regular maintenance plans.

Check 6: Pressure Switches, the Inducer Motor, and Blocked Flue Vents

Modern high-efficiency furnaces (80%+ AFUE) use a draft inducer motor to pull combustion gases through the heat exchanger and out the flue before the burners fire. Pressure switches — small round diaphragm switches mounted on the inducer housing — verify that the inducer is actually creating the correct negative pressure before allowing the gas valve to open. If the inducer is running but a pressure switch stays open, the furnace will not progress to ignition. Common causes include: a failed inducer motor (listen for grinding, humming with no spin, or complete silence when it should be running), cracked pressure switch hoses (the small rubber or silicone tubes that connect the switch to the inducer housing), a stuck-open or stuck-closed pressure switch diaphragm, or a blocked condensate drain on a 90%+ AFUE furnace causing water to back up into the pressure switch hose.

The flue vent is another critical item to check from outside the home. High-efficiency furnaces vent through plastic PVC pipes (typically 2–3 inch diameter) that terminate through a side wall rather than the roof. These terminations sit at or near ground level in most Canadian homes, making them vulnerable to obstruction by snow drifts, ice buildup, bird or rodent nests, and debris. A blocked flue creates a pressure differential that immediately trips the pressure switch and prevents ignition. After a heavy snowfall in Saskatchewan, Ontario, or anywhere in Atlantic Canada, walk around the perimeter of your home and confirm both the exhaust and intake pipes (usually side-by-side through the same wall) are clear and unobstructed. Clearing a blocked vent costs nothing and takes two minutes — not clearing it and calling a technician costs $150–$250 CAD for a service call. Our emergency furnace help page has guidance for after-hours situations.

Check 7: Gas Supply, Fault Codes, and When to Call a Professional

If the furnace passes all the checks above, verify that gas is actually reaching the appliance. Check whether other gas appliances in the home — the range, water heater, or gas fireplace — are operating normally. If none of them work, the issue is upstream: either your gas meter's emergency shutoff has been tripped (this can happen during a minor earth tremor or a flood in BC or Alberta), your gas company has interrupted service, or a manual shutoff valve in the gas supply line has been closed. The shutoff valve on the gas line feeding the furnace is usually a yellow-handled ball valve within a metre of the unit — confirm it is parallel (open) to the pipe, not perpendicular (closed). Never attempt to relight a pilot or force open a gas valve if you smell gas — evacuate immediately and call your gas utility from outside the home.

Most furnace control boards manufactured after 2005 store fault codes as a sequence of LED blinks on the board itself — typically visible through a small sight glass on the blower door panel without opening the furnace. The blink pattern (e.g., three blinks, pause, two blinks) corresponds to a specific fault code printed on a label inside the furnace door. Read this code before calling a technician — it tells you exactly which safety switch tripped and in what sequence, saving diagnostic time and potentially lowering your service bill. For furnaces that are more than 15–20 years old, a pattern of recurring faults — especially high-limit trips, cracked heat exchanger warnings, or repeated inducer failures — signals that repair costs are approaching replacement value. At that point, it makes sense to explore high-efficiency furnaces or use the furnace size calculator to spec a correctly sized replacement, and to look into available rebate programs through your provincial utility before committing to an expensive repair.

Checks 8 and 9: Condensate Drain Blockages and Control Board Failures

High-efficiency condensing furnaces (90%+ AFUE) extract so much heat from combustion gases that the exhaust condenses into liquid water — typically 2–5 litres per day in a Canadian winter. This condensate drains through a plastic trap and hose into a floor drain or condensate pump. When the drain line becomes blocked by algae, mineral deposits, or debris, water backs up into the secondary heat exchanger and triggers a float switch or pressure switch that prevents ignition. Signs of a blocked condensate drain include standing water around the base of the furnace, gurgling sounds during operation, and fault codes indicating a pressure switch fault on a furnace where the inducer motor sounds healthy. To clear the blockage, disconnect the drain hose, blow compressed air through it toward the drain (not toward the furnace), flush it with a dilute bleach solution (1 part bleach to 16 parts water), and reconnect. Annual maintenance plans should include condensate drain cleaning as a standard item.

Control board failure is the final check and the diagnosis of last resort. The furnace control board orchestrates the entire ignition sequence — reading thermostat signals, energizing the inducer relay, timing the ignitor pre-heat, opening the gas valve relay, monitoring the flame sensor signal, and managing safety lockouts. Boards can fail due to power surges, lightning strikes, moisture ingress, or simply component aging. Signs of a failed board include: the board's LED is completely dark even with confirmed power at the furnace, the board shows fault codes that don't correspond to any detectable hardware fault, or the board behaves erratically. Replacement boards cost $200–$500 CAD for the part and typically $150–$250 for installation labour. On a furnace over 12 years old, this repair cost approaches the threshold where replacement becomes a smarter financial decision. Use our efficiency savings calculator to model the operating cost difference between your current unit and a new 96% AFUE furnace — the annual savings in a cold Canadian climate frequently justify the upfront investment, particularly when combined with provincial rebates.