Losing heat in an Ontario winter is stressful, but the cause is often something straightforward. Our technicians find that roughly half of all no-heat service calls involve a simple issue the homeowner could have resolved before we arrived. Work through this list first.
8 Things to Check When Your Furnace Will Not Start
Check the Thermostat
Start here every time. Confirm the thermostat is set to HEAT (not COOL or FAN only), the temperature setpoint is higher than the current room temperature, and the batteries are not dead. Smart thermostat with a blank screen? The batteries or C-wire power supply may have failed. Replace batteries first.
Check the Air Filter
A completely blocked air filter is the single most common cause of furnace shutdowns. A clogged filter starves the furnace of return air, causing it to overheat and trip the high-limit safety switch. The furnace shuts off for safety, cools down, starts again, then shuts off again in a repeating cycle. Pull the filter out and hold it up to light. If you cannot see through it, replace it immediately.
Check the Circuit Breaker
Furnaces run on 120V electrical power in addition to natural gas. If the furnace breaker has tripped, the control board has no power and the furnace cannot operate. Go to your electrical panel and look for the furnace breaker. If it is in the middle position (tripped), push it fully to OFF first, then back to ON. If it trips again immediately, there is a wiring issue - call EcoFrost.
Check the Furnace Power Switch
The furnace has a dedicated power switch that looks exactly like a standard light switch, usually mounted on the furnace itself or on the wall nearby. It is often accidentally switched off by someone who did not realize what it was. Confirm it is in the ON position.
Check the Gas Supply
Locate the manual gas shut-off valve on the gas line leading to the furnace. The valve is open when the handle is parallel to the pipe and closed when it is perpendicular. Also confirm your home has gas supply - check if your gas stove works (if applicable). If no gas appliances work, there may be a supply interruption; call Enbridge at 1-866-763-5427.
Read the Error Code
On most gas furnaces, a LED on the control board blinks a pattern to indicate the fault. Count the blinks (a pause separates each sequence). Common codes: 4 blinks means overheating (usually a dirty filter), 7 blinks means ignition failure, 2 blinks means a pressure switch or venting issue. The wiring diagram inside the furnace door lists what each flash code means for your specific model.
Check the Exhaust Vent Outside
High-efficiency furnaces (96%+ AFUE) exhaust through PVC pipes that terminate at the exterior of your home, typically near the foundation. In Ontario winters, frost and ice can completely block the vent termination. Go outside and check both the exhaust (white PVC, may have steam) and intake pipes for ice or debris blockage. Clear any obstruction with your hand or warm water - never use a sharp tool on the PVC.
Try the Reset Button
Many furnaces have a manual reset button on the burner assembly (a red or yellow button). If the furnace attempted to start and failed (ignition lockout), pressing this button clears the lockout and allows a new startup attempt. You can find it on the burner assembly - check your owner manual for the exact location on your model. Only press the reset button once. If the furnace does not start after one reset attempt, further resets without diagnosis can damage components.
When to Call a Technician
Call EcoFrost if:
- You smell gas at any point during your checks
- The furnace shows a 5-blink or 6-blink code (gas valve or rollout switch fault)
- The CO alarm has activated
- The circuit breaker trips immediately after resetting
- The furnace starts, runs for a minute, then shuts down again repeatedly after you have changed the filter and cleared the exhaust vent
- There is no power to the furnace and all power checks appear normal
- The outdoor temperature is below -10C and you have not restored heat with the checks above
Preventing Future No-Heat Emergencies
- Change your air filter every 1-3 months during heating season. Set a calendar reminder.
- Book an annual furnace tune-up in September or October before the heating season starts.
- Keep extra replacement filters on hand so a filter change never requires a store trip at 2 AM.
- Test your thermostat batteries every fall. Many smart thermostats show a low-battery warning - do not ignore it.
- Clear snow and ice from the exterior vent terminations after heavy snowfalls.
- Install a CO detector on every floor and test it monthly.
Ready to take the next step?
Book Emergency Furnace Repair?Frequently Asked Questions
Why does my furnace start then stop after a few seconds?
My furnace turns on but no heat comes out - is it the blower?
How long does a furnace reset lockout last?
Can a furnace stop working because of cold weather?
EcoFrost Heating & Cooling
Toronto's Trusted HVAC Experts Since 2015
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