Furnace Not Heating: Who Should You Call?
Urgency: UrgentUpdated 2026-08-31
Short answer
Check the thermostat setting and batteries, confirm the furnace switch and breaker are on, and replace a clogged filter. If the furnace still will not produce heat, call a licensed HVAC technician. In an Ontario winter a no-heat call is treated as a priority service, and most companies will attend the same day.
Who to call: A licensed HVAC technician — gas work in Ontario requires a certified gas technician.
What to do right now
- 1Set the thermostat to Heat and at least 3°C above room temperature.
- 2Confirm the furnace switch (it looks like a light switch near the unit) is on and the breaker has not tripped.
- 3Pull the filter — if daylight will not pass through it, replace it and retry.
- 4If you smell gas at any point, leave the house and call your gas utility's emergency line before anyone else.
Most likely causes
- Clogged air filter
- The single most common cause. Restricted airflow overheats the heat exchanger and the limit switch shuts the burner down.
- Failed igniter or flame sensor
- The furnace tries to start, runs for a few seconds and shuts off — usually a dirty flame sensor or a cracked hot-surface igniter.
- Blocked exhaust or intake
- Snow, ice or a bird's nest in the sidewall PVC vents locks the furnace out for safety.
- Thermostat or control board fault
- Dead batteries, a loose C-wire, or a failed board that never signals for heat.
What it typically costs in the GTA
$120–$180 for a diagnostic visit. $200–$450 for an igniter or flame-sensor replacement. $600–$1,500 for a blower motor or control board. A full furnace replacement runs $4,500–$9,000 installed in the GTA.
Ranges are typical market pricing for the Greater Toronto Area and vary with access, age of the home, and time of year. Always get the scope in writing before work starts.
Furnace running but blowing cold air
If air is moving but it is cool, the blower is working and the burners are not. That points to ignition, gas supply, or a safety lockout — not the fan. Watch the status light on the furnace door: most units flash a diagnostic code, and the legend is printed on the inside of the panel.
Furnace short-cycling
Turning on and off every few minutes almost always means overheating from restricted airflow: a dirty filter, closed registers, or a blocked return. Left alone it cracks the heat exchanger — a repair that usually costs more than the furnace is worth.
What to have ready when you call
Have the furnace brand, model number, approximate age, the diagnostic flash code, and a description of what changed. It cuts diagnostic time and often the bill.
Get the right pro on it
Describe the problem in your own words. We’ll work out which trade you need and connect you with suitable local pros. Free, no obligation.
Frequently asked questions
- Can I fix a furnace myself in Ontario?
- You can change filters, reset breakers and clear vents. Anything involving the gas train, burners or venting must legally be done by a certified gas technician.
- Why does my furnace work sometimes?
- Intermittent heat usually points to a marginal flame sensor, a failing igniter, or a thermostat with a weak connection — all things a technician can confirm in one visit.
- How often should a furnace be serviced?
- Once a year, ideally in early fall before the first cold snap when technician availability is best.