There is nothing more frustrating than a freezing night only to find your vents are pushing out cold air. However, before you look for a fix, you must ensure your home is safe.
Below is a breakdown of the most common causes, what you can safely check in a few minutes, and when it’s time to call for licensed service.
Prefer to skip the troubleshooting and book a licensed tech? Visit our Repair Service page: Furnace Repair in Toronto or Call us at 1-855-795-1199.
Part 1: Quick Safety Checks (safe for homeowners)
Step 1: The Safety “No-Go” Check
Before touching your furnace or thermostat, look for these “red flags.” If any of these are present, stop immediately, leave the house, and call emergency services or a pro:
- The Smell of “Rotten Eggs”: A clear sign of a natural gas leak.
- Chirping CO Alarm: Carbon Monoxide is odorless and deadly; never ignore your detector.
- Visible Soot or Scorch Marks: This indicates dangerous combustion or venting issues.
- Violent Noises: Loud banging or screeching means a mechanical part is failing and could be dangerous.
Step 2: Check Your Thermostat & Fan Settings
If your home is safe, check the “brain” of your HVAC system:
- The “Auto” vs. “On” Rule: If your fan is set to “ON,” it blows air 24/7, even when the furnace isn’t heating. Switch it to “AUTO” so it only blows when there is actual heat.
- The Temperature Gap: Ensure the thermostat is set to “HEAT” and the target temperature is at least 3°C above the current room temperature.
If those quick checks don’t solve it, the issue is usually inside the heating sequence.
Step 3: Inspect Your Air Filter
- The Issue: A clogged filter is the #1 reason furnaces blow cold air. It restricts airflow, causing the system to overheat and shut off the burners as a safety precaution, while the fan keeps running to cool the unit down.
- The Fix: If the filter looks grey or furry, replace it. In the GTA, we recommend a fresh filter every 30-90 days during the winter.
Part 2: When the Issue is Internal (Professional Repair)
If Step 1-3 didn’t restore the heat, the problem is likely an internal component failure. In Ontario, TSSA regulations require a licensed technician to handle these gas-related components.
| What you feel/hear | Likely Technical Cause | Our Licensed Solution |
| Heat starts for a few seconds, then dies. | Dirty Flame Sensor | We clean or replace the sensor to ensure a stable “flame signal.” |
| Clicking sounds, but no fire. | Igniter Failure | We test electrical resistance and replace the igniter with a brand-authorized part. |
| Water pooling near the furnace. | Blocked Condensate Drain | We clear the lines and ensure the drainage is winter-proofed for the GTA climate. |
| Furnace starts and stops rapidly. | High-Limit Switch Trip | We diagnose airflow pressure to prevent the heat exchanger from cracking. |
Reliable Furnace Repair in the GTA
At Mas Air Home Comfort, we specialize in fast, honest diagnostics across Mississauga, Oakville, Vaughan, and the entire GTA. As an authorized dealer, we ensure your furnace is repaired safely and to manufacturer standards, keeping your warranty and home insurance valid.
Restore Your Home’s Comfort Today
- For Repairs & Emergency Service: +1 (855) 795-1199
- For New Installation Estimates: +1 (647) 360-6623
- Book Online: Request a Free Estimate or Service Visit
Related reading (keep troubleshooting smart)
- Furnace Not Heating — thermostat set but house still cold.
- Furnace Won’t Start — no power/ignition lockout scenarios.
- Noisy Furnace Operation — what different sounds mean and what to check first.
- Uneven Heating — balancing, duct/return issues, and room-by-room fixes.
FAQs
Most furnaces have a short blower delay at startup: the burners light and heat the exchanger for 30-60 seconds before the blower kicks in. This is normal. If cold air blows for more than 90 seconds after the thermostat calls for heat, you likely have an ignition failure or limit switch issue.
Technically yes, but we don’t recommend it. Flame sensors are fragile ceramic rods that can be damaged easily. Additionally, it might not be the sensor: the real problem could be a weak igniter, bad gas valve, or cracked heat exchanger. A licensed technician can test the flame sensor’s microamp signal to confirm it’s the problem.
No. If your furnace is repeatedly failing to ignite or shutting down mid-cycle, there’s a safety control detecting a problem. Resetting it repeatedly can cause gas buildup, damage the igniter, or hide serious issues like a cracked heat exchanger. Try resetting once; if it fails again within 24 hours, call a technician.
Repair costs in the Greater Toronto Area typically range from:
Thermostat or filter issues: $0–$150 CAD,
Flame sensor cleaning: $150–$250 CAD,
Igniter replacement: $200–$350 CAD,
Pressure or limit switches: $250–$400 CAD,
Inducer or blower motor: $400–$800 CAD,
Gas valve or control board: $400–$900 CAD.
If your furnace is 15+ years old and the repair estimate exceeds $1,000 CAD, replacement is often the better long-term investment, especially with available rebates on high-efficiency models in Ontario.
