Few things are more frustrating on a cold Ontario night than feeling cold air coming out of your vents. The good news: the cause is often simple. Here are the most common reasons a furnace blows cold air, what you can safely check yourself, and when it’s time to call a technician.
1. The thermostat fan is set to ON
If your thermostat fan is set to ON instead of AUTO, the blower runs all the time, even when the furnace isn’t heating. Between heating cycles, it moves cool air through the vents.
What to do: set the fan to AUTO and the system to HEAT. Make sure the temperature is set a few degrees above the room temperature.
2. The air at the start of a cycle is normal
When your furnace first turns on, the blower may push out cool air for a minute or so while the heat exchanger warms up. That’s normal. If the air stays cold after a few minutes, keep reading.
3. A dirty furnace filter
A clogged filter chokes the airflow. The furnace overheats, and a safety switch shuts the burners off to protect it. The blower keeps running, so you feel cool air. This is one of the most common causes we see.
What to do: check the filter. If it’s grey and dusty, replace it.
4. The furnace isn’t lighting (ignition or flame sensor)
Modern furnaces use an electronic igniter and a flame sensor. If the flame sensor is dirty, the furnace lights for a few seconds, then shuts off because it can’t “see” the flame. If the igniter has failed, the burners never light at all.
What to do: this needs a technician. A flame sensor cleaning or igniter replacement is a common, quick repair.
5. A blocked condensate drain or vent pipe
High-efficiency furnaces produce water and vent through white plastic pipes. If the condensate drain clogs, or snow and ice block the pipes outside, a safety switch stops the furnace from heating.
What to do: after a snowfall, check that the white pipes on the outside wall are clear of snow and ice. If they’re clear and the furnace still won’t heat, call us.
6. Gas supply or power problems
Check that the furnace power switch (it looks like a light switch, near the furnace) is on, and that the breaker hasn’t tripped. If you have other gas appliances, make sure they’re working.
If you ever smell gas, don’t touch any switches. Leave the house right away and call your gas utility’s emergency line from outside.
From the field
A Richmond Hill homeowner called us because their furnace was running but blowing cold air. We found a dirty flame sensor: the burners lit, but the furnace shut them off after a few seconds because it couldn’t detect the flame. We cleaned the sensor, tested a full heating cycle, and the heat was back on during the same visit.
When to call a technician
Call us if you’ve checked the thermostat, filter and vent pipes and the furnace still blows cold air, if it turns on and off repeatedly, or if you see error lights flashing on the furnace.
Air Net Heating & Air Conditioning has been repairing furnaces across Richmond Hill and York Region for 30 years. We service all major brands, and we quote an upfront flat rate before any work starts. Call 416-791-1609, open 8am to 8pm, 7 days a week.
Related service: Furnace repair in Richmond Hill