Why Is My Furnace Blowing Cold Air?

If your furnace is running but blowing cold air, the most common causes are a thermostat set to the wrong mode, a dirty filter choking airflow, or an ignition or flame-sensor problem stopping the burners from staying lit. Some of these you can check and fix yourself in a few minutes; others — anything involving the gas burners, ignition or a safety shutoff — need a Technical Safety BC-licensed technician. Here's how to work through it, starting with the simplest fixes, so you know whether you're looking at a quick reset or a service call in your Kelowna home.

Why is my furnace running but blowing cold air?

Work through these from easiest to most involved:

• Thermostat set to "on" instead of "auto." On "on," the fan runs constantly — even between heating cycles — so it blows room-temperature air. Switch it to "auto" so the fan only runs when the furnace is actually heating.
• Thermostat settings. Confirm it's set to "heat" and the target temperature is above the current room temperature.
• Dirty filter. A clogged filter restricts airflow, which can cause the furnace to overheat and shut off the burners as a safety measure — leaving the fan blowing cold. Replace a dirty filter and see if it recovers.
• Ignition or pilot problem. If the burners aren't lighting or staying lit, you'll get air with no heat. Modern furnaces use electronic ignition; older ones a pilot. Both should be diagnosed by a technician.
• Dirty flame sensor. A grimy flame sensor can cause the furnace to light briefly, then shut down — a very common, fixable cause of intermittent cold air.
• Overheating safety shutoff. Restricted airflow or a failing component can trip a safety limit switch, cutting the burners.

What can I safely check myself?

Homeowners can safely handle the first three: set the thermostat to "heat" and "auto," confirm the temperature setting, and replace a dirty filter. Those resolve a surprising share of cold-air calls. What you should not attempt yourself is anything involving the gas burners, ignition system, flame sensor or safety controls — that's licensed gas work, and getting it wrong is both a safety risk and, in BC, not permitted.

When should I call a technician?

If the thermostat and filter checks don't bring the heat back, it's time to call. Persistent cold air usually points to an ignition, flame-sensor or safety issue that needs proper diagnosis. If your furnace is also older and this isn't the first problem, it may be worth reading our guide on the signs your furnace needs replacing before pouring money into repairs.

For a diagnosis and repair, see furnace repair in Kelowna — we'll find the cause and get your heat back on, and tell you honestly if repair or replacement is the better call.