Why Your Furnace Might Be Blowing Cold Air

When you turn your furnace on during a cold day, you naturally expect it to blow warm air. Sometimes it doesn't. Sometimes you get an unpleasant blast of cold air instead. That blast of cold air is especially frustrating when the weather outside is cold, too. Find out why your furnace might be producing cold instead of hot air.

Problematic Thermostat

The first step you should take is to check your thermostat to make sure it's set to "heat" instead of "cool." If it's in "auto" mode, check what the temperature it starts cooling at — it may be too high for the current day. Likewise, make sure the fan isn't running without heat. If all those things are in order, the thermostat itself may be failing to take the temperature correctly.

Clogged Air Filter

Most furnaces draw air from the outside, heat it up, and circulate it through your house. To keep the interior of your furnace clean, it draws the air through a filter to remove dust and other debris. When the filter becomes clogged, your furnace may work overtime and overheat. Thus, you'll get only cold air. Experts recommend changing the filter every 1–3 months depending on your furnace usage.

Dirty Flame Sensor

The flame sensor is part of the burner assembly. Its job is to ensure that whenever the gas valve is open, a flame is lit. If the sensor doesn't detect a flame, it'll shut down the unit, which may leave your fan blowing with no heat. Carbon can build up on the sensor so it fails to sense the flame even when it's present.

Faulty Pilot Light

Similarly, the flame itself may be to blame. Maybe the flame sensor is working just fine and there really is no flame. In older furnaces, you don't have a flame sensor at all. So, your pilot light could be out while the furnace is left blowing cold air. You can relight the pilot light in either scenario. If it doesn't stay lit, you may have an issue.

Blocked Gas Valve

The furnace needs adequate fuel to run. Many furnaces run on natural gas. If you're getting cold air instead of hot, your gas valve may be closed. Check the valve to that end. If it's not closed, you may have a block or other issue — that's a job for professional repair.

Clogged Condensate Lines

New, high-efficiency furnaces have condensate lines. They remove the water that the heating process creates. When the lines become clogged, the situation triggers a sensor that prevents the burner from lighting. Thus, you only get cold air.

If you're getting cold air from your furnace, call local heating services to address the issue.