My Furnace is Blowing Cold Air: A Phoenix Tech Explains Why

During winter, people often ask us: “Why is my furnace blowing cold air?”

There are a variety of reasons this could be happening, like:

Below, we’ll explain these issues in more detail so you can determine what you need to do to get your heat back on.

If you notice that your furnace is blowing hot air at times and cold air other times, your fan is likely set to ON when it should be set to AUTO.

When your thermostat is set to ON, it means the furnace blower (the fan that blows warm air into your home) is running continuously...even when the burners are not running.

In simple terms, this means that your furnace is blowing air into your home constantly, even when that air hasn’t been heated.

The solution: Check your thermostat settings and ensure that the fan setting is turned to AUTO. Now the blower should only run when the burners are on.

If the blower still runs continually, there is another issue that an HVAC professional will need to solve.

Older furnaces use a permanently lit pilot light to ignite the gas for the burner. If the pilot light goes out, your furnace can’t heat any air.

The solution: Relight the pilot light. Instructions on your furnace should show you how to do this. If the pilot light keeps going out, you have another issue that a professional needs to solve.

Side note: Old furnaces waste tons of gas to keep the pilot flame lit. Consider upgrading to a newer, more efficient furnace that does not use a pilot light or to a heat pump that uses electricity.

Your furnace actually heats air via a part of the furnace called the heat exchanger. A heat exchanger is a group of metal coils that are heated. When air blows over the coils, the air picks up that heat.

Sometimes, the heat exchanger can get too hot and overheat. Your system can sense this overheating and will turn off the burners but keep the blower (fan) on to help the heat exchanger cool down.

If this is your issue, you’ll notice cool air blowing from your vents instead of heated air. You may also notice a sort of burning smell as well as a humming noise.

The solution: There are a variety of reasons that a heat exchanger can overheat, but typically anything that limits airflow over the heat exchanger can cause overheating. Our suggestion would be to check your air filter. If your air filter is clogged, it won’t pull in as much air as necessary to keep the heat exchanger from overheating.

If your air filter looks like the one on the right, it’s time to replace it.

However, if you replace your air filter and are still noticing cool air coming from your vents a few hours later, it’s time to call a professional. An overheated heat exchanger is not something you want to happen regularly, and a pro should be able to come take a look at your system and determine what the issue is.

Simply put, if there is no gas supply to your furnace, there will be no heat. Your furnace needs a consistent supply of gas to the burners to create heated air. But if the gas supply is off or weak, your pilot light won’t be able to light or stay lit, meaning you’ll have no heated air in your home.

The solution: First of all, ensure that the gas valve is turned on (make sure the valve lever is parallel/in line with the gas supply pipe). If your gas is on and you’re still feeling cold air coming from your vents, it’s time to call a pro.