Why Is My Furnace Blowing Cold Air?

If your furnace is blowing cold air, it could be a sign that there's a problem with your heat. It could be something simple and inexpensive to fix or a more involved issue that means a relatively expensive repair bill.

Let's look at some of the most common answers to the question, "Why is my furnace blowing cold air?" A furnace blows cold air due to:

While this list is not exhaustive, it covers the most common reasons your vents might still have solid airflow but no heat. This guide explains why these issues affect your heat and when to call in a professional.

A clogged air filter limits the amount of air flowing through your furnace. It slows air down, sometimes creating low air pressure at your vents. Air coming out might be warm, but if it's low pressure, there won't be enough warm air to get your home to a comfortable temperature.

If the air filter is partially clogged, it may only create drag in some places. One room might warm up properly, while another is cold. In most cases, you can and should change the air filter yourself. Many air filters need changing every 30 days, so keep a supply on hand and make changing the air filter your first step any time you notice a problem with your heat.

Leaking or sagging ducts can also impact airflow and where your heated air winds up. If your ducts leak, they may be wasting the heated air generated by your furnace, leaving you with cold air coming from some of your vents.

Do a quick visual inspection of your ducts. Do you notice any rust or spots where the mounting is loose? Run your hand along the seams to see if you feel any air exiting the ducts. If you think the ducts are the issue, that's often a job for the pros. Have an HVAC professional take a look and make recommendations about duct sealing.

The thermostat sensor is what tells your furnace when to turn on. If it's not working properly, it could mean your blower is running, but your furnace is off, essentially turning your heating system into a whole-house fan. The last thing you want is for the fan to run full blast during the middle of winter, so if the problem is the thermostat sensor, you'll need to get it repaired as quickly as possible.

Skipping oil changes can leave your car overheating on the highway. Similarly, a lack of maintenance can leave your furnace working much harder to do little heating. Maintenance involves testing major systems, cleaning your unit, checking for malfunctions, and lubricating any moving parts. Missing one or more maintenance visits can put a lot of extra stress on your furnace.

You should have maintenance done at least annually, though if you use your heat more than average, you might need two maintenance visits per year.

The blower motor is what distributes heated air from your furnace throughout your home. If it's not working properly, it might turn on when the furnace isn't heating, blowing cold air out of your vents. Without any heat, the air is at the same temperature as the outside air, cooling your house instead of heating it. If the blower motor doesn't turn on when the furnace is running, that can lead to overheating, triggering the automatic shutoff.

Either way, a malfunctioning blower motor is bad news regarding home comfort. Fortunately, it's relatively straightforward to repair and can usually be done in a single day. If you need a new blower motor, you can often be warm and cozy the day after you call in an HVAC pro.