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Why is my furnace blowing cold air?

Why is my furnace blowing cold air?

If your heater is blowing cold air you may be able to troubleshoot the problem yourself before having to call a professional. You’d be surprised how often the solution is an easy fix. The following are common reasons your heater is blowing cold air and what to do about it.


Your thermostat is the first thing you should check if your heater is not blowing warm air. Your thermostat should be set to “AUTO” and to the desired temperature. When your thermostat is set to “ON” the fan on your furnace will continually blow even when it isn’t heating the air. Also, if your thermostat is battery-powered, you should check to see if the batteries are still good. Even if the batteries are running low, it could cause your thermostat to act up.

Pilot light (electric heaters don’t apply)

A pilot light is a small flame that continually burns to ensure that your heater ignites easily. Sometimes the flame can get blown out for various reasons. If your pilot light is out you can relight it and if it stays on then you should have heat. If not, there may be a problem with the gas supply or with the thermocouple (the copper rod you see the pilot flame hitting). To check if there is an issue with the gas supply, turn on another gas appliance in your home to see if it works. If you don’t feel comfortable lighting your pilot light, you should call a professional to help.

Air Filter

If your air filter is dirty it can restrict airflow and cause your furnace to overheat. Safety controls on your furnace will shut off the burners. We recommend checking your air filter monthly and changing it when it is dirty. Keep in mind that repeated overheating can damage your furnace’s heat exchanger which is expensive to fix.  How to change your air filter.


Holes and leaks in your ductwork can cause cold air from your attic or a crawlspace to get into the ducts and cool the warm air that is being distributed throughout your home. Better insulation and sealing your ducts will fix this problem. Unfortunately, it’s not always easy to reach your ductwork or know what to look for. You may find that calling a professional to help is the best choice.

System Size

Size does matter when it comes to your heating system. Your heater should be specifically sized for your home. A furnace too big will heat your house too fast and never allow warm air to enter the insulation. This will cause the heater to turn on frequently throughout the day which increases energy costs. A furnace too small does not have enough power to heat your home. It will continue to run and jack up your energy costs because it won’t be able to reach your thermostat’s set temperature. A professional will be able to tell you if your furnace is not the right size for your house.

If your furnace is still blowing cold air we recommend calling a professional for help. Request service.