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What to Do When Your Furnace Blows Cold Air

Published May 24, 2016
By: Gary Sprague

Nobody wants a furnace that blows cold air, especially if the temperature outside is really chilly. Unfortunately, this is the sort of thing that only seems to happen on the coldest days of the year. So how do you troubleshoot this problem, aside from pulling out a stack of blankets? Before you make a call to an expensive technician, there are a few things you can check for yourself.

The Thermostat

Checking the thermostat may seem too easy, but sometimes it’s the most obvious problem that trips us up. Check to make sure that your thermostat fan setting is on AUTO, which will turn the blower off when the furnace is finished heating. If it is set to ON, the blower will run continuously, even when the furnace is no longer heating.

man in jacket adjusting his thermostat

While covering the easy problems, a furnace will blow cool air for a minute or two after it is first turned on, until the burners and ductwork are warmed up. So if your furnace is blowing cold air upon starting up, this is nothing to be concerned about. All that is required is a little patience. Even in today’s high tech world, heat isn’t instantaneous.

The Fuel Supply

Is there fuel in your oil or gas tank? Another simple thing to check, and one that is quite often the cause of the problem.

A furnace that isn’t getting fuel will not ignite and will blow only cold air. If you have a gas furnace, a lack of fuel will cause the pilot light to go out. This can be caused by something as simple as a strong draft. New furnaces are equipped with a spark ignitor or electronic ignition, which may require a technician to repair. But older gas furnaces can often be fixed by relighting the pilot.

Furnaces sometimes have written instructions on how to light the pilot, but if not you can light it by making sure the gas valve is turned to “Pilot”. Then push down the red reset button while holding a match to the pilot opening for several seconds, until the flame burns brightly. If there is still a problem with the pilot light going out, there may be a problem with the thermocouple, which will require a service technician.

If you have an oil furnace, the burner or the fuel line may be clogged. Usually in this situation the burner will go off on reset, but not always. If not, the furnace’s fan can continue to blow, even if there is no fuel. The first thing to do is to press the red reset button on the burner and hold it for a couple of seconds. This should start your furnace. If the furnace does not restart, a technician will need to be called.

Turn the Furnace Off and On

New furnaces are controlled by a computer board, and as with all computers, there can sometimes be glitches. This can cause the blower to run even if the fuel supply has been impacted. In this case, the problem can often be solved by locating the furnace’s red Off-On switch (there should be one on the furnace and one near the door to the basement or furnace room) and turning it off. Wait several seconds, and then turn it back on. Often a simple restart is all that’s needed.

Change the Filter

A dirty air filter can sometimes cause the furnace to overheat. In this situation, a limit switch will turn the burners off, but the blower fan will continue to run to cool the furnace down. Most professionals recommend changing a filter every 1-2 months. So, if like most of us you’ve gone far longer than that, now’s the time to change it. Instructions to change the filter can be found in your manual, but it’s a fairly simple procedure.

First, turn off the furnace. Then locate and remove the service panel. The filter should be located near the blower fan. Slide the old filter out, and then slide the new one in. There should be an arrow on the filter showing the proper direction of air flow, which is facing the blower. Close the service door and turn the furnace back on.

Leak in the Ductwork

A leak in your ductwork may cause the warm air that your furnace produces to be cool by the time it reaches the register. Screws or tape at the seams of the ductwork may come loose, causing the ductwork to separate at the seams. Over time, duct tape can start to peel. This problem can be fixed by running a new layer of duct tape around the leaking seam.

If you still can’t get the furnace to blow warm air after going through these steps, you may need to call a service technician to get your furnace running properly. And remember, prevention is a great way to stop the problem before it starts. This includes making sure you have enough fuel, changing your air filter regularly, and having your furnace cleaned every year or two.

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