What to Do When Furnace Blows Cold Air

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It’s winter, it’s cold, you go to switch on your furnace and all you get is cold air blowing out. Surely that makes no sense! A furnace doesn’t blow out cold air.
Don’t panic! It’s not an uncommon phenomenon and after a few simple checks you could be back on-track to a warm and cosy home.

Identify the cause.

Why is my furnace blowing cold air?

1. Your thermostat is set to “fan”

Before you jump to any conclusions about the furnace, the first and easiest check to make is the with thermostat. It could be as simple as the thermostat fan being set to “On” instead of “Auto” and that’s why the furnace isn’t warming up. This is a common mistake and easy to do so it’s worth checking.

2. Your vents aren’t open

If the thermostat isn’t the issue, check that the vents in your home are all open and allowing heat into the rooms.

3. Your air filter is clogged

Some people aren’t actually aware their furnace has an air filter, so it’s not an uncommon problem for it to become dirty and clogged.

A clogged filter can cause overheating which kicks in a safety control known as the limit switch. This will shut off the burners but won’t stop the fan running as this helps to cool the furnace and help prevent any damage.

It’s important to clean the air filter regularly and replace when needed. It’s generally recommended to replace 1-2 inch filters every three months, 4 inch filters every six months and 5 inch filters every 12 months.

4. You have serious duct problems

Sometimes it can seam like the furnace isn’t working properly, but in actual fact the cooling is coming from leaks in your ductwork. This can happen during the winter when cold air from the attic or your crawlspace enters the ducts; this is more likely to be a problem in older homes. If duct problems are suspected then your best bet is with an inspection. It should then be possible to provide a sealer to patch the leak and prevent cold air from blowing through your ducts.

5. You have ignition problems

If the furnace can’t light your fuel supply, you’re not going to get any heat. If you’ve ruled out other possibilities then you may have an ignition problem. With older furnaces this can be visible from a pilot light that’s gone out. If you have a modern furnace then it probably has an electric ignition which can be harder for non-professionals to diagnose.

Never try to light a furnace manually, this is not safe.

6. Your condensate drain lines are clogged

If you have a more modern, high-efficiency furnace then you may have condensate drain lines which help to remove water generated from the heat produced. If these lines become blocked then water can turn back up to the furnace a safety switch activates to stop the burners from lighting. It it rarely possible to spot the blockage yourself and be able to remove it. To be safe and sure it’s best to call a professional furnace specialist.

7. Your flame sensor is dirty

A common cause for your furnace appearing to blow cold air is a dirty flame sensor. Dirt that covers the flame sensor causes the burner to constantly switch on and off. This can be fixed by cleaning the sensor though sometimes a replacement is required.

8. Your furnace is running it’s defrost cycle

Some furnaces (especially those backed by heat pumps) have a defrost cycle that comes in after they’ve been running for some time. When the temperature outside is too cold on the outdoor unit’s heat exchanger then the moisture in the air freezes. To resolve this the furnace enacts a defrost cycle which can temporarily cause it to stop heating. Not all devices stop working during the defrost cycle, but some do so it’s worth checking if you have any documentation for your furnace. If your fans operating but the furnace isn’t heating then you’re going to get cold air not warm air.

How long the defrost cycle goes on for depends on the type of model, and the continual temperature outside.

Don’t fall victim to scam companies who sell you a solution to this problem, there isn’t one, it’s just a part of the process some furnaces go through and isn’t designed to be “fixed”. If the problem persists frequently then you may have an issue with your heater.

Other Causes

It’s not possible for homeowners to test all potential causes themselves. If you’ve done the above checks then you may need to schedule a professional furnace inspection and repair.