Why Is My Furnace Blowing Cold Air?

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Is your furnace blowing cold air? Here are some troubleshooting tips and simple DIY repairs for the most common causes for cold air from a furnace.

When winter rages and the temperature drops, your home’s HVAC system is more important than ever. There’s nothing worse than discovering your furnace blowing cold air, whether continuously or intermittently, in the middle of the most brutal stretch of winter

Luckily, there are a few things you can do to get your home warm and toasty again! We’ve rounded up six common conditions that cause furnaces to blow cold air, along with DIY fixes.

Incorrect Fan Settings

When troubleshooting, always start with the easiest possible solution. In this case, it’s the thermostat fan setting.

Check to see if the fan is set to ‘On.’ If it is, then it will blow air continually, whether or not the furnace produces heat. If your furnace always blows air, sometimes hot and sometimes cold, then it’s likely a fan setting issue.

This is more common on older thermostats with a physical fan control switch. These old-school switches can easily be bumped by an oblivious adult or a playing child. (I’ve seen this happen when moving furniture past a hallway thermostat.)

In this case, the fix is easy: Move the setting from ‘On’ to ‘Auto’ and see if that resolves the problem.

Dirty Filter

If your furnace or heat pump completes a full cycle but the air never quite warms up, it’s likely due to a clogged furnace filter.

This issue may seem counterintuitive. Most homeowners assume a blocked filter would prevent air from coming through the vents at all. But a furnace with a clogged filter still runs, and the blower still pushes air through the vents.

Of course, all that moving air comes from somewhere. The blower draws air through the path of least resistance. Normally, that’s through the filter and across the heating element. But if a dirty filter blocks it, the air will move through any crack or hole in the furnace body or vent system.

If your furnace is blowing cold air, check the filter to see if it should be replaced. After that, be sure to change your filter regularly and always use the properly-sized replacement. Clogged filters can cause any central heating system to blow cold air.

Pilot Light or Igniter Issues

Gas furnaces use a pilot light or igniter to spark natural gas into flame. If the pilot light goes out or the igniter fails, the heat will never kick on. Instead, the furnace will circulate cold air until the system recognizes the problem and shuts down. You’ll notice a strong, stable flow of cold air, then an abrupt stop.

Most newer furnaces don’t use a pilot light; they feature a solid-surface igniter that raises its temperature high enough to engage the natural gas. If your furnace has a bad igniter, it will need to be replaced. An extinguished pilot light can be relit; simply follow the directions on the furnace. If the pilot light keeps going out, it may need cleaning.

Dirty Flame Sensor

If your gas furnace begins to blow cold air, then cuts off abruptly, it could also be the flame sensor, which sits in the path of the flame. If the furnace cycles up but the sensor fails to detect the flame’s heat, it shuts down the furnace to prevent gas from invading your home.

It’s an essential safety feature, but the flame sensor can collect debris on its surface. If that happens, it will give a false alarm, ending the cycle before the circulating air can move from cold to warm.

Luckily, there’s a quick and easy fix. Turn off the power to the furnace, remove the flame sensor and clean it off with an emery cloth or fine steel wool. Use a light touch to avoid damaging the sensor, scraping along, not across, the surface. And never use a coarse grit sandpaper.

Once you get your furnace up and running again, order a replacement sensor. For most furnaces, they run around $10.

Flames sensors are found on gas furnaces, as well as heat pumps with a gas furnace as backup.

Clogged Condensate Line

All high-efficiency gas furnaces have a condensate line, as will any furnace connected to a central AC system. In all cases, this line allows excess water produced by the AC and furnace to drain away from the unit.

If dirt or debris block the line, water will back up. That will trigger a shut-off safety switch, or interfere with or damage the furnace. In either case, the furnace may not light. It will blow cold air, then shut down.

Fuel Issues

It’s possible a gas furnace won’t heat because it’s not getting a proper gas supply. This is another scenario where the furnace will blow cold air and then shut down.

Double-check the shut-off valves to make sure they’re open. If there’s still no gas reaching the furnace, it indicates an issue somewhere in that system. While it’s possible to troubleshoot gas lines, this kind of repair is best left to the pros. The risks are high and too many things could go wrong.

An electric furnace will blow cold or lukewarm air if one or more of the heating elements malfunctions. Replacing a heating element is complicated enough to warrant bringing in a pro.

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Dan Stout
Ohio-based freelance writer and author Dan Stout is a former residential remodeler, commercial site supervisor and maintenance manager. He’s worked on nearly all aspects of building and DIY including project planning and permitting, plumbing, basic electric, drywall, carpentry, tiling, painting and more. He also publishes noir fantasy thrillers, including The Carter Series, from Penguin imprint DAW Books.