FurnacesWhy Is My Furnace Blowing Cold Air?


The last thing you want on a chilly winter day is to deal with your furnace blowing cold air. Sadly, it can happen, but there could also be a simple fix that you can even take care of yourself. By learning about some common causes and solutions for this problem, you can keep your home nice and toasty through the coldest days. Here are some common reasons for cold air coming out of your furnace.

Common reasons for a furnace blowing cold air

1) Thermostat’s fan setting
2) Air filter
3) Pilot light
4) Condensate line

1) Thermostat’s fan setting

Thankfully, this could be a simple fix. It’s possible that the thermostat’s fan setting is set to ON. ON means that the blower will run 24/7. This means it will even run when the furnace is not heating, hence the cold air. We also cover this and other great furnace tips in this post: Your 7-Step Checklist For Fall Seattle Furnace Maintenance.

Fix: You can easily fix this by switching the fan setting to AUTO. In AUTO mode, the blower will only run when the furnace is heating.

2) Air filter

Keeping your filter fresh is a great habit for many reasons. A dirty furnace filter will decrease airflow and strain your system. When your system works harder, its parts will wear out more quickly. A dirty air filter also obstructs airflow over the heat exchanger, which causes it to overheat. When this happens, your furnace can activate a high limit switch, causing the furnace burners to shut off so the heat exchanger doesn’t crack. 

Fix: You can fix this issue by turning off the furnace, checking the filter, and changing it if it’s dirty.

3) Pilot light

If the pilot light isn’t lit, the furnace burners won’t light either.

Fix: You’ll need to go through the following steps to check your pilot light.

  1. Turn off the furnace.
  2. Find the pilot light assembly and the reset switch. These should both be located near the bottom of the furnace, but you can check your furnace’s manual if you’re having trouble finding them. Your reset switch should have a knob with the following settings: PILOT, OFF, ON. Turn the knob to OFF.
  3. Wait about 5 minutes.
  4. Turn the knob to PILOT while pressing down. This will restart the flow of gas to the pilot. While you’re pressing down on the knob, hold a lighter to the pilot opening until the flame is lit. Once it’s lit, it should be blue.
  5. Turn the knob to ON. Your furnace should be fired up now.
  6. Head to the thermostat and turn it back to HEAT. You should have hot air now!

4) Condensate line

Another reason for a furnace blowing cold air could be a blocked condensate line. When some furnaces run, they produce water that is drained out of a condensate line. But if the line gets blocked, water gets back up into the furnace. This causes an overflow kill switch to shut the furnace down. There could be a few things blocking the line, including dirt, mold, or even ice.

Fix: You’ll need to follow these steps to unclog the condensate line.

  1. Turn off power to the furnace at the breaker.
  2. Find the condensate drain pan.
  3. Remove water that’s in the pan with a wet/dry vacuum.
  4. Remove the pan and clean it with mild dish soap and water. Once it’s clean, replace the pan.
  5. Find the condensate drain pipe exit outside your home.
  6. Use your wet/dry vacuum to clear clogs at the opening.
  7. Turn the power back on at the breaker.


A furnace blowing cold air is the last thing you want to deal with when temperatures outside plummet. Your furnace shouldn’t feel like an air conditioner! If you know what to check, there’s a chance you can deal with the issue yourself. Some are simple fixes, and some have a few steps. If you’re still not sure why there’s an issue after you’ve checked all these parts, you can always contact us. Our experts will be able to pinpoint what the issue is and repair it so you can put your feet up in a warm home again.


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