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Furnace Blowing Cold Air? Do This

November 09, 2016

If your furnace is blowing cold air, there could be a million different things wrong with it.  But here are 4 common issues:

  • Thermostat setting is set to ON
  • The heat exchanger is overheating
  • The pilot light is out (old furnaces only)
  • Clogged condensate line (high-efficiency furnace only)

Thankfully, you can solve some of these issues through easy DIY solutions.

Check the thermostat

Does your furnace blow hot air sometimes and cold air other times?

The problem may be the thermostat fan setting. When set to ON, your furnace will deliver air 24/7, even in between heating cycles. So when the furnace isn’t heating that air, it will come out the vents as luke warm or cold.

Do this: Make sure that your fan setting is set to AUTO. Now your furnace will only blow heated air.

If the thermostat isn’t the issue, then...

Check the air filter

Your air filter may be so dirty that it caused your furnace to overheat and shut down the parts that heat the air.

Your see, your air filter protects your furnace’s internal parts from dust and dirt. But once that filter catches too much dirt, the filter starts blocking airflow to a critical part that heats the air: the heat exchanger.

The S-shaped hunk of metal is the heat exchanger.

So why is this a problem? Well, if the heat exchanger can’t pass enough heat to the air, the heat exchanger overheats. And when that happens, your system shuts down the burners as a safety precaution to protect the heat exchanger from cracking.  (Your technician might refer to this process as an “auto reset primary limit”.)

 At the same time, the blower keeps running to cool the heat exchanger down. 

So, that could be one reason why you’re getting cold air out the vents: the furnace burner has shut down but the blower keeps running.

Do this: Check and change the air filter if it looks like the air filter on the right:

Clean air filter (left) , Dirty air filter (right)

Note on the filter’s location: Your furnace’s air filter for the furnace may be located in the blower compartment or in the factory-supplied filter cabinet attached to the side or bottom of the furnace casing. 

If the filter isn’t the issue, then...

Check the pilot light

Furnaces made before 1990 have a standing pilot flame. If the flame goes out, then the burners won’t ignite. And if the burners won’t ignite, then the furnace can’t heat your air.

Do this: To relight most standing pilot flames, follow these instructions:

  1. Turn the power switch off. 
  2. Take off the furnace door.
  3. Look for a pilot flame. 
  4. If no flame, turn the furnace control valve to the “Off” position. 
  5. Wait 5 minutes. 
  6. Have a lighter on hand. 
  7. After 5 minutes, turn the control valve to “Pilot.” 
  8. Press the pilot button and hold it while using the lighter to light the pilot. Locate the pilot by following the tubing from the control valve.
  9. Once the pilot flame is lit, continue to hold the pilot button for one minute. 
  10. Release the pilot button. The pilot flame should stay lit. If it goes out, start the steps over.
  11. Stand back at arm’s length and turn the control valve slowly from “Pilot” to “On.” 
  12. Turn the furnace power switch on. 
  13. Look for the furnace burners to light. 
  14. Carefully replace the door. 

Clear the condensate line

High-efficiency furnaces (90% AFUE and up) create condensation, which is drained out via a drain line. 

Condensate drain line exiting a home.

However, if that line becomes clogged, water backs up into the furnace, causing a switch to shut down the burners. No burners, as you could guess by now, means no heat! 

Do this: Check around the furnace unit. Do you see any water on the floor? That might be water flowing up from a clogged condensate drain line.

The solution? Try unclogging the condensate drain line using this video tutorial

Furnace still not working? Call a furnace repair specialist

If you’ve tried all the above DIY options and you’re still not getting any hot air, then you’re going to need a professional’s help.

Do you live in the Denver, CO area? If so, we can help. Contact Plumbline Services for a furnace repair.