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Why is my Furnace Blowing Cold Air? A Denver Tech Answers

If you live in Denver, the last thing you want is a furnace that’s not doing its job. Some possible reasons your furnace is blowing cold air, include:

  • Thermostat is set to ON
  • Heat exchanger is overheating
  • Clogged condensate line (high-efficiency furnace only)
  • Major duct leaks
  • Inadequate gas supply

While some of these issues can be solved easily, others require a professional to diagnose and fix. Below, we’ll walk you through both (starting with issues you can fix on your own).

Rather have a professional diagnose and fix your furnace? We get it! Our team is experienced with all heating repairs and can help repair any issue quickly.

DIY Fixes

1. 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 lukewarm or cold.

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

2. 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 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.

If the heat exchanger can’t pass enough heat to the air, the heat exchanger overheats. This results in:

  • The system shutting down the burners as a safety precaution
  • The blower continues to run in order to cool the heat exchanger

If the furnace burner has shut down but the blower keeps running, this means cold air is being pumped into your home.

The solution: Check and change the air filter.

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

If you notice that your furnace is continuing to blow cold air or overheat even after you’ve replaced your air filter, it’s time to call a professional to diagnose your issue.

3. 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 means no heat!

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

Professional Fixes

Major duct leak

If there are leaks in your air ducts, cold air from your attic or crawl space can get sucked into your ducts. This cold air is then filtered into your house along with the warm air, resulting in air that’s cooler than the temperature you’ve set.

Example of a duct that runs through an attic

The solution: Call a professional. An experienced technician will need to inspect your air ducts and diagnose the issue, providing the best repair option. Repair leaks can be simple or complex depending on how extensive the leaks are.

Inadequate gas supply

If your furnace doesn’t get the amount of gas it needs to create warm air, it might lock down for safety purposes.

The solution: Check to see if your gas supply is turned off. If not, you likely have a severed line and a professional will need to complete this fix.

Furnace still not working? Call a furnace repair specialist.

If you’ve tried all these options and you’re still not getting any hot air, you're going to need a professional furnace repair.

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

Need help from a Colorado Plumbing, Heating, Cooling, or Electrical Specialist? 

For your convenience, you can request an appointment in one of two ways:

  • Call us at (303) 436-2525 for immediate assistance.
  • Click on the button below to schedule your appointment online.


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