9 Reasons Your Furnace Isn’t Blowing Hot Air

Having a furnace that isn’t blowing hot air can be very frustrating, especially when you need it to keep your home warm. There are several possible reasons why your furnace may not be operating properly. Here are 9 common reasons your furnace isn’t blowing hot air and what you can do about it.

1. The Furnace Filter is Clogged

One of the most common reasons a furnace isn’t blowing hot air is a clogged filter. The filter is responsible for removing dust and debris from the air before it circulates through your home. Over time, the filter can become clogged with particulates, restricting airflow. If the airflow is limited, the furnace may not be able to heat the air efficiently.

To fix this, turn off the furnace and carefully remove the filter. Examine it to see if it looks overly dirty or clogged. If it does, replace it with a new filter. Use filters with a MERV rating between 8-12 for optimal performance. Be sure to change the filter regularly as recommended by the manufacturer, usually every 1-3 months. Keeping the filter clean will allow proper airflow and let your furnace heat efficiently.

2. The Blower Motor is Not Working Properly

The blower motor is responsible for pushing heated air through the ductwork and into your home. If it is broken or working incorrectly, it can’t circulate the hot air properly. Blower motor problems could include:

  • Broken fan belt
  • Burned out motor
  • Obstructed fan blades
  • Faulty motor bearings

Turn off the power to the furnace and inspect the blower motor setup. Look for obstructions around the blades and any apparent issues with the belt or motor. You may need to call a technician to test the motor and determine if it needs to be repaired or replaced. Replacing a bad blower motor will get your furnace blowing hot air again.

3. There is a Blockage in the Ductwork

If the ductwork that distributes the heated air has a blockage, it will restrict airflow and prevent hot air from circulating normally. Blockages in the ductwork could include:

  • Built up dust, dirt, or debris
  • Collapsed ducts
  • Objects obstructing vents
  • Closed dampers

Inspect the various ductwork components from the furnace to the vents. Make sure dampers are open. Brush or vacuum accessible ducts to remove dirt. Check ducts for crushing or damage. You may need a technician to scope the ductwork if you can’t find or remove a blockage. Getting rid of obstructions in the ducts will get your furnace working properly again.

4. The Thermostat Batteries are Dead

One simple thing that can make your furnace stop blowing hot air is a thermostat with dead batteries. The thermostat controls the furnace based on the home’s temperature. If its batteries are depleted, it can’t accurately control the furnace.

Inspect your thermostat and replace the batteries if needed. Most models just use common AA or AAA batteries. Install fresh batteries and ensure the thermostat is functioning properly. With good batteries, the thermostat can correctly communicate with the furnace to start heating when needed.

5. There is a Pilot Light or Igniter Problem

For gas furnaces, issues with the pilot light or igniter can prevent the furnace from lighting and heating properly. The pilot light provides the flame needed to ignite the gas burner. Problems with the pilot light include:

  • Pilot light is out
  • Weak pilot flame
  • Dirty pilot orifice
  • Faulty pilot assembly

The igniter is used instead of a pilot light in many newer gas furnaces. Issues with the igniter that could prevent heating include:

  • Bad igniter
  • Faulty igniter wiring
  • Broken igniter switch

Inspect your furnace for pilot or igniter issues. Try relighting the pilot if it’s gone out. Clean any dirt or debris around the pilot. Have a technician inspect the igniter and replace parts as needed. Fixing the pilot or igniter will get the furnace burning gas and heating again.

6. The Flame Sensor is Dirty

The flame sensor is an important safety device in gas furnaces. It ensures the gas burner ignites properly and shuts off gas flow if there is no flame detected. If the sensor becomes covered in dust, dirt, or debris, it may not sense the flame and prevent the furnace from firing up.

Carefully clean the flame sensor with steel wool or fine sandpaper. Make sure not to damage the sensor in the process. After cleaning, try reigniting the furnace. The clean sensor should detect the flame properly and allow normal operation.

7. The Gas Valve is Faulty

The gas valve or regulator controls the flow of gas to the furnace burner. If it is stuck closed or not functioning properly, gas can’t reach the burner to be ignited. A bad gas valve requires replacement by a professional technician.

Signs of a faulty gas valve include:

  • Burner not lighting
  • Very small flames
  • Soot buildup

Have a technician inspect the gas valve. They will be able to diagnose problems and replace the valve if defective. The repaired or new gas valve will restore normal gas flow to the furnace.

8. The Thermocouple Needs to Be Replaced

The thermocouple is a heat-sensitive device that ensures the pilot flame stays lit. If it malfunctions, the pilot may go out and prevent the furnace from heating. Some signs of a bad thermocouple include:

  • Pilot frequently goes out
  • Weak or small pilot flame
  • Furnace keeps shutting off

Have a technician inspect the thermocouple and replace it if faulty. A properly working thermocouple will monitor the pilot light and keep the furnace running as needed.

9. The Fan Limit Switch is Defective

The fan limit switch turns the blower fan on and off based on furnace temperature. If the switch is broken, the blower won’t turn on to circulate heated air. Warning signs of a bad fan limit switch:

  • Blower not turning on
  • Burning smell from furnace
  • Short cycling

A technician can diagnose and replace the fan limit switch. The repaired switch will accurately control the blower fan again. With the fan cycling on and off properly, hot air will be pushed through your ducts.

Next Steps If Your Furnace Isn’t Blowing Hot Air

If your furnace isn’t blowing hot air, start by inspecting the filter, ductwork, thermostat batteries, and other components you can access safely. Attempt simple fixes like changing a clogged filter.

However, issues with motors, burners, valves, switches, and other furnace parts should be left to a professional HVAC technician. They have the skills and tools to diagnose and repair these issues correctly.

Don’t take chances with problems like:

  • Gas valves
  • Thermocouples
  • Blower motors
  • Flame sensors
  • Igniters

Call a qualified technician for assistance if you can’t determine the cause of the problem or feel unsafe handling repairs. They can pinpoint why your furnace isn’t blowing hot air. With the necessary repairs, your furnace can work properly again and keep your house comfortable.

Frequently Asked Questions About Furnaces Not Blowing Hot Air

What should I check first if my furnace isn’t blowing hot air?

The first step is to check your air filter. A clogged, dirty filter is the most common reason a furnace won’t blow hot air. Turn off the furnace, remove the filter, and inspect it. Replace the filter if it looks dirty or is overdue for a change.

Why does my furnace keep shutting off?

Frequent shutdowns are often caused by a problem with the flame sensor, thermocouple, or gas valve. These parts help monitor gas flow and the pilot flame. If faulty, they can repeatedly shut off the furnace for safety. Have a technician inspect and replace them if needed.

How can I get my pilot light to stay lit?

A pilot that won’t stay lit usually indicates issues with the thermocouple or gas supply. Clean the thermocouple contacts and make sure gas valves are fully open. Have a professional service the pilot assembly if problems persist.

What if my furnace igniter keeps clicking?

The igniter cycles on and off to light the furnace burner. If it clicks rapidly without igniting the gas, the igniter itself is likely faulty. A technician can assess the igniter and replace it if defective.

Why does my furnace keep blowing cold air?

If a furnace blows cold air, it typically means the burner isn’t lighting properly. Causes can include pilot or igniter problems, a faulty gas valve, or a malfunctioning flame sensor. A technician should diagnose the specific issue.

Can a dirty filter make my furnace run all the time?

Yes, a restricted filter reduces airflow, making the furnace work harder and run longer to try to heat your home. Replacing a clogged filter can improve efficiency and prevent the furnace from running continuously.

Why does my blower fan not turn on?

If the blower doesn’t activate, suspect issues with the fan limit switch, blower motor, or wiring. A technician can test components and repair or replace defective parts getting the blower going again.

How do I know if my gas valve is bad?

Signs of a faulty gas valve include flames burning differently than normal, the pilot or burner not lighting, soot buildup around the burner, and the furnace making odd noises or smells. Have a pro inspect the valve.

What should I do if the pilot light keeps going out?

Frequent pilot outages often mean the thermocouple needs replacement. A professional technician can confirm the issue and install a new thermocouple to fix the problem.


Having an underperforming furnace that doesn’t provide adequate heat is uncomfortable and frustrating. In many cases, common issues like a dirty filter, blocked ducts, or dead thermostat batteries can be DIY repairs. However, problems with the blower, burners, valves and other integral components should be left to a trained HVAC technician. They have the expertise to correctly diagnose why your furnace isn’t blowing hot air and get it working at full capacity again. With the right repairs, your furnace can once again keep your home warm all winter long.