Why is My Gas Furnace Blowing Cold Air and Not Shutting Off?
If your gas furnace is blowing cold air and won’t shut off, it can be a frustrating and concerning issue. There are several potential reasons why this may be happening, and understanding the underlying causes can help you address the problem effectively.
Specific Reasons Why Your Gas Furnace is Blowing Cold Air and Not Shutting Off
Is the Thermostat Malfunctioning?
- Incorrect Thermostat Settings: If the thermostat is set to “ON” instead of “AUTO,” the fan will run continuously, even when the furnace is not heating. This can cause the furnace to blow cold air without shutting off.
- Thermostat Communication Issues: If the thermostat wires are shorted or there is a communication issue between the thermostat and the furnace, it can cause the furnace to run continuously without producing heat.
Is There an Issue with the Gas Supply?
- Low Gas Supply: If the gas supply line is clogged or there is a low supply of gas, the furnace may deactivate and blow cold air. This issue typically requires professional intervention.
Are There Problems with the Furnace’s Internal Components?
- Dirty or Clogged Air Filter: A dirty air filter can block airflow, causing the furnace to overheat and blow cold air. Replacing the air filter can resolve this issue.
- Pilot Light Issues: If the pilot light is out or damaged, the furnace will not produce heat. For older furnaces with pilot lights, relighting the pilot or cleaning the pilot’s orifice may resolve the issue.
- Flame Detector Problems: Modern furnaces use flame detectors. If the flame detector is covered in dust, dirt, or grime, it can cause the furnace to blow cold air. Cleaning the flame sensor can fix this issue.
- Condensate Lines Clogged: Blockages in the condensate lines can deactivate the burners, causing the furnace to blow cold air.
- High Limit Relay or Roll Out Switch: These safety switches can trip if the furnace overheats or if there is an issue with the burners. If these switches trip, they may not reset automatically, causing the furnace to blow cold air continuously.
Possible Causes of a Gas Furnace Continuously Running and Blowing Cold Air
Limit Switch
- Overheating: If the furnace overheats, the limit switch will activate, turning off the burners but allowing the blower to continue running. This can cause cold air to be blown into the home.
Blower Motor
- Continuous Operation: If the thermostat is set to “ON” or if there is an issue with the thermostat’s communication with the furnace, the blower motor may run continuously, even when the furnace is not producing heat.
Ignition System
- Igniter or Pilot Light Issues: Problems with the ignition system, such as a faulty igniter or a pilot light that won’t stay lit, can prevent the furnace from producing heat, leading to cold air being blown.
Troubleshooting and Fixing a Gas Furnace That Won’t Stop Running and is Blowing Cold Air
Step-by-Step Instructions for Diagnosing the Issue
- Check Thermostat Settings:
- Ensure the thermostat is set to “AUTO” instead of “ON”.
- Verify that the temperature setting is correct and not set too low.
- Inspect Air Filter:
- Check if the air filter is dirty or clogged. Replace it with a new one if necessary.
- Check Pilot Light or Flame Detector:
- For older furnaces, check if the pilot light is out and relight it if necessary. Clean the pilot’s orifice if it’s dirty.
- For modern furnaces, clean the flame detector if it’s covered in dust or grime.
- Inspect Condensate Lines:
- Check for blockages in the condensate lines and clear them if necessary.
- Check for Tripped Safety Switches:
- Look for tripped high limit relays or roll out switches. These may need to be manually reset.
- Check Ductwork:
- Inspect the ductwork for holes or cracks that could be allowing cold air to enter the system.
Necessary Tools and Safety Precautions
- Tools: A screwdriver, a vacuum cleaner for cleaning components, and a new air filter if needed.
- Safety Precautions: Ensure the power to the furnace is turned off before performing any maintenance. Use protective gloves and safety glasses when working with electrical or gas components.
Estimated Costs for Parts or Repairs
- Air Filter: $10-$30
- Pilot Light Assembly: $50-$100
- Flame Detector: $20-$50
- Professional Repair: Varies widely depending on the issue and location, but typically ranges from $100 to $500 or more.
Reference Links
- Snell Heating and Air: 7 Reasons Your Furnace is Blowing Cold Air & Ways to Fix
- Reddit – HVAC Advice: Furnace blowing cold, fans won’t turn off
- Bell Brothers HVAC: What To Do About A Furnace That Won’t Turn Off And Is Blowing Cold Air
