Furnace Blowing Cold Air and Won't Shut Off

Quick answer

If your furnace is blowing cold air and won't shut off, first check the thermostat settings and air filter. Turn off the power to reset the system. If the issue persists, inspect the limit switch or contact a professional HVAC technician.

Confirm the symptom

• Furnace fan is running continuously
• Air coming from vents is cold
• Thermostat is calling for heat but no warm air
• System won't shut off even when desired temperature is reached

Tools and prep

• Screwdriver set
• Flashlight
• New air filter (if needed)
• Thermal tape or replacement wire (if needed)
• Safety gloves and goggles
• HVAC service contact information

Step-by-step troubleshooting

1. Check Thermostat Settings
   Do: Ensure the thermostat is set to 'Heat' and the desired temperature is higher than the current room temperature.
   Observe: Thermostat settings are correct.
   Means: Thermostat is not the issue.
   Next: Check air filter
2. Check Air Filter
   Do: Remove and inspect the air filter for dirt or blockage.
   Observe: Air filter is dirty or clogged.
   Means: A clogged filter restricts airflow and can cause the furnace to blow cold air.
   Next: Replace the air filter
3. Reset the Furnace
   Do: Turn off the power to the furnace at the circuit breaker for 30 seconds, then restore power.
   Observe: Furnace resets and starts functioning normally.
   Means: The issue may have been a temporary electrical glitch.
   Next: Monitor the system
4. Inspect Limit Switch
   Do: Locate the limit switch (usually on the heat exchanger) and check if it's tripped or damaged.
   Observe: Limit switch is tripped or appears damaged.
   Means: A faulty limit switch can prevent the furnace from igniting and cause continuous fan operation.
   Next: Replace the limit switch
5. Check Ignition System
   Do: Look for signs of a failed ignition system, such as a cracked igniter or no spark.
   Observe: Ignition system is faulty.
   Means: A failed igniter prevents the furnace from heating the air.
   Next: Contact a professional HVAC technician
6. Monitor System Operation
   Do: Allow the furnace to run for a few minutes after any repairs or resets.
   Observe: Furnace continues to blow cold air or won't shut off.
   Means: The issue may require professional diagnosis.
   Next: Contact a professional HVAC technician

Common causes

Parts matching

Common parts needed for a furnace blowing cold air and won't shut off include:

• Air filter
• Limit switch
• Thermostat
• Ignition system
• Electrical components
• Blower motor
• Control board

Always ensure the furnace is turned off and cooled down before replacing any parts. If unsure, consult a professional HVAC technician.

When to call a professional

• The furnace continues to blow cold air after replacing the air filter and resetting the system.
• You notice a faulty limit switch or ignition system but are unsure how to replace them.
• The furnace still won't shut off after checking all components and following troubleshooting steps.
• You smell gas or notice unusual noises coming from the furnace.
• The issue involves electrical components or control board failure, which require professional expertise.

FAQ

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