Furnace Blowing Cool Air When On Heat

Quick answer

If your furnace is blowing cool air when on heat, first check the thermostat settings and air filter. Ensure the gas or power supply is active, and verify the pilot light is lit. If these don't resolve the issue, the problem may require professional attention.

Confirm the symptom

• Furnace is set to 'heat' mode
• Thermostat is calling for heat
• Cool air is blowing from vents when furnace is running
• Furnace fan runs but no warm air is produced

Tools and prep

• Screwdriver
• Flashlight
• New air filter (if needed)
• Thermometer (to check air temp)
• Safety gloves and glasses
• Gas leak detector (if applicable)

Step-by-step troubleshooting

1. Check Thermostat Settings
   Do: Ensure the thermostat is set to 'Heat' and the temperature is higher than the current room temperature.
   Observe: Furnace should start heating if settings are correct.
   Means: Incorrect thermostat settings can prevent the furnace from heating.
   Next: If settings are correct and furnace still blows cool air, move to the next step.
2. Inspect Air Filter
   Do: Remove and inspect the air filter for dirt or blockage.
   Observe: A dirty filter restricts airflow and can cause the furnace to blow cool air.
   Means: A clogged air filter reduces efficiency and may prevent proper heating.
   Next: Replace the filter if it's dirty and check if the issue is resolved.
3. Verify Gas or Power Supply
   Do: Check if the gas valve is open (for gas furnaces) or if the circuit breaker is on (for electric furnaces).
   Observe: The furnace should have a consistent power or gas supply.
   Means: A lack of fuel or power prevents the furnace from heating.
   Next: If the supply is correct, proceed to the next step.
4. Check Pilot Light (Gas Furnaces)
   Do: Look for the pilot light in the furnace's burner compartment. If it's out, relight it following the manufacturer's instructions.
   Observe: A lit pilot light indicates the furnace is ready to heat.
   Means: An extinguished pilot light prevents the furnace from igniting.
   Next: If the pilot light is lit, move to the next step.
5. Test Furnace Limit Switch
   Do: Locate the limit switch (usually near the heat exchanger) and check if it's tripped. Reset it if necessary.
   Observe: The furnace should restart and begin heating.
   Means: A tripped limit switch can shut off the furnace to prevent overheating.
   Next: If the limit switch is not the issue, consult a professional technician.
6. Inspect Blower Motor
   Do: Check if the blower motor is working correctly and if the belt is intact and properly tensioned.
   Observe: A faulty blower motor or loose belt may cause the furnace to blow cool air.
   Means: The blower motor circulates air, and issues with it can prevent proper heating.
   Next: If the blower motor is damaged, contact a professional for repairs.

Common causes

Parts matching

If your furnace is blowing cool air when on heat, you may need to replace or repair the following parts:

• Air Filter – Replace if dirty or clogged to improve airflow and heating efficiency.
• Limit Switch – Reset or replace if tripped to restore heating function.
• Blower Motor or Belt – Repair or replace if damaged to ensure proper air circulation.
• Gas Valve – Check or replace if there is a gas supply issue.
• Thermostat – Calibrate or replace if settings are incorrect or not responding properly.

Always consult the manufacturer's instructions or a professional technician when replacing or repairing furnace parts to ensure safety and proper function.

When to call a professional

• The furnace still blows cool air after checking and correcting all basic issues.
• You smell gas or hear unusual noises from the furnace.
• The pilot light repeatedly goes out or is difficult to relight.
• The limit switch continues to trip after being reset.
• The blower motor or belt is damaged and you are not comfortable replacing them yourself.
• The furnace is old and frequent repairs are needed.

FAQ

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