Furnace Blower Runs Constantly? 7 Things to Check Before Calling for Service

A furnace blower that never seems to shut off can be annoying, increase your electric bill, and make you think something major has failed.

Fortunately, a blower that runs continuously is often caused by a thermostat setting, control issue, or airflow problem rather than a failed furnace.

1. Check the Thermostat Fan Setting

This is the first thing to verify.

Most thermostats have two fan settings:

• AUTO
• ON

If the fan is set to ON, the blower will run continuously.

Fix:

Switch the fan setting to AUTO and see if the blower shuts off after a few minutes.

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1. Verify Program Settings

Some smart thermostats have circulation modes that periodically run the fan.

Examples include:

• Fan circulation
• Air cleaning mode
• Comfort mode

Review thermostat settings and disable these features if desired.

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1. Check for a Stuck Fan Relay

The furnace control board uses a relay to start and stop the blower motor.

If the relay sticks:

• The blower may run constantly
• Heating and cooling may still function normally

A failed relay often requires control board replacement.

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1. Inspect the Air Filter

A severely clogged filter can create airflow issues.

In some systems, overheating protection may affect blower operation.

Signs:

• Weak airflow
• Excessive dust
• Furnace running longer than normal

Fix:

Replace the filter with the correct size and rating.

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1. Check the Limit Switch

The high-limit switch protects the furnace from overheating.

If the switch fails:

• The blower may stay on
• Heating cycles may become irregular

A technician can test the switch for proper operation.

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1. Look for Control Board Problems

Modern furnaces rely on electronic control boards.

A failed board may:

• Keep the blower running
• Ignore thermostat commands
• Cause unusual operating behavior

Inspect for burn marks or diagnostic LEDs if accessible.

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1. Verify Thermostat Wiring

Incorrect wiring can cause the fan to run continuously.

Common causes include:

• Miswired thermostat
• Loose connections
• Shorted wiring

This is especially common after thermostat replacement.

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Common Causes Ranked

The most common reasons a blower runs constantly are:

1. Thermostat fan set to ON
2. Smart thermostat circulation settings
3. Faulty fan relay
4. Failed limit switch
5. Thermostat wiring issues
6. Control board failure

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Estimated Repair Costs

Repair Estimated Cost
Air Filter $10-$40
Thermostat $25-$250
Limit Switch $20-$100
Control Board $100-$500
Professional Service $150-$600

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Final Thoughts

A furnace blower that runs continuously doesn’t always indicate a major repair. Start with the thermostat settings and simple airflow checks before assuming expensive components have failed.

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