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HVAC Diagnostic Guide LOW URGENCY

Furnace Blower Won't Turn Off

Indoor blower fan runs constantly, never shuts off between cycles or when heating isn't called for.

Quick Check Before You Call a Pro

  1. Check thermostat — is FAN set to ON instead of AUTO?
  2. Try lowering the thermostat below current room temp — does blower eventually stop?
  3. Note whether burners cycle on/off normally while blower runs
  4. Listen for relay chatter at the furnace (clicking sounds = relay issue)

01 · Most Likely Causes (Ranked)

60%

Thermostat Fan Set to ON

What's happening

Easiest cause and most common. The thermostat has a separate switch for FAN. When set to ON, blower runs continuously regardless of heating/cooling demand. This is by design — some people prefer constant air circulation.

How to check

Look at thermostat for FAN setting. Should be AUTO for normal operation. ON means blower runs always.

How to fix

Switch FAN setting from ON to AUTO. Blower will continue for ~60 seconds for cool-down after current heat cycle, then stop until next heating call.

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15%

Stuck Fan Relay on Control Board

What's happening

The relay on the furnace's control board that controls the blower has welded contacts or stuck mechanically. The board sends the 'stop' signal but the relay won't open.

How to check

Thermostat is set to AUTO but blower never stops. May hear clicking from control board area. Sometimes power-cycling the breaker temporarily resets the relay.

How to fix

Control board replacement. Match manufacturer part number. $150-400 part.

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10%

Limit Switch Stuck Closed

What's happening

Some furnaces use the high-limit switch's signal to control blower operation. If the limit switch has failed in a closed position, the blower stays energized.

How to check

Multimeter test across limit switch. If still showing continuity hours after furnace last ran, the switch is stuck.

How to fix

Replace limit switch. Match manufacturer part by temperature setting and physical size. $25-50 part.

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8%

Wiring Short Between G and R

What's happening

A short circuit between the G (fan) and R (24V) wires in your thermostat wiring causes constant fan call regardless of thermostat setting.

How to check

Disconnect the G wire at the furnace control board. If blower stops, you have a short somewhere in the wiring run. Check wherever wires pass through metal (chafing) or near recent construction.

How to fix

Trace and repair shorted wire. May require running new thermostat cable. Job for a tech.

7%

Failed Thermostat

What's happening

Thermostat is calling for fan continuously even when set to AUTO. The internal relay or microprocessor has failed.

How to check

Disconnect the G wire from the thermostat (not from the furnace). If blower stops, the thermostat is faulty. If blower keeps running, problem is in the furnace wiring.

How to fix

Replace thermostat. $20-200 depending on model.

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ThermostatsControl BoardsLimit Switches

02 · Frequently Asked Questions

Is it bad if my furnace blower runs all the time?
Not necessarily — some people deliberately set FAN to ON for better air filtration and even temperature distribution. But it does use more electricity (~$10-30/month extra) and reduces motor life. If it's running constantly without your wanting it to, find and fix the cause.
Why does my blower run after the furnace turns off?
Normal — most furnaces have a delay-off feature that runs the blower 60-120 seconds after burners shut down. This extracts the last residual heat from the heat exchanger and improves efficiency. Should stop after that delay.
What's the difference between FAN ON and FAN AUTO?
AUTO: blower only runs during active heating/cooling cycles plus brief delay-off. ON: blower runs 24/7 regardless. ON improves air circulation/filtration but uses more electricity and shortens motor life.
Can a stuck blower damage the furnace?
Not the blower itself — but constant operation accelerates wear on motor bearings, capacitors, and the motor windings. If it's been running for days or weeks straight, motor life is being consumed. Diagnose and fix it sooner rather than later.

03 ·Related Symptoms

Furnace Blowing Cold AirThermostat Blank or Not WorkingFurnace Not Blowing Air
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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