A furnace fan that never stops usually means Fan is set to “On,” a stuck fan limit, or a bad relay—start with the thermostat.
Fast Checks Before You Call
Start simple at first. In many homes, that’s the thermostat. Set the thermostat to Heat and Fan to Auto. Drop the set point below room temperature. If the blower keeps running, you’re chasing a control issue, not a normal post-heat cooldown.
Confirm the Fan switch isn’t on purpose. On many thermostats, Fan “On” runs the blower nonstop by design. Honeywell’s help page explains the Auto vs On choice; Auto runs the fan only during heating or cooling cycles, while On runs it continuously for circulation. Link the fan setting to how your system behaves, then switch to Auto if you don’t want constant airflow. If you use a smart thermostat, check the app for a fan circulation feature. Disable it during troubleshooting to avoid confusion.
Next, check the filter. A clogged filter overheats the heat exchanger and can trip safety devices. Replace the filter if it looks dirty or past its date. Open supply and return grilles, and make sure doors and service panels are latched. Many furnaces won’t behave right with panels loose.
| Symptom You See | Likely Cause | First Action |
|---|---|---|
| Blower runs all the time | Fan set to On; stuck fan limit; welded relay on control board; G wire short | Set Fan to Auto; pull thermostat G wire test; inspect limit switch |
| Fan stops only when power is cut | Fan relay stuck closed; control board fault | Power down, restart; if unchanged, board/relay needs service |
| Blower runs after power restore | Normal short purge; or status code for “blower on after power up” | Wait 90 seconds; watch the board LED for codes |
| Cool air while fan runs | Burners off from high-limit trip; airflow restricted | Swap filter; open registers; clear returns |
| Fan runs with thermostat removed | Short on G circuit; relay stuck | Disconnect G at furnace; if still on, suspect relay/board |
When The Furnace Fan Won’t Turn Off: Causes That Fit
Thermostat Setting Or Wiring
Many “won’t shut off” calls end at the wall. If Fan is set to On, the blower is doing exactly what it was told. Switch to Auto. Some thermostats also offer circulate or fan schedules that keep the blower running in small intervals each hour. Review those features and disable them during heat calls if you don’t want constant airflow.
If Fan is already on Auto and the blower still runs, pull the G wire at the thermostat. If the fan stops, the thermostat or cable is the suspect. If it keeps going, move to the furnace end.
Dirty Filter Or Blocked Airflow
Restricted airflow drives temperatures up inside the furnace. Safety switches react by killing the burners, while the blower may keep moving air to cool the cabinet. That can look like a fan that never ends. Swap the filter, open closed registers, and clear return grilles. If airflow improves, the blower should behave.
Fan Limit Switch Stuck Or Misadjusted
The fan limit switch is the traffic cop between heat and airflow. It turns the blower on once the furnace is warm, and off after the burners stop and the heat exchanger cools. It also has a high-limit function that shuts the burners down if things run too hot. If the switch sticks closed, the blower can run continuously. If it’s mis-set, the blower may never receive the stop signal.
Control Board Or Fan Relay Fault
Modern furnaces use a control board with a relay to power the blower. When that relay welds shut, the blower runs even without a call from the thermostat. Some boards also run a short fan cycle after power is restored during a heat call; that’s normal. The Carrier service manual notes a brief “blower on after power up” window. If yours never stops, note the LED code and wiring shorts and plan a board check.
G-Circuit Short Or Add-On Accessory
The green “G” circuit commands the blower. A pinched wire, a humidifier coil miswired to G, or a loose splice can hold the fan relay on. Disconnect G at the furnace control. If the blower stops, hunt the short between the furnace and the thermostat or any accessory tied into that circuit.
Step-By-Step Troubleshooting You Can Do Safely
1) Set The Thermostat To Auto And Standby
Set Heat, Fan to Auto, and a set point below room temperature. Wait two minutes. If the blower stops, you’re done. If not, continue.
2) Try The G-Wire Test
Turn power off at the furnace switch. Remove the thermostat face. Disconnect the G wire from the thermostat terminal and cap it. Restore power. If the blower stays off, replace or reprogram the thermostat or repair the cable. If the blower still runs, go to the furnace.
At the furnace, open the blower door and find the control board. Kill power again. Remove the G wire from the G terminal on the board and keep it isolated. Restore power. If the fan still runs with G removed, the board’s fan relay is likely stuck closed.
3) Swap The Air Filter And Open Vents
Even if it looks okay, try a fresh, correctly sized filter. Open supply registers and verify cold-air returns aren’t blocked by furniture or bags. Better airflow helps temperature controls behave.
If you struggle to remember filter dates, set a phone reminder. The ENERGY STAR maintenance checklist suggests inspecting filters monthly and changing them as needed. That simple habit prevents many fan and limit headaches.
4) Check The Blower Door Switch And Panels
With many brands, the board acts oddly if the blower door switch isn’t fully engaged. Seat the door firmly. Latch any burner compartment panels. Loose doors can fool safeties.
5) Inspect The Fan Limit Switch
Older units use a dial or slide-lever fan limit. Make sure the lever is in Auto, not Manual. If the blower stops when you move the lever to Auto, you’ve found the reason. On adjustable models, the fan-off setting sits below fan-on, and there’s a higher safety stop for the high limit. If the device looks damaged, don’t force it—plan a replacement.
Newer furnaces read temperature with a probe tied to the board. You won’t see a separate dial, but the function is similar: blowers start once heat builds, then stop after a short cool-down. If the probe fails or the board logic locks, the fan may run with no heat call.
6) Watch The Control Board
Kill power for one minute, then restore. A normal board may spin the blower briefly after power returns during a heat call, then stop. If the blower starts and never stops, note the LED code and wiring. A welded fan relay or shorted output will keep power flowing to the motor.
7) Call A Pro When You See These Signs
Burnt smells, tripped breakers, melted harness plugs, repeat high-limit trips, or any work that exposes live wiring are jobs for a licensed tech. The goal is a safe fix, not a quiet fan.
| Task | DIY Or Pro? | Typical Time/Cost |
|---|---|---|
| Set Fan to Auto; reprogram thermostat | DIY | 5 minutes; $0 |
| Replace standard 1-inch filter | DIY | 10 minutes; low cost |
| G-wire test and cable repair | DIY/Pro | 15–60 minutes; cable varies |
| Replace fan limit switch | Pro | 1–2 hours; part + labor |
| Replace control board or fan relay | Pro | 1–2 hours; part + labor |
| Blower motor diagnosis | Pro | 1–2 hours; varies by model |
Care That Keeps The Blower From Sticking On
Change Filters On A Schedule
Dirty filters block airflow and lead to nuisance limit trips and long fan run-downs. A good rule: check monthly during heavy use and replace when loaded with dust. Your utility bills and indoor air will thank you.
Use Auto As Your Default Fan Mode
Auto limits run time to actual heating or cooling calls. Use On only when you want constant mixing, like evening out temperatures after big gatherings or filtering during dusty projects. Then flip it back.
Keep Registers And Returns Open
Shut registers make airflow drop. That strains the furnace and can confuse limit controls. Leave most of them open, especially on floors with long duct runs.
Schedule A Yearly Tune-Up
A technician can test the limit switch, clean the blower, check amp draw, and log board codes. Catching weak relays and worn motors early prevents stuck-on fans later.
Myths And Misreads
“The fan should stop the instant the burners stop.” Not always. Many furnaces keep the blower on for a short cool-down to pull the last bit of heat from the exchanger. That saves energy and protects the heat exchanger from hot-spot stress.
“It must be the motor.” Motors do fail, but a stuck-on blower often traces back to controls, not the wheel that spins. Start with settings and safeties before buying a motor you don’t need.
Safety Notes You Shouldn’t Skip
Always cut power before removing doors, pulling wires, or reaching near the blower. Never bypass a limit switch. If a board shows scorch marks or a motor smells like burnt insulation, stop and book service. Gas furnaces have other safeties that need to stay intact.
Helpful References
See your thermostat’s Fan Auto vs On explanation from Honeywell for how the fan switch changes blower behavior. For routine care like filter changes, the ENERGY STAR maintenance checklist lays out simple steps any homeowner can do.