A nonstop furnace blower usually points to fan set to ON, a stuck limit switch, or a bad relay—start by switching the thermostat to AUTO.
When the blower never stops, your home feels drafty, bills climb, and parts wear early. This guide gives a clear path to find the cause fast and stop the endless spin without guesswork.
Rapid Checks Before You Dig Deeper
Start with the easy wins. Many run-on cases come from a simple setting or a small airflow issue. Work top to bottom, then move to deeper tests.
| Likely Cause | What To Check First | Risk If Ignored |
|---|---|---|
| Fan set to ON | Thermostat FAN mode; pick AUTO (or CIRC) | Higher bills, extra wear |
| Smart stat circulation | Look for a “circulate” or hourly fan program | Fan runs by design |
| Manual override at limit switch | Older furnaces have a push/pull override | Fan stays on until switched back |
| Dirty filter or blocked return | Replace filter; clear vents/returns | Overheat trips, poor airflow |
| Stuck fan limit switch | Fan keeps running after heat is off | Continuous blower, short cycling |
| Stuck blower relay or board fault | Fan runs with thermostat removed | Damaged board, wasted energy |
| Shorted stat wiring (G call) | Inspect green fan wire for shorts | Fan energized nonstop |
| Heat pump defrost or fan timer | Brief runs are normal during defrost | Normal if intermittent |
Blower Keeps Running After The Heat Cycle — Common Causes
Thermostat Fan Mode And “Circulate” Schedules
The plainest cause is the fan set to ON. In that mode the blower runs all day. Switch to AUTO so the fan stops between calls. Some models add a “circulate” option that spins the fan for part of each hour. That setting also keeps air moving when there’s no heat call. If you want the blower to stop, turn that feature off. See the official Honeywell fan modes guide for the exact steps on common models.
Manual Override On Older Fan Limit Controls
Many legacy furnaces use a mechanical fan/limit control with a small push-in button for constant fan. If pushed in, the blower runs no matter what the thermostat says. Pull it back out to return to automatic. If the override is broken or the dial sticks, the blower can stay on until the part is replaced.
Dirty Filter, Closed Vents, Or Starved Return
A clogged filter or blocked return starves airflow and can trigger high temperature inside the cabinet. Burners shut off to protect the heat exchanger, yet the fan may keep running to pull heat out. Replace the filter, open supply registers, and clear the return grill. Government guidance backs frequent filter checks during heavy use; see the DOE EnergySaver filter advice.
Fan Limit Switch Stuck Or Misreading
The fan limit switch tells the blower when to start and stop. When it fails, the blower may never stop. Other signs include no warm air or short cycles. A tech can test the sensor and confirm if the switch is stuck closed or the dial is out of range. Replacement is routine on gas and oil units.
Blower Relay Or Control Board Fault
The thermostat sends a “G” signal to energize the fan relay. If the relay welds shut or a board trace fails, the fan stays powered even with no call. A fast clue: remove the thermostat faceplate to drop all calls. If the blower keeps spinning with the stat off the wall, the issue lives at the furnace, not the thermostat.
Thermostat Wiring Short
A nicked green wire can touch R and backfeed the fan circuit. Inspect the cable at the furnace and behind the thermostat. Separated copper strands or staples through the jacket are common culprits. Re-terminate or replace the run if damaged.
Safe Setup Before You Work
Kill power at the service switch or breaker before taking off panels. Let the cabinet cool. Wear gloves and eye protection when reaching around sheet metal. If you smell gas, stop and call a licensed tech.
Step-By-Step Fix That Solves Most Cases
1) Confirm Fan Mode
Set the thermostat to AUTO. Turn off any circulate or hourly fan programs. On some stats, this sits under Fan, Equipment, or Comfort settings. Many apps have a separate Fan tab.
2) Power-Cycle The Furnace
Flip the furnace switch off for 60 seconds, then on. If the fan stops and returns to normal timing, you likely cleared a short-term control hang.
3) Swap The Filter And Clear Vents
Install the right size filter with arrows toward the blower. If you’re not sure on change cycles, monthly checks during heavy heating is a safe rule, and more often with pets or dust.
4) Check The Fan/Limit Control
Open the burner door. Look for a round dial or a rectangular control with temperature marks. If there is a push/pull button, set it to automatic. If the dial doesn’t move off the same point, or the fan never drops out even after a long cool-down, the switch may be stuck.
5) Rule Out The Thermostat
Remove the thermostat face to kill all calls. If the blower stops, the thermostat or its wiring is involved. If it keeps running, look at the relay or control board next.
6) Test The G Circuit
At the furnace low-voltage block, remove the green wire. If the fan stops, the issue is upstream. If it keeps running with G removed, suspect a stuck relay or a board fault.
7) Inspect The Blower Relay Or Board
Many boards have a cube relay labelled FAN. If contacts weld shut, they hold the blower on. Some ECM blowers have an internal module that can fail and send power to the motor at all times; module replacement is the cure.
Why The Blower Runs After Heat Stops
A short post-burner run is normal. The blower keeps moving air to pull heat from the exchanger and raise efficiency. The run time can be a few minutes and is set by temperature or a board timer. What is not normal is an hour-long run with no new heat call.
Signs You’ve Found The Root Cause
- Setting fix worked: Fan stops between calls after changing to AUTO or disabling circulate.
- Airflow fix worked: After a new filter and open vents, the unit heats without tripping and the blower shuts down smoothly.
- Part fault: Fan still runs with thermostat removed and G wire pulled. That points to relay, board, or limit switch.
Repair Paths, Effort, And Cost Ballparks
Costs vary by region and model. These rough ranges help plan the fix. If your system uses a variable-speed ECM motor, parts can run higher than single-speed gear.
| Issue | Typical Fix | DIY/Pro & Cost Range* |
|---|---|---|
| Fan set to ON or circulate | Change mode to AUTO | DIY, $0 |
| Dirty filter / blocked return | Replace filter, clear grilles | DIY, $10–$40 |
| Manual override left on | Return switch to automatic | DIY, $0 |
| Stuck fan limit switch | Replace limit/control | Pro, $150–$350 |
| Stuck relay / bad control board | Replace relay or board | Pro, $200–$800 |
| Shorted thermostat wire | Repair or replace cable | Pro, $120–$300 |
| ECM blower module fault | Replace module/motor | Pro, $400–$1,200 |
*Ranges are general and not quotes.
Prevent The Next Run-On
Keep Filters Fresh
Match the correct size and MERV for your unit. Check monthly in heavy heating and sooner with pets or renovation dust. A clean filter keeps the heat exchanger within target temps and protects the limit control.
Leave Vents And Returns Open
Closing too many registers can push static pressure high and cause overheating. Keep furniture clear of returns. If a room needs less heat, crack the register rather than shutting it.
Service The Blower
Vacuum dust from the blower cabinet during yearly service. Dust on blades and the motor blocks airflow and adds drag.
Seal And Size Ducts Right
Leaky or undersized ducts force longer run times and can trigger safety trips. A pro can test static pressure and correct sizing or leakage.
Know Normal From Not-Normal
Many systems run the blower for a short time after burners shut off. Heat pumps can also run brief cycles in defrost. Those are normal. What you’re watching for is a blower that runs for long stretches with no new heat call.
ECM Motor Notes
Variable-speed motors use an electronic module that meters power. When that module fails, the motor can run with no call. If your model ramps up and down smoothly in normal times yet now runs at one odd speed for hours, flag the module for testing.
When To Call A Technician
Get a pro involved if the blower stays on with the thermostat removed, if you smell gas, if breakers trip, or if you see scorch marks inside the cabinet. A licensed tech can test the limit switch, relay, and board under load and confirm safe operation before leaving.
Quick Reference Fault Tree
If The Fan Runs Nonstop
- Set FAN to AUTO; disable circulate.
- Power-cycle the furnace.
- Replace filter; open vents and returns.
- Set any manual fan override at the limit to automatic.
- Pull the thermostat. If the fan stops: reprogram or replace the stat. If it keeps running: go to the control.
- Remove G at the furnace. If the fan still runs: relay or board fault; call a pro.
Final Fix That Actually Works
Most run-on cases end with a mode change, a fresh filter, or one small part. Work the list once, in order. If the fan still spins with no stat attached and G removed, the furnace controls need service. After the repair, set a filter reminder and enjoy quiet cycles again.