When an air handler runs nonstop, start with fan mode, then check for a stuck relay, a tripped limit switch, or a wiring short.
Your blower should cycle. If the fan never rests, you pay for run time and parts wear. Use the checks below to find the cause fast and fix it safely.
Rapid Checks That Stop A Constant Blower
Work from simple to detailed. Cut power at the service switch or breaker before opening panels. Restore power only when a test needs it.
| Symptom | Likely Cause | Quick Check |
|---|---|---|
| Fan runs even with thermostat “off” | Stuck fan relay or shorted G wire | Remove thermostat face; if fan keeps running, suspect relay or board |
| Fan runs only in cooling season | Thermostat fan set to “On” | Switch fan to “Auto”; many systems are designed to cycle the fan |
| Heat works, fan never stops after a heat call | Fan limit switch stuck or high-limit trip | Watch furnace LED codes; inspect fan limit if accessible |
| Fan starts and stops slowly but never fully rests | Variable-speed mode or “circulate” feature | Check thermostat menu for circulate or cycle fan settings |
| Fan runs with outdoor unit off | Condensate safety or control board logic | Check float switch; clear drain if the switch is engaged |
Why The Air Handler Keeps Running — Common Causes
Thermostat Fan Mode Set To “On” Or “Circulate”
Many thermostats have three fan choices: Auto, On, and a cycle feature that runs the blower on a timer. Auto runs the fan with a heat or cool call. On runs nonstop. The cycle feature runs the fan on a schedule even when there’s no call. Use Auto for daily use.
Official guidance for heat pumps warns that continuous fan can hurt performance on many systems. The U.S. Department of Energy recommends Auto unless you have a high-efficiency variable-speed motor. See the DOE page on operating and maintaining a heat pump.
Stuck Fan Relay Or Welded Contacts On The Control Board
The fan relay is an electronic switch. Shorts, power surges, or age can weld the contacts. If you pull the thermostat faceplate and the blower still runs, the G signal from the thermostat is out of the picture, which points to the relay or board.
High-Limit Trip Or Fan Limit Switch Problem
In gas furnaces and some handlers with heat kits, a fan limit switch controls blower timing and protects against overheating. If the limit trips, the fan may run to cool the unit. A stuck manual fan switch on older controls can also keep the fan on.
Shorted Thermostat Wiring
A nicked thermostat cable, a staple through the jacket, or exposed copper touching metal can energize the G circuit. Inspect the cable run near the air handler and at the wall. Look for crushed sections and loose strands bridging terminals.
Condensate Float Switch Or Safety Circuit
Blocked drains trigger a float switch to prevent water damage. Some setups leave the fan powered while locking out cooling. Clear the trap, flush the line, and verify the switch opens when the pan is empty.
Step-By-Step: Find The Culprit Without Guessing
1) Read The Thermostat’s Fan Setting
Set Fan to Auto and disable any circulate timer. Clean filters keep airflow in range for normal cycling.
2) Bypass The Thermostat To Separate Controls From Hardware
Pull the thermostat face straight off. Leave the base on the wall. If the fan stops, it’s settings, wiring at the stat, or the stat itself. If the fan keeps running with the face removed, move to the air handler; suspect relay, board logic, or a safety circuit.
3) Inspect The Air Filter And Return Air
A clogged filter restricts airflow. That can cause long blower run time after a heat call or trigger a high-limit. Swap a 1-inch pleated filter every 1–3 months based on dust and usage. Large media cabinets last longer, but still need checks. Make sure returns are clear of furniture and dust mats.
4) Look For A Stuck Fan Limit Or Manual Fan Switch
On older furnaces, open the blower door and locate the limit control. If it has a manual fan switch tab, set it to Auto. For modern boards, read LED codes on the window for limit faults.
5) Test The G Circuit For A Short
Power off. At the air handler control board, remove the G wire. Restore power. If the fan stops, the thermostat or wiring is pulling G. If the fan still runs with G disconnected, the relay is stuck or the board commands the fan for another reason.
6) Check The Condensate Drain And Float Switch
Locate the trap near the indoor coil. If water sits in the pan or the float is raised, the safety is active. Clear algae with a cleaning solution, flush the line, and confirm the float drops.
7) Note Heat Pump Defrost Or Electric Heat Behavior
During a defrost cycle or with electric heat kits, the blower may run longer by design. If the fan runs well past a cycle, treat it as a fault.
Safe Tools And Simple Tests
What You Need
- Non-contact voltage tester
- Multimeter with AC volts and continuity
- Nut driver set and a flashlight
- Replacement filter sized for your return
- Small brush and a cup for trap cleaning
Door Switch And Breaker Basics
Most handlers cut power when the panel opens. Do not defeat the door switch. Secure the panel before live checks.
Text-Based Flow You Can Follow
- Set Fan to Auto and disable circulate.
- Pull the thermostat face. If the fan stops, recheck settings and wiring at the stat base.
- If the fan keeps running, move to the air handler and remove the G wire at the board.
- If the fan stops now, repair the cable or replace the thermostat.
- If the fan still runs, check the float switch and drain.
- No drain issue? Suspect a stuck relay or a control board command tied to a limit.
- Restore a clean filter, confirm blower speed taps or settings, and test again.
Fixes By Scenario
Fan Runs Constantly Even With Thermostat Removed
Likely a stuck relay or board. With power off, pull the motor lead from the fan terminal. If the fan stops on power-up, replace the board or relay. If it still spins, trace wiring for a miswire.
Fan Runs After Heat Call Ends
A heat exchanger needs a short cool-down, but endless run points to a limit issue. Clean the filter, open supply registers, and confirm the blower speed matches the heat tap. If airflow is good, test the limit switch.
Fan Runs With Outdoor Unit Idle In Cooling Mode
Check the stat’s fan setting and any circulate feature first. If set to Auto and still running, check the float switch. Clear the drain, then reset power.
Pro-Level Clues That Save You A Second Visit
These details help a service team diagnose faster:
- Exact model numbers for indoor and outdoor units
- Thermostat brand and model
- Photo of the control board with labels visible
- Video showing the fan behavior and stat display
- Any recent work: new thermostat, duct cleaning, electrical changes
Energy, Comfort, And Fan Settings
Setting the fan to On moves air between cycles, but it uses power and can re-evaporate moisture on the coil. Auto is the better daily setting on many systems unless a pro programmed a low-speed profile.
When A Constant Fan Is Normal
Some variable-speed systems use low airflow between calls to smooth room temps and filter air. That setting comes from the installer profile. If your unit was commissioned for continuous low speed, you’ll still see clear starts and stops during calls, plus short idle periods for defrost or board tests. If the fan never rests and airflow feels full speed, treat it as a fault.
Quick Math: What A 24/7 Fan Costs
You can estimate cost with a simple formula: blower watts × hours ÷ 1000 × electricity rate. Check the data label or your manual for blower watts. If the number isn’t listed, a clamp meter reading on the blower circuit gives a field value.
Simple Wiring Tests Without Guesswork
Thermostat cable uses standard colors, but only labels matter. Green is common for the fan call (G). At the board, the G terminal is next to R and C. Power off, take a photo, then remove G and insulate the loose end. Restore power. If the fan stops, the short is upstream. If it keeps running, the board is driving the fan or the relay is stuck. Do not jumper unknown terminals with power applied.
Deep Causes And Lasting Fixes
| Part Or Setting | What It Does | DIY Or Pro? |
|---|---|---|
| Thermostat fan mode | Commands blower to Auto, On, or timed cycle | DIY: Set to Auto; disable circulate |
| Fan relay/control board | Switches blower based on calls and safeties | Pro: Diagnose/replace board or relay |
| Fan limit switch | Controls heat fan timing; protects from overheating | Mixed: Inspect wiring; replace is a pro job |
| Thermostat wiring (G circuit) | Sends fan call from stat to handler | DIY: Visual check; Pro for shorts and rewiring |
| Condensate float switch | Stops cooling when drain backs up | DIY: Clear drain; test float |
| Variable-speed settings | May run low airflow between calls | Pro: Program airflow profiles |
| Duct sealing and returns | Low restriction prevents high-limit trips | DIY: Change filters; Pro: Seal ducts (ENERGY STAR duct sealing) |
Prevent A Repeat
Keep Filters Fresh
Check a 1-inch filter monthly and swap when dark or loaded. Larger media filters last longer but still need regular checks. Clean filters protect the heat exchanger and coil and cut run time.
Seal Duct Leaks
Leaky ducts waste air and can pull dust into the system. Sealed ducts boost comfort and save energy. Use mastic or UL-listed foil tape on joints you can reach; leave hidden runs to a pro.
Schedule Seasonal Service
A tune-up checks safeties, blower amps, coil cleanliness, refrigerant charge, and firmware on smart stats. During service, ask the technician to confirm blower programming, fan delay, and drain protections. Keep notes of dates, filter sizes, and any parts replaced so patterns are easy to spot next season.
When To Cut Power And Call A Technician
- Breaker trips when the fan starts
- Burnt smell from the handler cabinet
- Water under the indoor unit
- Fan speed surges or squeals
- Any sign of melted wiring or scorched terminals
FAQ-Free Bottom Line
A nonstop blower usually comes down to a fan mode set for continuous run, a stuck relay or control, a limit switch issue, a wiring short, or a drain safety. Start with the thermostat and filter, isolate the G circuit, check the drain, and read board codes. If it still runs with the stat face off and G pulled at the board, the control board or relay needs service.