A furnace blower that runs non-stop is one of the easiest diagnoses in HVAC — 90% of the time it’s a thermostat setting. The other 10% is a stuck fan limit switch, which is an easy repair once identified.
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Check 1: Thermostat FAN setting (30 seconds, $0)
Look at your thermostat. Find the FAN control. It has two positions:
- AUTO: blower runs only when the furnace is actively heating (or when AC is cooling). Between cycles, blower is off.
- ON: blower runs continuously, regardless of whether heating or cooling is active.
If FAN is set to ON: switch it to AUTO. The blower will stop at the end of the current fan cycle.
This is the cause in the vast majority of “blower never shuts off” calls. It’s an easy switch to accidentally hit — especially on smart thermostats where the FAN ON mode can be set through an app or accidentally tapped on the touchscreen.
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Check 2: Is the furnace actually heating, or just blowing?
Feel the air coming from the supply vents. Is it warm or room-temperature/cool?
- Warm air: the furnace is in an active heating cycle, which may be running a long time on a cold day. This is normal. Once the thermostat is satisfied and the burners shut off, the blower will run 2–4 more minutes to cool the heat exchanger, then stop. This is not a problem.
- Room temperature or cool air: the furnace isn’t heating, but the blower is running. Either FAN is set to ON (Check 1), or the fan limit switch is stuck.
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Check 3: Fan limit switch (older furnaces)
Applies to: furnaces with a mechanical fan limit switch — typically furnaces manufactured before the mid-1990s, though some early 2000s models also used mechanical switches.
The fan limit switch is a round or rectangular device mounted inside the furnace cabinet, near the heat exchanger plenum. It has a bimetal element that responds to heat exchanger temperature and a manual adjustment dial.
Most fan limit switches have an override lever on the face of the switch. If this lever is in the MANUAL or CONTINUOUS RUN position, the blower runs regardless of temperature. Check whether the override lever has been moved.
If the override isn’t set and the blower still runs continuously: the switch itself may be stuck in the run position — the bimetal element or contacts are welded closed. Test by turning off the furnace power for 1 minute and restoring. If the blower starts again immediately (without the heat exchanger warming), the limit switch is stuck.
Replacement: a mechanical fan limit switch is a $20–40 part and a straightforward swap — remove the mounting screws, unplug the wires, and install the replacement. An HVAC tech can do this in 20–30 minutes.
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Check 4: Control board relay (modern furnaces)
Applies to: furnaces with electronic control boards (most furnaces post-1995).
On modern furnaces, the control board controls the blower using relays. A relay that has welded contacts will keep the blower running continuously.
Signs: FAN setting is on AUTO, no heat call active, but blower runs. Cycling the power switch off and on doesn’t stop it.
This requires a control board replacement — a tech job. Control board costs vary widely by brand ($80–350 for parts).
Common furnace control-board and blower-motor brands include Carrier, Trane, Lennox, Goodman, Rheem, and York — parts pricing and availability both depend heavily on matching the exact OEM part number for your furnace’s specific control board revision, not just the brand.
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When the blower running constantly is intentional
Some homeowners run the furnace blower continuously on purpose for:
- Air filtration: circulating air through the filter more often
- Even temperature distribution: moving air between rooms when not heating
- Dehumidification: running through a whole-house dehumidifier inline
If someone in the household set FAN to ON for this reason, the blower running constantly is working as intended. The only downside is motor wear from higher run hours.
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Diagnostic summary
| Observation | Cause | Fix |
|---|---|---|
| FAN set to ON at thermostat | Setting | Switch to AUTO |
| FAN set to AUTO, warm air from vents | Normal heating cycle in progress | Wait for cycle to complete |
| FAN set to AUTO, cool air, blower won’t stop | Fan limit switch stuck or board relay welded | Check limit switch manual override; call tech if failed |
| Power cycle stops the blower, but it starts again immediately | Fan limit switch stuck or board relay | Replace limit switch or board |
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FAQ
Is it bad to run the furnace fan continuously? It shortens the blower motor’s life somewhat (higher run hours) and uses more electricity. With a variable-speed ECM motor, continuous fan is very efficient (50–100W). With a PSC (single-speed) motor, continuous running draws 300–500W and costs roughly $20–30/month in electricity at average rates.
My blower runs but the heat is off. Could it be summer AC mode? Yes — in the summer, the blower runs when the AC is cooling. If the thermostat is set to COOL and there’s a cooling call, the blower runs. This is normal. If you’re in summer and the blower runs without any cooling output, the thermostat’s FAN setting may be ON, or the thermostat isn’t sending a cooling call to the AC even though it’s running the blower.
Can the blower running constantly cause any damage? Beyond normal motor wear, running the blower without heating or cooling circulates unconditioned air, which can make the house feel less comfortable. If the high-limit switch is stuck and causing the blower to run after repeated limit trips, there may be an underlying airflow problem that needs fixing — but the blower itself running isn’t damaging.
What’s the difference between ECM and PSC blower motors, and does it matter for this problem? ECM (electronically commutated motor) and PSC (permanent split capacitor) both run continuously when the fan limit or thermostat calls for it. The main difference for this problem: ECM motors have their own control electronics that can sometimes cause fan-run faults independent of the main control board. An ECM that’s running continuously when it shouldn’t may indicate an issue with the ECM motor’s own control board, not the furnace control board.
If the blower motor itself needs replacement, see our guide to furnace blower motor replacement cost, including the PSC vs. ECM distinction that drives the price range.
