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Furnace Blower Won't Stop Running? The Limit Switch or Thermostat Fan Is Stuck

Your furnace blower keeps blowing cold air 24/7 and won't shut off. The fan limit switch is stuck or the thermostat fan setting is on ON instead of AUTO — two free fixes.

Category:HVAC

Difficulty:Easy

Time:15 min

Success:50%

Updated:May 22, 2026

quick_referenceQuick Answer

For Furnace Blower Won't Stop Running? The Limit Switch or Thermostat Fan Is Stuck, start with "Check if your thermostat fan is set to ON instead of AUTO": Stop assuming it's a furnace problem. Walk to your thermostat and look at the fan setting. If it says ON, switch it to AUTO. Wait 2 minutes. If the blower stops, you've been paying to circulate cold air for no reason and your furnace is fine. The fan ON setting runs the blower 24/7 for air filtration — useful if you have a high-end filter, but a waste of electricity otherwise. Stop DIY if the furnace is tripping the high-limit safety (burner cycles on and off rapidly) — this indicates an airflow problem like a clogged filter or collapsed duct, not a limit switch issue. This is listed as a easy recovery and usually takes about 15 min.

verifiedGuide Snapshot

Repair areaHVAC

Estimated time15 min

DifficultyEasy

Stop conditions4

Last updated May 22, 2026. Review the stop conditions before continuing.

account_treeRecovery State

DeviceFurnace

Current stateBlower Runs Constantly

Specific stateLimit Switch Stuck

Failed stepBlower Control

Likely failure typeSensor Fault

DIY boundaryDIY recovery first

paymentsCost decision

help1. Understand the Problem

Furnace blowers are controlled by a fan limit switch — a combination temperature sensor and timer inside the furnace. When it fails in the closed position, it sends constant 120V to the blower motor regardless of burner status. The other common cause is simply the thermostat fan switch set to ON instead of AUTO, which runs the blower continuously even when there's no call for heat.

build_circle2. Try This First

Best First Step

Check if your thermostat fan is set to ON instead of AUTO

Stop assuming it's a furnace problem. Walk to your thermostat and look at the fan setting. If it says ON, switch it to AUTO. Wait 2 minutes. If the blower stops, you've been paying to circulate cold air for no reason and your furnace is fine. The fan ON setting runs the blower 24/7 for air filtration — useful if you have a high-end filter, but a waste of electricity otherwise.

visibility3. Visual Guidance

See what's happening and how to try the first recovery step.

Shut off furnace power and locate the fan limit switchTurn off the furnace at the disconnect switch or breaker. Remove the furnace front panel. Look for the fan limit switch — a rectangular metal box (about 2x3 inches) with a dial or metal disc on the front and wires going to the blower motor. It's usually mounted on the heat exchanger housing with a bi-metal probe extending inside.

Test the limit switch with a multimeter — it should read open when coldWith the furnace cold and power off, test continuity across the blower terminals on the limit switch (usually marked BLOWER or FAN). A properly functioning cold limit switch reads OPEN (infinite resistance). If you read continuity (near 0 ohms) with the furnace cold, the switch is stuck closed and sending constant power to the blower. Replace it.

Replace the fan limit switch — match the insertion length exactlyOrder the exact replacement by model number. Remove two screws holding the switch and carefully pull the old switch out with its probe. Slide the new switch's probe into the same hole — it must go to the same depth as the old one or the temperature readings will be wrong. Tighten the two mounting screws, reconnect the wires exactly as labeled, and replace the front panel. Restore power and test.

autorenew4. If That Doesn't Work

Try the next recovery options.

electrical_services

Check for a stuck blower relay on the control boardSome newer furnaces use a relay on the control board instead of a mechanical limit switch to control the blower. If you don't have a fan limit switch (the furnace is less than 10 years old), find the blower relay on the board and tap it gently with the back of a screwdriver. A stuck relay sometimes frees up — but it will stick again and should be replaced.

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tune

Adjust the fan-off temperature setting if blower runs too long after heat cycleOn the limit switch dial, there are adjustable pointers for FAN ON, FAN OFF, and LIMIT. If the blower runs for 5+ minutes after the burner shuts off, the FAN OFF setting may be too low. For most furnaces, FAN ON should be around 120-130°F and FAN OFF around 90-100°F. Adjust with a flathead screwdriver.

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Check the thermostat wiring if the fan runs with thermostat removed from wallRemove the thermostat from its wall plate. If the blower stops immediately, the thermostat is shorting the fan circuit internally. If the blower keeps running with no thermostat connected, the problem is in the furnace — either the limit switch or a shorted G-wire touching R.

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Frequently Asked Questions

Does running the blower 24/7 damage the furnace?expand_more

No, but it wastes electricity (a blower motor draws 400-800 watts continuous) and can increase humidity in summer if the AC coil is wet. Some HVAC pros recommend ON for better air filtration, but AUTO is more energy efficient for most homes.

Why does my blower run for 2 minutes then shut off and restart repeatedly?expand_more

This is short-cycling, usually caused by a clogged air filter. The furnace heats up too fast because airflow is restricted, hits the high-limit safety, shuts off the burner, cools down, then restarts. Replace the filter (1-inch filter every 1-3 months, 4-inch every 6-12 months).

Can I just unplug the blower until I fix it?expand_more

If the furnace isn't heating and the blower is just running cold air, yes — kill power at the breaker for safety and to stop the noise. But if it's heating season and the burner is firing, the blower MUST run or the heat exchanger will overheat and crack.

warning5. Stop DIY If

Don't continue if any of these apply.

reportThe furnace is tripping the high-limit safety (burner cycles on and off rapidly) — this indicates an airflow problem like a clogged filter or collapsed duct, not a limit switch issue.

reportYou smell natural gas — evacuate immediately.

reportThe blower motor is smoking, sparking, or has a burning smell — the motor is failing and drawing excessive current.

reportThe furnace is more than 25 years old — parts may be obsolete and the heat exchanger may be cracked. Consider replacement.

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