Furnace Blower Runs Constantly: Causes and Fixes

A furnace blower that runs constantly can mean several things — the simplest being that your thermostat fan is set to 'ON' instead of 'AUTO'. If that's not the issue, a stuck limit switch or relay is usually the cause. A blower that runs without producing heat may also indicate a safety condition the furnace is protecting itself from.

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Common Symptoms

Fan runs continuously day and night without cycling off
Fan runs even when thermostat is satisfied and no heat call is active
Cool air blowing from vents when furnace isn't in a heating cycle
Furnace won't complete a heating cycle — fan runs but burner goes off early
Higher than normal electric bill from continuous fan operation

Most Likely Causes

1
Thermostat Fan Set to ON (Most Common — Free Fix)
The thermostat has a 'FAN' switch with two settings: ON and AUTO. When set to ON, the blower runs 24/7 regardless of whether the furnace is heating. Set it to AUTO and the fan only runs during heating (and cooling) cycles.
2
Limit Switch Stuck Open
The furnace limit switch is a safety device that shuts off the burners if the furnace overheats. If it gets stuck in the open (tripped) position, the furnace will lock out the burner but keep the blower running to cool itself down. A clogged filter is the most common reason the furnace overheats and trips the limit switch.
3
Fan Relay Stuck Closed
The fan relay is an electrical switch on the control board that turns the blower on and off. If the relay contacts weld themselves closed from age or a power surge, the blower runs constantly regardless of thermostat commands.
4
Furnace Short-Cycling with Dirty Filter
A clogged filter causes the furnace to overheat, trip the limit switch, shut off the burners, and run the fan to cool down. This repeats in a short-cycle pattern. Replacing the filter often resolves this entire chain of events.

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Quick DIY Checks

Caution

A furnace blower running constantly due to a tripped limit switch may indicate a heat exchanger crack — a serious safety hazard that can allow combustion gases to enter your home. If you smell fumes or the furnace repeatedly trips the limit switch even with a clean filter, call an HVAC technician immediately.

1Check the thermostat FAN setting first: if it's set to ON, change it to AUTO. The fan should stop within a minute of the current heating cycle ending.
2Replace the air filter if it's dirty. A severely clogged filter causes the furnace to overheat and run the blower continuously to protect itself.
3Locate the limit switch on the furnace — it's usually a small oblong device on the furnace plenum or heat exchanger panel with a button or reset. Press the button to manually reset it. If it trips again after the next heating cycle, the furnace is overheating (likely due to a clogged filter or blocked vent).

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4Check for blocked supply or return vents in the house. Closed or blocked vents increase static pressure and can cause overheating, tripping the limit switch and causing continuous blower operation.
5If the fan relay is suspected, have an HVAC technician inspect the control board. A stuck relay is easily visible as it cannot be reset — it requires component replacement.

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Repair vs Replace

✓ Worth Repairing

Thermostat fan setting change is free. Filter replacement costs $10–$30. Limit switch replacement runs $20–$50 in parts. Relay or control board issues may cost $100–$200 to repair professionally. Replacement is only warranted if the heat exchanger is cracked — which is a safety issue requiring immediate action.

Est. Repair Cost

$0–$200 depending on cause

Est. Replacement Cost

$3,000–$6,000 for a new furnace

Recommended Tools & Parts

Furnace Air Filter (MERV 8–11)
High-quality pleated furnace filter. Replace every 1–3 months to maintain airflow and prevent limit switch trips.
$10–$25
Buy on Amazon →
Furnace Limit Switch
Replacement fan/limit switch for furnace overtemperature protection. Models vary — verify your furnace model before ordering.
$20–$50
Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.