Furnace Blower Fan Won't Stop Running? Portland Troubleshooting Guide
"My furnace blower won't turn off!" - This annoying problem wastes electricity and indicates an electrical or control system fault. From faulty limit switches to thermostat wiring issues, we diagnose and fix the eight most common causes of continuous blower operation in Portland homes.
NORMAL OPERATION
- • Runs during heating cycles
- • 30-120 seconds after heat stops
- • Stops when thermostat satisfied
- • Quiet, normal operation
- • Only when fan switch calls for it
PROBLEM SIGNS
- • Runs continuously 24/7
- • Won't stop after heat cycle
- • Runs when thermostat off
- • Loud or unusual noises
- • Higher electric bills
Electrical diagnosis • Control board testing • Portland furnace experts
Owner & Lead HVAC Technician
Safe Checks First
Quick DIY Checks (Safe to Do Yourself)
Check thermostat fan switch - Ensure it's set to "Auto", not "On"
Turn thermostat completely off - Blower should stop within 2 minutes
Check circuit breaker - Reset if tripped, blower should stop
Remove thermostat from wall - Check for loose or damaged wiring
⚠️ STOP - Don't Do These!
- • Don't unplug the furnace (damages control board)
- • Don't bypass safety switches
- • Don't work on live electrical circuits
- • Don't ignore continuous blower operation
Why Furnace Blowers Run Continuously
The furnace blower (fan) should only run when heating or cooling, plus a brief cool-down period after heating stops. Continuous operation wastes electricity and indicates a control system problem. Portland's humid climate can accelerate electrical component failure, making this a common issue in our area.
The eight causes below are ordered from most common to most complex. Start with the simple thermostat checks, then move to professional electrical diagnosis. Most blower issues are electrical rather than mechanical, requiring professional expertise for safe repair.
Portland-Specific Blower Issues
- Humidity corrosion affects electrical contacts and switches
- Older homes have outdated control systems prone to failure
- Crawlspace installations expose components to moisture
- Power quality issues from older electrical systems
- Dust and pollen can interfere with sensors and switches
Cost of Continuous Blower Operation
A typical furnace blower uses 300-500 watts continuously. At Portland electric rates (~$0.12/kWh), this costs $30-50 per month in wasted electricity. Plus the noise disturbance and potential motor wear make it worth fixing promptly.
Faulty Blower Fan Limit Switch
The fan limit switch controls blower operation based on heat exchanger temperature. When it fails, the blower continues running after the heat cycle ends, often for hours. Common in older Portland furnaces where humidity corrodes electrical contacts.
Portland Fix:
Replace fan limit switch ($150-$250). Test temperature settings and electrical continuity. Professional diagnosis required to avoid safety risks.
Stuck Blower Fan Relay
The relay that controls blower power gets stuck in the "on" position. Can be caused by electrical arcing, power surges, or component failure. The blower runs continuously regardless of thermostat settings.
Portland Fix:
Replace blower relay or control board ($200-$400). Test electrical system for shorts or voltage issues. Modern furnaces have more reliable solid-state controls.
Thermostat Wiring Problem
Wiring between thermostat and furnace gets crossed or shorted. "Fan" wire touching "hot" wire causes continuous blower operation. Common during thermostat replacements or when rodents chew wiring.
Portland Fix:
Check thermostat wiring at furnace and thermostat. Use voltage tester to identify shorts. Rewire correctly ($100-$200). Consider wireless thermostat to eliminate wiring issues.
Failed Furnace Control Board
The main control board that manages all furnace functions develops a fault. Modern boards use microprocessors that can fail from power surges, age, or component wear. Results in blower running continuously or erratically.
Portland Fix:
Replace control board ($300-$600). Professional diagnosis required - don't guess at board replacement. Transfer settings from old board if possible.
Airflow Obstruction/Sensor Failure
Pressure switches or airflow sensors detect proper air movement. When they fail or detect restrictions, the control board may keep the blower running trying to establish airflow. Dirty filters or blocked vents trigger this.
Portland Fix:
Clean/replace filter, check for duct obstructions. Test pressure switches and airflow sensors. Repair or replace failed components ($150-$300).
Manual Fan Switch Set to "On"
Some thermostats have a "Fan" switch with "Auto" and "On" positions. If accidentally set to "On" (or stuck), the blower runs continuously. Easy to overlook but common cause of "blower won't stop" calls.
Portland Fix:
Check thermostat fan switch position. Set to "Auto" for normal operation. If switch is stuck, thermostat may need replacement ($50-$150).
Overheating Protection Circuit
Safety circuits designed to prevent overheating can malfunction, keeping the blower running to cool the system. Occurs when temperature sensors fail or heat exchanger gets too hot from airflow restrictions.
Portland Fix:
Clean heat exchanger, check for airflow restrictions. Test temperature sensors and safety circuits. Replace failed components ($200-$400).
Power Supply Issues
Electrical problems like low voltage, dirty contacts, or failing transformers can cause control board confusion. The blower may run continuously as the system tries to maintain operation despite electrical instability.
Portland Fix:
Test voltage at furnace, check transformer output. Clean electrical contacts, replace failing components. Professional electrical diagnosis ($150-$300).
Safe Steps to Stop Blower Immediately
Turn thermostat to OFF position - blower should stop within 2 minutes
If blower continues, turn off power at main breaker (label which one!)
Wait 5-10 minutes for control board to reset, then restore power
If problem persists, call professional - do not operate furnace
Portland Furnace Blower FAQs
Stop the Annoying Blower - Get Professional Electrical Diagnosis
Continuous blower operation wastes money and indicates a control system problem. Our certified technicians diagnose and fix electrical issues safely, from faulty switches to control board failures. Stop the noise and start saving electricity today.