Furnace Blower Fan Won't Stop Running | Complete Portland Diagnosis Guide
Furnace blower fan won't stop running? Complete diagnosis guide for Portland homes. Thermostat settings, limit switch problems, fan relay failure, electrical issues. Continuous fan operation causes.
Read Time
15 min
Service Area
Portland Metro
License
CCB #187834
Owner & Lead HVAC Technician
A furnace blower fan that won't stop running is a common but concerning problem that homeowners face. While the blower fan is designed to run for a short time after heating cycles to prevent condensation, continuous operation indicates an underlying issue that needs attention. This problem can stem from simple thermostat settings to serious electrical or safety system failures.
In Portland's mild climate, where furnaces don't run as continuously as in colder regions, this issue becomes more noticeable and wasteful. Continuous fan operation not only wastes electricity but can also indicate safety problems like limit switch failure that could lead to overheating or other dangerous conditions.
This comprehensive guide will help you understand why your furnace fan won't stop, how to diagnose the problem, and when to call for professional help. We'll cover the most common causes, safety considerations, and Portland-specific factors that can affect blower fan operation.
Normal vs Abnormal Fan Operation
Normal: Fan runs during heating cycles and continues 1-3 minutes after to cool the heat exchanger. Abnormal: Fan runs continuously (more than 5-10 minutes after heating stops) or never stops at all.
Most Common Causes of Continuous Fan Operation
Thermostat Fan Switch Set to 'ON'
Symptoms:
Fan runs continuously, regardless of temperature
Diagnosis:
Check thermostat fan switch position
Solution:
Set thermostat fan switch to 'AUTO' position
Danger Level:
Low - just wastes electricity
Portland Cost: $0 (DIY fix)
Faulty Limit Switch
Symptoms:
Fan runs continuously, furnace may overheat
Diagnosis:
Limit switch doesn't open when it should
Solution:
Replace limit switch ($150-300)
Danger Level:
Medium - can cause overheating
Portland Cost: $200-400 with labor
Stuck Fan Relay
Symptoms:
Fan won't stop when heating cycle ends
Diagnosis:
Relay contacts stuck in closed position
Solution:
Replace fan relay or control board ($200-600)
Danger Level:
Low - electrical component failure
Portland Cost: $300-700 with labor
Control Board Failure
Symptoms:
Fan ignores thermostat commands, erratic operation
Diagnosis:
Control board not sending proper signals
Solution:
Replace control board ($300-600)
Danger Level:
Medium - affects multiple functions
Portland Cost: $400-800 with labor
Wiring Issues
Symptoms:
Intermittent fan operation, doesn't respond properly
Diagnosis:
Loose or damaged wiring connections
Solution:
Repair/replace wiring ($100-300)
Danger Level:
Medium - electrical hazard
Portland Cost: $200-500 with labor
Understanding Thermostat Fan Settings
AUTO Mode
Description:
Fan only runs when heating or cooling is needed
Best For:
Energy efficiency, normal operation
Indicator:
Fan stops when heating/cooling cycle ends
Common Issue:
If fan runs constantly, switch is likely set to 'ON'
ON Mode
Description:
Fan runs continuously for air circulation
Best For:
Improved air quality, constant filtration
Indicator:
Fan runs all the time, regardless of temperature
Common Issue:
Wastes electricity, but not a furnace malfunction
CIRCULATE (some models)
Description:
Fan runs intermittently for air circulation
Best For:
Balanced air quality and energy savings
Indicator:
Fan runs briefly every 10-20 minutes
Common Issue:
Normal operation, not a malfunction
Step-by-Step Diagnosis Process
Thermostat Check
Action:
Locate thermostat and check the fan switch. Ensure it's set to 'AUTO', not 'ON'. Also check for 'FAN' or 'CONTINUOUS FAN' settings.
Tools:
None - visual check
Indicates: If set to 'ON', that's the cause
Power Supply Verification
Action:
Ensure furnace has power (green light on). Check circuit breaker hasn't tripped. Verify thermostat has good batteries.
Tools:
None initially
Indicates: Power issues can cause erratic fan operation
Fan Relay Test
Action:
Listen for clicking when thermostat calls for heat. Fan should start when heating begins and stop shortly after.
Tools:
None - auditory check
Indicates: No clicking or continuous operation = relay failure
Limit Switch Inspection
Action:
Professional only - checks if limit switch opens/closes properly. Tests continuity and calibration.
Tools:
Multimeter
Indicates: Failed switch prevents proper fan shutdown
Control Board Diagnosis
Action:
Check for error codes, test voltage outputs, verify timing sequences. May require furnace model-specific tools.
Tools:
Multimeter, diagnostic tools
Indicates: Board failure if signals are incorrect
Safety Considerations & Risks
Overheating Hazard
Description:
Continuous fan operation can mask limit switch failure, leading to heat exchanger overheating
Warning:
Can cause heat exchanger cracks, carbon monoxide leaks
Immediate Action:
Shut down furnace immediately if you suspect overheating
Prevention:
Annual professional maintenance includes limit switch testing
Electrical Fire Risk
Description:
Faulty relays or control boards can cause electrical shorts or arcing
Warning:
Risk of electrical fire, especially with damaged wiring
Immediate Action:
Don't attempt electrical repairs yourself
Prevention:
Licensed electrician for any wiring work
Carbon Monoxide Danger
Description:
Cracked heat exchanger from overheating can leak CO
Warning:
Invisible, odorless gas - install CO detectors
Immediate Action:
Evacuate if CO detector alarms, call emergency services
Prevention:
Annual heat exchanger inspection
Energy Waste
Description:
Continuous fan operation wastes significant electricity
Warning:
Can increase monthly electric bill by $50-100
Immediate Action:
Address immediately to save energy costs
Prevention:
Regular thermostat and system checks
Portland-Specific Contributing Factors
Older Home Wiring
Local Issue:
Many Portland homes have outdated electrical systems
Impact:
Increases risk of wiring failures causing fan issues
Solution:
Professional electrical inspection
Cost:
$150-300
Humidity Effects
Local Issue:
Portland's humidity can corrode electrical contacts
Impact:
Relay contacts stick, control boards fail prematurely
Solution:
Keep furnace area dry, annual electrical cleaning
Cost:
$50-100 annual maintenance
Power Quality
Local Issue:
Variable power quality in some Portland neighborhoods
Impact:
Can cause control board failures or erratic fan operation
Solution:
Install surge protector or power conditioner
Cost:
$100-200
Cold Weather Stress
Local Issue:
Rare but severe cold snaps stress electrical components
Impact:
Can cause limit switch or relay failures
Solution:
Winterization service includes electrical checks
Cost:
$100-200
Energy Cost Impact in Portland
Continuous fan operation in Portland can add $50-100 to your monthly PGE bill. At current rates of $0.15-$0.20 per kWh, a blower motor running 24/7 consumes significant electricity. In our mild climate where heating demands are lower than colder regions, this waste becomes even more noticeable and expensive.
Address continuous fan operation quickly - it's both a comfort issue and an energy waste problem.
When to Call a Professional
🚨 Emergency Situations
- •Furnace shows signs of overheating
- •Unusual burning smells
- •Carbon monoxide detector alarms
- •Loud noises or vibrations
- •Visible sparks or flames
🔧 Regular Service Situations
- •Thermostat check confirms fan set to 'AUTO'
- •Fan runs longer than 10 minutes after heating
- •Problem persists after thermostat adjustment
- •Furnace is over 5 years old
- •You're uncomfortable diagnosing electrical issues
Why Professional Diagnosis Matters
Continuous fan operation can indicate serious safety issues that require specialized diagnostic tools and training. Licensed HVAC technicians can properly test electrical components, measure voltage, and identify problems that aren't visible to homeowners. In Portland, where humidity can accelerate electrical component failure, professional service ensures your furnace operates safely and efficiently.
A $150 diagnostic visit can prevent thousands in potential damage or safety hazards.