Furnace Blower Running Constantly
Fan runs non-stop even when thermostat is off
55% chance this is caused by:
Thermostat fan setting on 'ON' or stuck fan relay
Furnace blower running constantly is a easy-difficulty repair. The most likely cause is thermostat fan setting on 'on' or stuck fan relay, and replacing the failed part takes approximately 10-15 minutes. The part costs about $40 — compared to $2,500-6,000 for a full replacement.
Fix it this weekend. Most parts ship same-day on Amazon.
A service call costs $150-300. This repair takes about 10-15 minutes and the part runs $40. Order the part tonight, fix it Saturday morning.
✓ Good call investigating this. Most homeowners wait until the problem gets worse — which typically means a higher repair bill. This is a easy-rated repair — 90% of homeowners with basic tools complete it successfully. A $40 part saves you a $150–300 service call.
The exact part you need is listed below — order it now and fix it this weekend.
Safety Warning
Turn off gas supply at the shutoff valve before servicing. If you smell gas at any point, evacuate and call your gas company. Furnaces use high voltage and contain combustible gas. Hire a professional for any work involving the heat exchanger, gas valve, or ignition system.
🔧 The Exact Part You Need
Total parts cost: $40. Same-day shipping available on most items.
What happens if you don't fix this
A $40 part now can prevent a much bigger problem later. Left unfixed, this issue can cascade into a $$2,500-6,000 replacement — that's × the cost of the part. The repair gets more expensive the longer you wait.
Find the Right Part for Your Furnace
Parts vary by model. Search with your model number for the best match.
Tip: Add your model number (e.g. "GE GSS25GSH") for more precise results.
🔥 Parts for Furnaces move fast on Amazon — especially common failure parts like these.
🔨 Tools You'll Need
Repair Summary
| Appliance | Furnace |
| Problem | Blower Running Constantly |
| Diagnosis Confidence | 55% |
| Most Likely Cause | Thermostat fan setting on 'ON' or stuck fan relay |
| Difficulty | easy |
| Estimated Time | 10-15 minutes |
| Parts Cost | $40 |
| Replacement Cost | $2,500-6,000 |
📋 Step-by-Step Instructions
Check thermostat — fan should be set to AUTO, not ON
If thermostat is on AUTO and fan still runs, the fan relay is stuck
Turn off power to furnace
Locate the control board inside the furnace
Find the fan relay — it may be visibly stuck or burned
Replace the relay board
Test by setting thermostat to heat
Don't put this off.
A Furnace that's failing can cause water damage, electrical issues, or food spoilage — each costing 10x what the replacement part costs. The fix is straightforward. The part is cheap. The risk of waiting isn't.
🔍 Why This Happens
Root cause analysis for your furnace's blower running constantly
The fan relay board in your furnace is an electro-mechanical component that fails through electrical arcing, contact pitting, or winding breakdown. These components typically fail after years of repeated switching cycles — the contacts degrade gradually until they stop making a reliable connection. At 55% match rate for your symptom, this is the component to replace.
55%
Diagnostic match rate
High
DIY success rate
10-15 minutes
Typical repair time
🛡️ Prevention Tips
How to prevent your furnace's blower running constantly from happening again after this repair.
Replace the air filter every 1-3 months during heating season. A dirty filter is the #1 cause of premature furnace component failure.
Have the system inspected annually before heating season. A technician can catch wear on the igniter, flame sensor, and heat exchanger early.
Keep all vents and registers open and unblocked. Closed vents create pressure imbalances that stress the blower motor.
Don't store items near the furnace — restricted airflow and combustible materials near the unit are safety and performance hazards.
Before installing the new fan relay board, inspect the wiring connectors for corrosion, melting, or loose fits. Electrical failures often damage the connector, not just the component.
Other Furnace Problems
Common Questions About This Repair
How much does it cost to fix a furnace that's blower running constantly?
How long does this furnace repair take?
What if this isn't the problem?
Should I just replace my furnace instead?
🔧 Repair Data Sources
Repair guides are compiled from:
- • Manufacturer service manuals and technical bulletins
- • Common failure pattern data from appliance repair technicians
- • OEM parts databases for exact replacement compatibility
Difficulty ratings are based on the average homeowner's skill level, not professional standards. Confidence percentages reflect the likelihood that the listed cause matches the observed symptom. Updated May 2026.
Disclaimer: This repair guide is for informational purposes only and is based on manufacturer service manuals and published repair data. TheFixGuide is not responsible for any damage, injury, or loss resulting from attempted repairs. If a repair involves gas, high voltage, stored energy (capacitors), or refrigerant, we strongly recommend hiring a licensed professional.
🔨 Need tools? Get parts on Amazon
Get Parts →Dex A. Varland
Master Appliance Technician · 15+ Years Experience
Dex A. Varland is a master appliance technician with 15+ years of hands-on experience across all major brands. He has diagnosed and repaired thousands of household appliances and contributes to TheFixGuide to help homeowners tackle repairs with confidence.
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