Last updated: 2026-05-01
🏠
Furnace Blower Running Constantly
Fan runs non-stop even when thermostat is off
55%
55% chance this is caused by:
Thermostat fan setting on 'ON' or stuck fan relay
🔧 The Exact Part You Need
Total parts cost: $40. Same-day shipping available on most items.
🔨 Tools You'll Need
Easy · 10-15 minutes
Parts: $40 vs $$2,500-6,000 to replace
📋 Step-by-Step Instructions
1
Check thermostat — fan should be set to AUTO, not ON
2
If thermostat is on AUTO and fan still runs, the fan relay is stuck
3
Turn off power to furnace
4
Locate the control board inside the furnace
5
Find the fan relay — it may be visibly stuck or burned
6
Replace the relay board
7
Test by setting thermostat to heat
Other Furnace Problems
Frequently Asked Questions
How much does it cost to fix a furnace that's blower running constantly?
The part costs about $40. If you do it yourself, that's your total cost. A professional service call would add $100-200 in labor, bringing the total to $190.
How long does this furnace repair take?
This repair is rated "easy" and typically takes 10-15 minutes. The most time-consuming part is usually accessing the component, not the actual swap.
What if this isn't the problem?
Our diagnosis is 55% likely based on the symptom you described. If replacing this part doesn't fix it, the next most common cause for "fan runs non-stop even when thermostat is off" would be related to the wiring harness or control board. In that case, consider calling a professional.
Should I just replace my furnace instead?
A new furnace costs $2,500-6,000. If yours is well under 15-20 years old, a $40 repair is far more cost-effective. If it's near end-of-life and has had multiple issues, replacement starts making more sense.