Furnace Fan Won’t Turn Off 
A blower fan or motor may fail to shut off on a natural gas or propane furnace for a number of reasons. This is actually a very common occurrence, and it can be easily remedied with at-home troubleshooting, without the need to call a technician. Read this article, and if we haven’t given you a solution on how to fix your furnace fan, it may be best to call a technician.
669 Heat HVAC technicians are available 24/7/365 days a year! That includes after-hours, weekends, and holidays at no extra cost to you! Book your appointment today!
Why is my furnace fan blowing constantly?
There are a few reasons why a furnace fan won’t turn off, and those are typically due to:
- The fan select switch on the thermostat is set to “Fan ON,” rather than auto.
- A fan limit control, fan sequencer relay, bad relay, or circuit board has its contacts sticking – although this issue depends on the age and type of furnace.
- A wiring problem where the wiring on the insulation is worn off due to age or neglect.
- An issue with the limit switch opening or with the limit switch inherently.
- A clogged or dirty air filter can also restrict airflow, which can cause the furnace to overheat. As a safety measure, the fan might run continuously to cool down the system. Check and replace the air filters.
- HVAC zoning, if your home has multiple zones, can mean you have separate thermostats controlling different parts of the house. A zoning system might cause the fan to run continuously if one zone is calling for heating.
However, these might not be the only issues. Call a technician if you are unable to determine the root cause.
Why is my fan running when my thermostat is off?
If your furnace is running even when the thermostat is set to “off,” there could be several reasons for this unexpected behaviour. One possible explanation is a malfunction in the thermostat itself. Thermostats rely on sensors to detect temperature changes and send signals to the furnace to turn on or off. If these sensors are faulty or the thermostat’s internal wiring is damaged, it might inaccurately signal the furnace to run despite being set to the “off” position.
Another potential cause could be a wiring issue within your HVAC system. Crossed or short-circuited wires could send continuous signals to the furnace, bypassing the thermostat’s control. This could lead to the furnace running even when the thermostat is telling it to stay off.
In some cases, a malfunctioning relay or control board within the furnace itself could be responsible. These components help manage the furnace’s operation and respond to signals from the thermostat. If they’re not functioning properly, they might keep the furnace running regardless of the thermostat’s setting.
Provided the complexity of most HVAC systems (plus their potential hazards) we always recommend consulting a professional technician to identify and then resolve the issue. They will be more accurate in identifying the root cause of the issue and can take any necessary steps to repair the matter to ensure your furnace operates correctly.
How do you fix a fan that keeps running?
Before you decide to call a technician to diagnose and remedy the issue, you may want to follow a couple of troubleshooting steps to clear up the matter first. If you notice any issues beyond what you’re capable of handling during troubleshooting, however, call a tech.
1. Thermostat Check.
While it may sound a bit silly, it might be a surprise to you how few people think to turn their thermostats to the correct temperature and mode. The fan may be set to continuously run if the thermostat is in the “Fan ON” mode rather than auto.
2. Power Cycling
Turn off the furnace at the circuit breaker or main power switch. Wait a few minutes, then turn it back on. This can sometimes reset any electronic glitches causing the fan to run constantly.
3. Check Fan Limit Switch
The fan limit switch is a safety device that tells the furnace fan when to turn on and off. If this switch is malfunctioning, it could cause the fan to run non-stop. Consult your furnace’s manual to locate the switch and follow any troubleshooting steps provided.
4. Air Filter Inspection
A clogged air filter can cause the furnace to overheat, leading to the fan running continuously. Check and replace the air filter if it’s dirty or blocked.
5. Thermostat Wiring
Inspect the wiring connecting the thermostat to the furnace. Loose or damaged wires could be causing incorrect signals to be sent to the fan.
6. High Limit Switch Check
Similar to the fan limit switch, the high limit switch detects when the furnace is too hot. If it’s faulty, it might keep the fan running to cool down the system. Refer to your furnace’s manual for guidance on checking and troubleshooting the high-limit switch.
7. Check for Obstructions
Make sure there are no obstructions around the furnace’s intake and exhaust vents. Restricted airflow can lead to overheating, which might trigger the fan to run continuously.
8. Inspect Relays and Controls
Inside the furnace, there are relays and control boards that manage the fan’s operation. A malfunctioning relay or control board could cause the fan to run continuously. Look for any visibly damaged components or burn marks.
9. Call a Professional
If you’ve gone through the above steps and the issue still persists, it’s time to call a qualified HVAC technician. They have the expertise and tools to diagnose and fix complex furnace problems safely.
Is your furnace on the fritz? If so, 669 Heat is here to help! Our technicians are available 24/7/365 days a year! That includes after-hours, weekends, and holidays at no extra cost to you! Furthermore, if your current furnace is deemed beyond repair, make sure to ask us about our current discounts and financing options, so you can save on your new furnace! You’ll also receive a complimentary new system purchase quote from a home comfort advisor!
