If your blower motor doesn’t turn, you likely have a bad blower motor. Maybe the motor is dead or defective. However, a blower motor that won’t turn may also point to other issues such as underlying circuitry or power supply challenges.
For example, maybe the lines supplying your fan are broken, or the wires are loose. Alternatively, you may have a faulty fan limit switch. Let’s discuss the most causes in detail to better understand what you can do.
Reasons Your Furnace Blower Won’t Turn On
1. It’s a Thermostat Setting Issue
Most thermostat issues originate at the thermostat, and blower motor issues aren’t different. If your blower isn’t turning, you most likely have a faulty thermostat. You see, even after you turn on the furnace, it will not produce heat until the thermostat calls for heating.
Sometimes the blower may run even with heating off. However, most people set the fan to AUTO for convenience. In this case, the fan will only run when heating is ongoing and go off when heating stops – even when the furnace is still switched on.
So, it means that your blower fan is off because the heating is off. If you want it to run even when heating is off, you need to set it to the ON position on the thermostat.
Solution: Check your thermostat settings to determine whether your fan is set to ON or AUTO and change the settings accordingly. If you want the fan to continue running even when the furnace isn’t heating, set it ON.
2. The Thermostat is Faulty
Still, on the thermostat, you may also want to diagnose it for internal damage. If the thermostat is broken, the furnace may not work. Therefore, the blower motor won’t run either.
The reason is as explained earlier. Furnaces only heat when the thermostat says so and will remain idle for as long as the thermostat doesn’t call for heating. So, if the thermostat is broken and can’t call for heating, your furnace will stay off no matter how many times you try to switch it on.
However, remember that the fan is also set at the thermostat. So, if the thermostat breaks, you completely lose control of the fan too. So, the fan may stay off until you fix the thermostat.
Solution: The easiest way to check whether you have a broken thermostat is to bypass it for a moment. Turn the power off, unplug the red and white wires from the thermostat, and connect the wires directly. Then turn the furnace back on and see if heating resumes.
If so, you need to fix or replace the thermostat.
3. The Fan isn’t Getting Power
Hopefully, you’ve confirmed that the rest of the furnace has power. You’ll know you have power if the burners light up when you turn on the furnace. The next step is to check whether the fan is getting power.
The furnace blower motor gets power from two sources. Primary power to run the motor comes from the furnace’s main circuit board. It’s typically 24V derived from an onboard step-down transformer. Additionally, the motor uses a capacitor to store starting power as 24V is too little to kickstart the motor.
However, since the capacitor also gets power from the circuit board, the first place to check if you suspect an electrical malfunction in the connection to the mainboard or the board itself.
Solution: Is the circuit board in perfect working condition? Are the wires leading to the blower motor intact? Or do you have a few loose wires and short circuits? You may need an HVAC pro or an electrician to diagnose the issue fully.
4. The Blower Motor Capacitor is Defective
We’ve mentioned that the blower motor gets starting power from a special capacitor. These capacitors are fairly powerful, typically ranging from 200V to 450V. That’s a lot more than the furnace’s circuit board’s 24V.
The extra power provides the extra thrust necessary to start the motor. Now, imagine if the capacitor is dead. The blower capacitor wouldn’t have the power to start.
So, it will stay off even when the furnace is running. Usually, you’ll hear the motor humming, but the blades are not moving. The same applies if the capacitor is dying away.
Maybe it’s too old and losing the capacity to store charge. In this case, the fan may attempt to start and fail repeatedly. So, you may hear it blow for a few seconds and die out repeatedly.
Solution: The first step is to test the capacitor’s charge storage capacity. If it’s dead or aging, replace it. The capacitor is dead if it’s black or has a burning smell.
5. The Fan Limit Switch is Stuck Open
The furnace blower motor is controlled by a temperature limit switch which determines when the fan comes on and when it goes off.
The switch closes several seconds after you turn on the furnace when the furnace temperature hits the ON setting, typically 100°F, and shuts down several minutes after you shut down the furnace when the temperature at the furnace drops below 90°F.
Unfortunately, the limit switch isn’t fail-proof. It can break down due to damage or owing to age. It may also be affected by electrical shorts.
If the switch experiences damage that causes it to get “stuck” when open, it will not run until you reset it. It can also get stuck due to dirt buildup.
Solution: Regular maintenance is the ultimate solution to fan limit switch issues. However, if the issue has already happened, try to reset it. But if that doesn’t work, call a technician to diagnose the wiring and extent of wear to determine whether to replace the switch.
6. You have Airflow Challenges
This simply means that warm and cold air isn’t moving freely through the furnace and around your home as something impedes the flow.
For this case, you want to check your air filter and the furnace vents specifically. The furnace blower is located right after the filter next just beyond the return air vents.
This setup allows it to pull cold air into the furnace. However, it cannot pull any air if the filter is blocked, perhaps due to the buildup of dirt and debris.
As a result, the furnace may experience overheating within the internal chambers, causing the limit switch to shut down the heating process and, by extension, the blower fan.
The same will happen if the furnace vents, supply or return, are partially or fully blocked. The furnace will experience overheating and consequently shut down, shutting down the fan too.
Solution: Again, regular maintenance is the key. Make sure to schedule seasonal furnace inspections so the technician can tune up the appliance for optimal functioning throughout the heating season.
7. The Switch is Defective or Dead
Finally, the furnace blower may also refuse to turn on if it’s defective or damaged. Defective here means that one of the components isn’t performing normally. A typical example is when the drive belt (where applicable) is damaged or too worn.
A worn drive belt can become too loose to rotate the blower motor. The blower motor can also become defective or die out. For instance, if the wire coils become loose, the motor cannot function normally and may eventually refuse to work.
Solution: Inspect the furnace closely to find out if the different parts are functioning normally. Are the fan blades straight and unobstructed? Is the motor alive?
Some of these parts can be repaired or repacked by a professional. However, if the motor is dead, you must replace the entire blower system.
Summary
The blower motor is a critical part of the furnace, without which the furnace cannot work. Therefore, it can be worrying when you find out that the furnace blower won’t turn on.
Fortunately, you can personally troubleshoot most of the reasons why a furnace blower motor may fail. Just make sure to call the pros whenever you feel overwhelmed.