The furnace fan limit switch is located within the furnace panel cover, typically in the upper right corner, from where it controls the operation of the furnace blower.
It turns on the fan when the furnace is running normally and turns it off if the heating process is interrupted or when you shut down the furnace. It can also shut the furnace if there’s overheating.
So, you can see why the switch is critical to smooth furnace operation. You’d be in serious trouble if the switch malfunctions.
Read on to learn how to troubleshoot the switch, ways to test it, and how to reset your fan limit switch in case it malfunctions.
What Does the Furnace Limit Switch Do?
The furnace limit switch is a relay circuit within the furnace that determines the operational state of the furnace blower. It decides when to turn on the blower and when to turn it off. The on and off switching happens at specified temperatures.
It will kickstart the blower when the furnace reaches a specified minimum temperature and shut it down when the furnace reaches a specified maximum temperature.
It’s one way the furnace regulates heating in your home. The temperature limit switch works alongside the thermostat to ensure that the home’s occupants enjoy a warm, cozy environment without subjecting the furnace to extreme strain or the occupants to unnecessary overheating risk.
How the Limit Switch Works
When you turn on the furnace, the thermostat sends a signal to the control board that turns on the furnace and initiates the heating process. However, the limit switch doesn’t close immediately.
Instead, it remains off until the minimum temperature limit is reached, at which point the switch, essentially a relay circuit, closes, allowing electric current to flow to the fan.
The fan will then spring to life. The blower keeps running until temperatures either exceed the upper limit or lower limit. So, it will keep running even when the furnace cycles off.
Remember that all furnaces run in cycles, typically 15 minutes on, then 15 minutes off, and so on. However, the fan doesn’t follow the cycle. Instead, it keeps running to ensure even heating throughout the home. If overheating causes the furnace to exceed the high-temperature limit, the limit switch will trip, causing the fan to shut down.
Otherwise, it will run until you turn off the furnace. The typical low and high limits are 90°F and 200°F, respectively. This means that the switch will turn off the blower fan if furnace temperatures are 90°F or lower or 200°F or higher.
Most furnace limit switches also have an ON setting, i.e., the temperature at which the limit switch will activate the fan. Although the value varies from one manufacturer to the next, it’s 100°F in most furnaces.
Do All Furnaces Have a Fan Limit Switch?
Yes, all furnaces have a high limit switch as it’s critical to the operation of the furnace. For one, the limit switch is solely responsible for turning the blower motor on and off. There’s no other way to operate the blower motor.
Additionally, the limit switch serves as a safety mechanism in case of furnace overheating. It instantly trips to stop the heating process, thus preventing potentially deadly consequences, such as heating fires or internal furnace damage.
All furnaces rely on the blower motor to disperse warm air throughout the home and draw cold air into the combustion chamber.
Where is the Furnace Fan Limit Switch Located?
The furnace fan limit switch is located inside the furnace panel cover, near the upper right corner. To access it, proceed as follows;
Turn off the furnace
Switch the furnace off at the circuit breaker. Then switch off the power at the furnace too.
Remove the access panel
Most panels come off without removing any bolts or screws. However, you need to remove one or two bolts/screws. Refer to your furnace manual to find out how to remove your panel.
Locate the fan limit switch
The switch is mounted on the furnace air plenum and is easily recognizable due to the wiring. It has at least two pairs of wires. The first set goes to the limit switch while the other pair ends at the fan switch.
How Do I Know if My Fan Limit Switch is Bad?
It’s best to have the limit switch checked during annual furnace fine-tuning. This allows HVAC technicians to fix potential issues in advance for trouble-free heating throughout the winter period. However, you must also continuously watch for signs of trouble whenever you turn on the heater.
You want to watch out for the following particularly;
- The furnace fan doesn’t come on: You’ll know that the furnace blower motor is off if you can’t feel warm air coming from the supply vents. A common reason for this is a failed or defective limit switch. Or maybe the low limit temperature is set too high.
- The furnace fan runs nonstop: The limit switch is supposed to open the circuit, thus turning off the fan when temperatures drop below the lower limit, typically when you shut down the furnace. So, whenever the fan seems to run nonstop, it may point to a limit switch issue. Perhaps the sensors responsible for detecting low temperatures are defective. Or maybe the switch is “stuck” in an ON position.
How to Bypass the Furnace Fan Limit Switch
If you’re wondering if you can bypass the furnace limit switch, the answer is yes. You can easily remove it from the equation and let the rest of the furnace run without it. However, beware that bypassing the furnace limit switch is dangerous and highly unrecommended.
For one, bypassing the limit switch means you’re completely exposed in case of furnace overheating. You wouldn’t be able to count on the protection of the high-limit switch. Additionally, bypassing the limit switch can cause damage to the switch, ultimately necessitating limit switch replacement.
That said, though, sometimes bypassing the fan limit switch may be the only way to prove beyond doubt that your limit switch is defective. If that’s the case, disconnect the wires to the limit switch and connect them directly.
How to Test a Furnace Fan Limit Switch
An even better way to verify whether your furnace limit switch is working normally is to test it. The ideal type of test depends on the type of limit switch on your furnace.
Testing an electromechanical limit switch
Electromechanical switches have a circular dial with a mechanical low set-point and high-set point. Both set points are adjustable. One side of the switch has 120 volts for the fan, while the other has 24 volts for the gas valve. Your job is to determine whether there’s continuity when the limit switch reaches the preset temperatures on the high and low sides.
Infinite resistance means the limit switch is bad.
Testing solid-state limit switches
The solid-state limit switch is a small circular device. Most models have a reset button. It’s the more common type of limit switch in modern furnaces.
Here too, you need to check for continuity. However, some technicians may use a jumper wire to jump the connections on the solid-state limit switch to determine if the switch is defective. We strongly advise against jumper switches unless you’re an HVAC professional.
How to Replace the Furnace Fan Limit Switch
If the continuity tests confirm that you have a bad furnace fan limit switch, the only solution is to replace it. It’s a simple process if you know how to remove the furnace limit switch.
Locate the limit switch and unscrew the switch plate. Then remove the malfunctioning limit switch and install the matching replacement.
The switches cost between $4 and $25, while professional replacement costs start at $65. Just make sure to purchase original equipment manufacturer parts.
Now you know where the fan limit switch is located on a furnace. You also know how to troubleshoot a bad fan limit switch and test and replace a bad switch. This knowledge can be invaluable down the line.