What is a Fan Limit Switch?
A furnace has two switches called a fan limit switch and a high limit switch, these switches can be separate or in one package called a combination switch.
High limit switch
A high limit control switch will turn off the gas to a furnace if the temperature becomes top high. For example if the blower motor stopped running then no air would move through the unit. The flames would raise the temperature inside the unit and have no place to go getting hotter and hotter. As a safety the high limit switch turns the gas off when the temperature becomes too high.
Fan control limit switch
The fan switch reads the temperature inside a furnace and once the temperature becomes warm enough turns the blower motor on sending warm air into a home. This is used so that cold air is not blown in to a house waiting for the gas flames to heat up enough before the blower turns on.
Depending on the furnace these two switches can be separate or together in what is called a combination switch.
Some furnaces will have two or even three high limit switches wired in series and checking the temperature at deferent points in the furnace.
Example Limit Switch On Amazon
Honeywell L4064B2236 Combination Fan and Limit Furnace Control
Things To Check First
The furnace filter. A clogged up filter will block air flow into a furnace and raise the temperature inside it. If the temperature inside the unit becomes to high the High limit switch will turn the gas off to the unit. Be sure the filter is good.
The next thing to check is if the blower motor is working or not. If the blower motor is not working and no air is being blown through the unit it will overheat and the high limit switch will turn the gas off.
One way to check this is go to the thermostat and turn the FAN setting from AUTO to ON. If the blower motor blows air out the vents when manually set to ON then you know that the blower motor itself is good and something else such as the limit switch is causing the problem. If the blower motor doesn’t turn on than that doesn’t mean the motor is bad, other components can cause a blower motor to stop working.
Before testing the limit switch power off the furnace wait a few seconds and power it back on. Some furnaces will try a set number of times to start (Usually three) if it fails then it will have to be reset with a power off before it works again.
Keep in mind other things can also cause this problem we are simply trying to eliminate the blower motor and filter as possible problems, as these are the most common.
To quickly rule out the thermostat read this post.
Testing The Limit Switch
Once you know the blower motor is working, and filter is clean then the limit switch will need to be tested.
Types of limit switches
There are different types of limit switches the first is the electromechanical switches that have a circular dial with a mechanical low set point and high set point that can be adjusted. With these type of switches one side will have 120 volts for the fan and the other side will have 24 volts for the gas valve. To test it a multimeter will have to be used to see if there is continuity when it reaches the preset temperatures on the low and high side.
Another common type is the solid-state high limit switch which is smaller an circular in design. The solid-state switches sometimes have a reset button that should be checked for before replacing. These also should be tested with a multimeter for continuity. Although not recommend some service techs in the field will use a jumper wire to jumper the connections on these type of limit switches, to see if the switch has gone bad. Don’t use a jumper switch unless you know what you are doing.
If both the blower motor and limit switch test good than other possible problems may be the contactors, circuit board, or gas valve and possibly the thermostat.
Where can you buy a Fan Limit Switch
Fan limit switches can be bought at a HVAC supply store. Some HVAC supply stores do not sell to the public because of the high returns while others will but at elevated prices.
If you can wait a few days I would buy from Amazon as they have a big selection.