The high-temperature limit switch is a small but important device used in forced-air furnaces powered by natural gas, LP, or heating oil. Also known as the fan limit switch or simply the limit switch, it has two functions in the furnace's operation: It turns the fan on and off during the normal heating cycles of the furnace, and it senses heat and shuts down the furnace if the interior temperature gets too high.
Identifying the Limit Switch
The limit switch is normally accessed by removing the furnace's cover panel. A typical limit switch consists of a long temperature sensor probe attached to a mounting plate. The probe extends through the wall of the furnace, and the plate fastens to the outside of the furnace's hot air supply plenum, above the combustion chamber or heat exchanger. The mounting plate usually contains two or more terminals that receive control wires for the blower fan and the furnace's gas valve.
It's possible that your furnace will have two or even three limit switches that check temperatures at different parts of the furnace. The terminals for these switches may each have their own mounting plate, or they may be part of a single combination unit.
On some furnaces (usually older models), the limit switch mounts on the outside of the furnace rather than inside the service panel. These types usually have a rectangular cover plate that protects the wire connections.
How the Limit Switch Works
The limit switch plays an important role in every normal cycle of the furnace. When the thermostat calls for heat, the furnace burners ignite and begin heating the heat exchanger. Initially, the air above the heat exchanger—in the supply plenum—is not warm enough to heat the house, so the limit switch keeps the blower off. When the air in the plenum reaches the lower setting on the limit switch, the switch turns on and activates the blower fan, circulating air through the heat exchanger on its way to the house while simultaneously pulling cool air from the house through the air returns and into the furnace.
When the house temperature reaches the designated setting on the thermostat, the burners turn off, but the limit switch keeps the blower running for a little while to extract as much heat as possible from the heat exchanger. When the air in the supply plenum drops back to the lower setting on the limit switch, the switch shuts off the fan until the next cycle begins.
The other important function of the limit switch is to shut off the burners if the heat exchanger gets too hot—a condition that can crack the piping in the exchanger, effectively ruining the furnace. Overheating can occur if there is a problem with the blower fan or if the furnace filter is so dirty that it restricts airflow through the furnace and exchanger so that the exchanger doesn't cool as it should.
Limit Switch Malfunctions
One common symptom of a malfunctioning limit switch is a blower fan that doesn't shut off. This happens when the switch fails to shut off after the burners have stopped and the air exchanger is sufficiently cool.
A bad switch also can prevent a furnace from operating at all. If the switch has completely failed and is stuck in the open circuit (off) position, the furnace won't run. The same effect can result from repeated overheating. If a limit switch (doing its job) hits its top limit and has to shut off the burner four or so times, the control computer of the furnace may go into a "hard shutdown" mode so the furnace won't turn on until the unit is serviced.
Replacing a Limit Switch
A bad limit switch is usually handled by a furnace repair professional, but it can be replaced by a DIYer with a basic understanding of electrical issues and who understands how to use a multi-meter. The repair involves disconnecting and removing the switch, then checking it for continuity. If the multimeter shows that resistance is infinite, it means the switch is bad; replacement is a matter of simply inserting a new limit switch, attaching the mounting screws, then reconnecting the wires.
If you attempt your own replacement, make sure you buy a limit switch that is an exact duplicate, with the same voltage ratings and temperature range.