A furnace's fan pushes warm air through the duct system and into a home. However, if the fan runs continuously, it will end up pushing not-yet-heated air into the home. To keep this from happening, the fan is designed to come on only after the air in the furnace plenum has reached a certain temperature.
A fan limit control switch is what helps to ensure that the fan only runs when there is enough warm air to blow into a building. What this means is that if you notice your furnace blowing cold air into your home, the first thing that you should check is the fan limit control switch. Here is what you should know about how your furnace's fan limit control switch is to blame for your problems.
An obstructed fan limit switch
The fan limit switch has a dial. This dial usually turns as the air in the plenum heats up. And when this dial reaches a certain point, it causes the fan to start running.
Unfortunately, there are times when this dial jams. It may also fail to turn because of installation mistakes such as poorly locating the sheet metal screw. All these things that mess with the dial's movement interfere with its responsiveness as far as the rise or drop in plenum temperature is concerned. As a result, the fan will likely end up running even when it shouldn't, something that will then cause the furnace to blow cold air into your home.
A malfunctioning limit switch temperature sensor
The fan limit switch depends on the limit switch temperature sensor to tell whether the temperature in the plenum has risen high enough to warrant the fan running.
If this sensor gets clogged with dirt, its ability to detect temperature changes in the plenum area will diminish. As a result, it may end up feeding the fan limit control switch wrong readings, something that may then lead to the fan running when it shouldn't. This is something that can be detected by simply looking at the heat and helix sensors. If they are dirty, they are likely to blame for the sensor problem. Cleaning them will restore their functionality and will thus solve your problem.
However, there are times when the sensor feeds wrong readings to the fan limit control switch simply because it is defective. In such cases, the only way to solve the problem is to replace the defective sensor. A local home furnace repair technician can help with this.
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