What Is A Furnace Fan Limit Switch?

We have heard this question many times and we thought that it was time to address this question!  Let’s start by defining a furnace fan limit switch.  This switch is a component of a furnace system that determines when the furnace’s blower assembly for hot air turns on and off.  In simpler terms, this is what determines when hot air should be blown through your ventilation system to warm your home to the temperature that you have set.  However, it has has two other functions.  This switch also determines when the blower assembly turns off as well as determining if the furnace needs to be shut down due to overheating.

How does a furnace fan limit switch work?

The fan limit switch itself consists of the following parts:

• An on/off switch for the blower fan
• A temperature sensor within the furnace
• A control which turns the blower fan on or off dependent on temperature in the furnace
• A high-limit setting that allows it to turn off the heat source/burner in the event that the furnace overheats to dangerous temperatures
• Blower fan Manual ON switch that allows the blower fan to be set to run continuously.

When you turn on your furnace that it calls for the heat to be distributed through the home, the burners turn on and heat up the air in the air plenum.  Then the air is blown across the heat exchanger and through the duct work and into the various rooms of your home.  The fan limit switch is what communicates to the blower assembly that it is OKAY to blow the heated air.  When the fan limit switch has sensed that the air in the air plenum is the right temperature it allows the blower to distribute the air.  When the pre-determined thermostat setting has been reached, the furnace is shut off.  However, the blower will continue to blow the air until the fan limit switch signals that the internal temperature of the furnace has fallen low enough to disengage.

In the event that the furnace fan breaks or malfunctions and the burner has continued to burn fuel causing the internal temperature of the furnace to rise, the fan limit switch will cut power to the fuel valve.  This is the safety feature of this fan limit switch that helps to prevent possible fires or explosions.

What is the function of a fan limit switch?

The fan limit switch controls when the furnace blower fan turns on and off. This control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace get too high.

Fan/Limit switches are used on all types of heating furnaces.

A combined fan limit switch provides at least three heating blower fan temperature controls that respond to the temperature in the furnace.

1- FAN OFF: the low temperature at which the furnace blower fan will STOP to prevent blowing cool air onto room occupants.

Typical factory setting: 90°F

2- FAN ON: the temperature at which the blower fan will turn ON to begin to deliver heat to the building’s occupied spaces.

Typical factory setting: 100°F

3- FAN LIMIT: the high temperature at which the high limit switch will open (turn OFF) to stop the burner to prevent overheating of and damage to the heat exchanger. Such damage would make the furnace unsafe.

Typical factory setting: 200 °F

By keeping the fan off until the supply air plenum is warm, the heating furnace limit switch prevents the furnace blower from sending chilly air into the building if the oil or gas burner has not sufficiently heated up the furnace heat exchanger and supply air plenum.

Watch out: when adjusting the temperature settings in this control don’t press on, bend, twist nor in any other way fool around with the rotating metal disc. Doing so can bend its moving parts and spring, rendering the control both inaccurate and unsafe.

Fan limit switch benefits

So why is this fan limit switch beneficial to you and your home?  Well, there are a few reasons both safety and comfort related.  When speaking about safety, it is easy to see that it helps the furnace from dangerously overheating.  It helps to prevent potential home fires or explosions as described above, as well as preventing damage to the furnace itself.  Because of it’s communication and control of the blower assembly of the furnace it also helps to maintain the temperature regulation in your home.  This allows for a reduction in cold air drafts as the system runs meaning that your home is much more comfortable.

Fan limit switch problems

With any part of your HVAC system you can run into problems here and there.  Here are the most common problems that we have seen:

• the switch sensor becomes dirty and stops working as a result
• the furnace filter gets clogged which raises the temperature in the plenum too early, tripping the sensor
• the blower motor malfunctions and stops working causing overheating that trips the switch to shut off the furnace
• problems withing the contractors, circuit board, gas valve or thermostat that cause the fan limit switch to stop working properly.

If you notice that your furnace continually runs yet have no heating from the system, you may have a problem with the fan limit switch.  It is also possible to notice a problem if the blower does not run, or does not stop running when it normally should.  If you find that your furnace is not functioning properly, it is always best to call a HVAC professional to come take a look so that repairs can be made before an even bigger problem arises.

Fan limit switch problem prevention

The very best way to prevent fan limit switch problems is with regular HVAC maintenance.  Regular cleaning and an annual inspection can help identify a problem before it’s too late and can save you hundreds if not thousands of dollars in repairs.  This also adds to the longevity of your system and keeps it running at peak efficiency.

While it is possible to fix or replace a switch by the homeowner, the only way to be truly sure that it is done correctly and that there are not other underlying causes of problems in your system is to call a professional!  If you are having any doubts about your system and it’s ability to keep you warm this winter make sure you contact us right away!  We are your hometown team of experts that will keep you warm!

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