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. A furnace fan limit 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, the furnace fan limit switch 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.
The fan limit switch itself consists of the following parts:
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.
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.
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:
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.
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 fan limit 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!