What is a Furnace Limit Switch?

November 16, 2022
Furnace Repair in

Understanding the Role of the Furnace Limit Switch in Central Heating Systems

Gas furnaces have many different components that all work in conjunction to enable the unit to produce heat and circulate it throughout the building. Today we’re going to take a closer look at one of the most important of these components — the furnace limit switch. The limit switch serves two important roles in a gas heating system, and if the switch wears out or malfunctions, it can prevent your heating system from working entirely. With that in mind, here is a full overview of what exactly a furnace limit switch is and what it does.

How the Limit Switch Controls the Blower Fan

The furnace limit switch is located inside the supply plenum, which is a large metal box located either above or next to the furnace. The supply plenum is where the hot air created by the furnace exits the unit, and it is directly in between the furnace and the main trunk of the duct system. Hot air travels out of the furnace into the plenum and then circulates through the trunk and all of the ductwork branches.

The limit switch works to signal the blower fan to run when the furnace turns on and then to shut down whenever the furnace stops heating. The entire system is controlled by the thermostat, which constantly measures the indoor temperature and signals the furnace when it needs to turn on and off. Once the thermostat signals the furnace, the limit switch then activates to signal the blower fan to run also.

If you turn the fan setting on your thermostat to “On” instead of “Auto,” then the thermostat will signal the limit switch to switch over to manual mode. This makes it so that the blower fan runs constantly even when the furnace is off.

How the Limit Switch Prevents Damage From Overheating

The limit switch is also an essential safety mechanism that works to prevent possible damage to the furnace caused by overheating. Furnaces can overheat for a variety of different reasons, but most commonly it occurs due to air supply issues such as a clogged air filter, leaking ductwork, obstructed or closed vents, etc. All of these issues will restrict how much air flows through the ductwork. If there isn’t sufficient air supply, the system won’t be able to disperse the hot air out of the furnace quickly enough. This results in the heat building up inside the furnace and eventually causing it to overheat.

If the system overheats, it has the potential to seriously damage the components in the furnace and the blower fan. The biggest issue is that all of that heat could damage the electrical wiring and potentially create a fire hazard. The limit switch works to make sure that this doesn’t happen.

Whenever the furnace is running, the limit switch constantly measures the temperature of the furnace and blower fan. If the temperature ever rises above a set level, the limit switch will activate and automatically shut down the furnace and the blower fan to prevent them from overheating. The limit is usually set to trigger if the temperature in the furnace reaches 200 degrees Fahrenheit or the blower fan temperature goes above 130 degrees.

Once the limit switch is triggered, the furnace won’t start up again until it has had enough time to fully cool down, which can often take up to an hour or more. In most cases, the threshold for the limit switch to allow the system to run again is 100 degrees.

How Annual Furnace Maintenance Can Ensure the Limit Switch Works Correctly

Like most other components, the furnace limit switch can fail due to age. Should this happen, it will prevent the blower fan from running and you will have no heat until the switch is replaced. The more common issue is that the sensor on the limit switch gets dirty and makes it so that the switch can’t properly measure temperature. This is a more serious issue since it could enable the furnace to continue running even after it overheats.

The easiest way to make sure that the limit switch and the rest of your heating system are working correctly is to have your furnace maintained every year. When maintaining the system, a technician will fully clean any grime and dust off the limit switch to make sure the sensor can properly measure the temperature inside the system. They will also clean and inspect all of the other important components, including the furnace burners and flame sensor and the entire blower fan assembly.

If you need furnace maintenance or any other HVAC service in Deptford or anywhere else in southern New Jersey or southeastern Pennsylvania, JAMCO Heating & Air Conditioning has you covered. We maintain and repair all brands and types of furnaces, and we also specialize in installation for both residential and commercial HVAC systems. Our team installs and services air conditioners and ductless mini-splits as well. To schedule your annual furnace maintenance or any other heating or cooling service, give us a call today.

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