As you know a gas furnace has multiple components that work together to keep your home warm in winter. A small but critical component of them is a furnace limit switch. It’s also called a high limit switch or a fan limit switch. If your furnace keeps running constantly or turns on and off repeatedly, there may be an issue with the limit switch. You can inspect the furnace and limit switch, but to be on the safe side, consult furnace repair services.
Here we will explain what is a furnace limit switch, how it works, and what are signs to replace it.
What Is A Furnace Limit Switch?
A limit switch is a small safety device installed in your furnace that has two jobs:
Job #1: Overheating Prevention
Primarily, a furnace limit switch is responsible for keeping the furnace from overheating. This means it can turn off the furnace (the burners) when it detects that the temperature of the furnace is higher than the manufacturer’s set value.
This makes it an important device because if a furnace keeps overheating, it can cause a fire or an explosion. Even when there is no fire or explosion, overheating can damage the heat exchanger, allowing carbon monoxide to leak into your house.
Keep in mind that carbon monoxide (CO) is a dangerous gas that is undetectable by sight or smell and can be fatal if someone inhales it for longer periods of time.
Job #2: Signaling The Fan Blower To Turn On Or Off
Moreover, a furnace limit switch also takes part in the heating process as it signals the fan blower to turn on or off. But what does a blower fan do? For this, if we explain how a gas furnace works, describing the role of a limit switch and the blower fan in this will be easier:
When a gas furnace is tuned on, the burners in the combustion chamber receive natural gas to turn on and create heat.
- This heat then enters a metallic tube called a heat exchanger.
- Cold air from your home enters the furnace and flows over the outer surface of the heat exchanger. As the heat exchanger is hot, it warms up the air.
- This warm air goes to the supply-air plenum, which is a chamber for holding warmed air that will be supplied to the house via ducts. But when the furnace is turned on, the air in the supply plenum is cold or not warm enough. This cold air should not be blown inside your house. This is where a furnace limit switch comes in.
- The limit switch is usually located under the supply plenum for detecting its temperature. The limit switch doesn’t allow the fan blower to turn on for the air to reach your home until the temperature of the air in the supply plenum reaches a set value. When the air inside the supply plenum is warm enough, the limit switch signals the fan blower to turn.
- When the blower fan is activated, it supplies the air to ductwork and you receive warm air inside the house through the supply ducts.
- Once the desired temperature is reached, the limit switch signals the fan blower to turn off.
If we explain it precisely, the fan limit switch keeps cold air from reaching your home and ensures that you receive warm air. It does it by controlling the fan blower.
Signs Your Furnace Limit Switch Is Malfunctioning
No Airflow
A common sign is no airflow from the supply ducts because the limit switch will be stuck or broken and won’t signal the blower fan to turn on when required. So, you won’t notice any air blowing from the vents.
Cold Air Coming From The Vents
A faulty fan limit switch can also cause the fan blower to keep running. This means you will keep receiving air through the vents whether it’s warm or cold because the device that was there to stop air until it’s warm is broken.
The Furnace Will Keep Switching On And Off (Short Cycling)
In some cases, as the blower fan keeps running, its motor overheats and shuts off even when not required and the desired temperature set through the thermostat is yet to be reached. So, there is inconsistent heating.
The Furnace Runs Constantly
In contrast to the above point, some cases have been reported where the furnace was running constantly and the culprit was a malfunctioning limit switch.
Overheating Of The Furnace
An important function of a limit switch is to keep the furnace from overheating by turning off the burners when the inside temperature of the furnace reaches a certain level. If the limit switch is broken, it won’t perform its job and the furnace may overheat.
This is a major problem because an overheating furnace can cause a fire. Even when the overheating doesn’t lead to a fire, it damages the components of the furnace and causes many kinds of problems. In some cases, the components are damaged to the extent that your only option is to replace the furnace.
Other signs of a faulty fan limit switch that are also caused due to the above-mentioned reasons are inconsistent heating, inefficient heating, and carbon monoxide leaks.
One thing to remember is that all the signs mentioned above don’t necessarily mean that the furnace limit switch is bad. There may be an issue with any other furnace component. So, if you notice any problem with your furnace or heating capabilities, have it inspected by an expert so they determine the exact reason for the issue.
Causes Of A Bad Furnace Limit Switch
Age
A furnace limit switch can last as long as your furnace, but it’s not a rule. It’s possible for the limit switch to malfunction due to various reasons. Some are below:
Blocked Furnace Filter
A furnace filter is there to keep dust and contaminants in the air to reach the system. This means it gets dirty quickly and you have to clean it depending on your usage. If the furnace filter is not cleaned regularly, it restricts the airflow. This causes the components of the furnace to overheat and malfunction. It can trip or damage the limit switch.
Blocked Vents
If the vents are blocked, they also restrict the airflow and cause overheating.
Ignoring Maintenance
Furnace maintenance includes inspection, cleaning, and repairing the components. As many parts work together, an issue with one part can cause the other part to malfunction as well.
Many times, the limit switch is not faulty but doesn’t function properly due to dust buildup on the sensor or probe. If you maintain your furnace regularly, the limit switch will be cleaned as well so many limit switch issues won’t happen.
Plus, if there is an issue and you suspect the limit switch is causing it, clean the limit switch if you can open the furnace cover and access the limit switch after turning off the furnace.
Conclusion
A furnace limit switch is a small but important device that protects your furnace from overheating and also takes part in the heating process. Usually, if your furnace short cycles or overheats, the limit switch is either dirty or faulty. If you notice any problems with your furnace, contact HVAC companies Arlington so that an expert visit your place and thoroughly inspect the furnace for problems.
