High Limit Switch Furnace: Several Facts And FAQs

In this article we will discuss about the High Limit Switch Furnace. To operate a residential gas furnace safely and efficiently the use of a High Limit Switch is essential.

High Limit Switch, also known a “Fan Limit Switch” or simply “Limit Switch” plays an important role from safety point of view. The main function of a Limit Switch is to sense the temperature in the air plenum of the gas furnace and shuts off the burner as temperature goes to a limit which may cause a dangerous or uncomfortable condition. 

Why Does High Limit Switch Trip?

The suspected reason for High Limit Switch tripping is lack of airflow through the furnace, due to which temperature becomes high and the Limit Switch trips off.

An airflow issue through the furnace generally leads to Limit Switch tripping. Lots of reasons are there which prevent an adequate airflow throughout the system.

  • Air Filter: A dirty filter is one of the most common issue which prevents the flow of air through the furnace, resulting a very hot heat exchanger causing tripping of the Limit Switch. On the body of the air filter certain arrows are marked for the direction of air flow, these arrows must be followed properly to avoid any kind of air blockage.
  • Blocked Register: Through the registers, air flows out of a furnace, therefore the register should not be blocked to ensure proper movement of air.
  • Exhaust Vent: After the burning of gases inside the furnace, the exhaust gases should be vented out. The blockage of exhaust vent may cause overheating.
  • Blower Motor: If the blower motor is not working efficiently and air flow is not fast enough, then the heat exchanger get overheated.
  • Blocked ‘A’ Coil : ‘A’ Coil used to flow air from the system is sometimes clogged with debris.
  • Defective Limit Switch: Sometimes the Limit Switch itself going bad, this may be another reason for tripping. Then it has to be replaced or repaired for proper and safe functioning of the furnace.

Open High Limit Switch Furnace

A Furnace High Limit Switch is designed in such a manner so that it can activate and deactivate the furnace blower assembly as per the requirement.

Open High Limit Switch Furnace indicates the heating system turns off the fuel supply to prevent the overheating which may damage the furnace as well as create a dangerous situation inside the residence.

High Limit Switch Location

High Limit Switch Furnace is a small but very essential device used in air furnaces powered by natural gas, LP or heating oil.  

To detect the furnace Fan Limit Switch, cabinet cover should be removed. Generally it is attached to the plenum of the furnace above the combustion chamber or heat exchanger. Location of the Switch may vary as per the make and model of the Furnace.

high limit switch furnace
Components of A Gas Furnace; Image credit:flickr

How Does A Furnace High Limit Switch Work?

Furnace High Limit Switch protects the furnace as well as the home or business area by automatically bringing the temperature of the furnace down when it goes too high.

Acting as the key component of the furnace safety system, the two main functions of the High Limit Switch are:

  • Regulation of Fan Motor: It gives the blower fan an indication when to turn on and off. When the air is heated enough, it senses the supply air temperature and tells the fan to turn on in order to circulate hot air into the home. It allows the blower to keep on running until all heated air is distributed inside the residence completely.
  • Prevents from Overheating: It also identifies the supply air temperature, when the supply air temperature is too high, the components inside the furnace may get damaged. At this moment, the Limit Switch trips to shut down the system and stops furnace functions to control the temperature. In most extreme cases overheating may lead to fire hazards.
Furnace High Limit Switch; Image Credit: Flickr

Bypass High Limit Switch Furnace

It is advice able that Fan Limit Switch or Limit Switch should be bypassed only for a temporary basis.

Limit Switch should be bypassed only for an essential reason like troubleshooting or testing purpose. A Furnace High Limit Switch acts as an essential part during the operation of a furnace by regulating the temperature, thereby it prevents the damages that may occur to the internal components of the furnace as well as maintain safety of the human being.

If we run a residential gas furnace without the Limit Switch for a long time, heat exchanger may damage and toxic gases may enter the living area of the residence. 

High Limit Switch Furnace reset

After detecting the problem associated with High Limit Switch and resolving it, the Limit Switch has to be reset for normal functioning of the furnace.

The steps to be followed for resetting the High Limit Switch Furnace are :

  • At the beginning we have to close the gas valve and cut off the power supply to deactivate the system. This step is done completely for safety purpose.
  • After that panel should be opened with the help of a screw driver to find the furnace’s heating duct.
  • Identify the white or silver Limit Switch, remove the cover to see the body of the switch with two metal arms.
  • These two arms are responsible for the high and low temperature range of the furnace.We have to identify the component in between these two arms which control the fan.
  • Fix the temperature of the central component at approximately 105-115 degrees Fahrenheit. Try to ensure the fan switches off and on at accurate time.
  • Now set the temperature on the left component at around 90 degree Fahrenheit, which will give indication to the left arm to stop blowing cold air when it reaches 90 degrees ,similarly set right arm at 200 degree Fahrenheit, which indicates the furnace to shut off when the burner reaches this temperature.
A home high energy efficient furnace in a basement; Image Credit: Unsplash

How to Replace?

The Furnace High Limit Switch helps to ensure your safety and the comfort of your home.

The steps to be followed to replace a defective High Limit Switch Furnace are:

  1. At first we have to deactivate the furnace using service switch or circuit breaker to avoid any kind of accident.
  2. After that replace the old one and fix the new Limit Switch by wiring it exactly the old Switch. It is quite simple since it has only two wires coming off and two screws are there to hold it in place. For further issues the instruction manual provided along with the Limit Switch can be followed.
  3. After setting up the new Switch, furnace can be activated and regulate the thermostat to get accurate signs of the heating process of the furnace. Sufficient amount of hot air through the vent will justify smooth replacement of the Limit Switch.

How to Test High Limit Switch Furnace?

After checking all the possible reason of tripping up of the High Limit Switch Furnace and it is noticed that blower motor is working, filter is clean enough then the Limit Switch is need to be tested.

Testing of High Limit Switch Furnace is a DIY task which can be easily done with the help of a screwdriver and a multi-meter. Steps Followed to test High Limit Switch Furnace are:

  • First of all deactivate the system either by turning off the serviceman switch or shutting the power off at the breaker panel.
  • Opening the panel and identify the white or silver High Limit Switch.
  • Disconnect the wires attached to the switch, try to remember the connection to reconnect them.
  • Now with the help of a multi-meter, test if there is continuity between the two arms.

If the High Limit Switch Furnace is faulty, replace it with a new one.

In continuity test of a High Limit Switch Furnace, a multi-meter set is used to measure resistance in ohms, small electric current is sent between the switch terminals and measures the resistance between them.

Generally two possible rest results are: Zero (or near to zero),indicating switch is closed and current is flowing. Another possibility is Very High or Infinite, indicating the switch is open.

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