Furnace High Limit Switch Keep Tripping? Troubleshooting & Fixes

Furnace High Limit Switch Keep Tripping

If your furnace is not functioning properly, it might be because of a high limit switch tripping problem. A vital part of the furnace monitors temperature in the air plenum and shuts off the furnace burner when the temperature reaches maximum level.

A dysfunctional high limit switch cannot manage temperature and make indoors uncomfortable. This article will talk about high light switching, reasons, and solutions to troubleshoot the problem for a comfortable stay.

What is a Furnace High Limit Switch?

Mechanical equipment comes with certain specific safety features to keep your house protected. Furnace High Limit Switch is one of the crucial safety features of HVAC equipment. It observes the burner and switches it off when the temperature reaches an extremely high level. Burner gets switched on when temperature decreases to normal levels.

Furnace High Limit Switch Keep Tripping Impact on Furnace

If the high limit switch trips multiple times in succession in some HVAC products, the furnace goes into lockout mode. It needs repair and reset to turn on again. If you want to DIY, reset the unit by switching off and switching it on after some time.

The overheated furnace can also damage the heat exchanger. A cracked or damaged heat exchanger can be dangerous for your home because it will allow combustion gases such as Carbon mono Oxide to enter your home, and the furnace will become dysfunctional.

How to Know High Limit Switch is Tripping?

If you experience ‘short cycles’ with furnace operation, it might be high limit switch tripping. The unit typically runs without producing the required heat or does not reach the set temperature limit and then automatically switches off. This process repeats every couple of minutes.

TOP 4 Reasons For Hight Limit Switch Tripping (With Solutions)

There are broadly four main reasons for tripping;

  • Dirty Flame Sensor Rod
  • Overheated Heat Exchanger and Low Air Flow
  • Defective High Limit Switch
  • Oversize or and Undersized Furnace

1. Dirty Flame Sensor Rod (Step by step Cleaning)

Step 1: Locate a high-limit switch to clean the flame sensor rod. It is generally attached with a metal plate that is bolted to the exterior of the furnace plenum. A furnace plenum is a big metal box where the air gets heated. 

Step 2: Remove the furnace cover to locate a high-limit switch. The high limit switch uses a flame sensor rod to monitor temperature through the furnace plenum.

Step 3: Remove two screws that hold the rod with the plenum to clean it by yourself.

Step 4: Clean it gently using sandpaper or steel wool. There is another option to replace it if required. It is advisable to consult a professional to replace the flame sensor rod if you have any concerns or doubts.

2. Overheated Heat Exchanger and Low Air Flow

Heat inflow and outflow are essential for a comfortable indoor atmosphere. It optimizes temperature for a cozy stay. When airflow decreases, the speed of heat outflow is affected, leading to heat exchangers overheating. This is also a reason for frequent high-limit switch tripping.

Three solution for Overheated Heat Exchanger and Low Air Flow

1) A dirty furnace filter reduces airflow and causes heat exchanger overheating. In addition, dirty filters with harmful particles can cause health problems for you and your family. The best way to restrict this is to check and clean furnace filters regularly (monthly).

2) To increase airflow and outflow of heat, you can open the air supply vents of all rooms. Air from all sides will push the excess heat outside and can balance the heat exchange

3) Restricted airflow also happens because of the dirty furnace blower. Accumulated dirt and debris clog the blower and slow down airflow that leads to the overheated heat exchanger. You can call for annual maintenance for cleaning.

3. Defective High Limit Switch

The defective high limit switch can also cause tripping. It is a common problem with older furnaces because of the year of use and operation. A dysfunctional switch can trip because of low temperature, frequent short cycles, and sudden lockouts. Ask for professional support to replace the high limit switch.

4. Size of the Furnace

The size of the furnace could also be a reason for tripping. If the furnace is too big for your home, it will heat the house in a short time and turn off & on frequently.

Solution for the improper size of the furnace includes ‘load calculation’ to assess size requirement for your home. You can also determine if the size is the reason behind frequent ‘short cycles.’

Consult authorized and professional support for load calculation. If calculation proves that your unit is oversized or undersized for the home, you might need to replace it with a unit suitable for the home.


High-limit switch tripping ruins the furnace. Temperature does not reach the set limit, and it is highly uncomfortable during winters. Follow the abovementioned reasons and solutions to repair your unit for convenient indoor air and temperature. You can try ‘DIY,’ but it is highly advisable to contact professional help if there is any confusion about furnace technicalities.

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