• Dirty air filter

 Dust and debris can build up in filters over time, restricting airflow and causing heat to build up in the furnace. Some high performance filters may also restrict airflow.

• Cracked Heat Exchanger

A cracked heat exchanger can cause a furnace's high limit switch to trip to prevent overheating. When a heat exchanger overheats, it can cause stress cracks and premature metal fatigue.

•  Blower Motor or Blower Wheel

Many scenarios with the blower motor can cause the main limit to trip. A dirty blower motor or wheel can cause the reduction of airflow moving across the heat exchanger. When there is not enough air flow moving across the exchanger the main limit will trip when the limit temperature is reached. These temperature are identified on the switch.

• Incorrect Fan Speed Selected

When a furnace is installed the company should have performed a start up sheet. One element of the start up sheet is fan selection. If the incorrect fan speed is selected the fan may not move enough air across the exchanger. This will cause the system to overheat and trip the main limit. 

• Defective Control Board

A defective control board could also cause the main limit to trip.  Fan relays are built on the control board and are separated by heating, cooling, and fan only operation. Simply check the terminals connecting to the blower motor for voltage. If there is proper voltage at the terminals then the board is operating properly and will not need to be replaced.  This would indicate a replacement of the blower motor.  

• Clogged Evaporator Coil

A dirty evaporator coil will trap the heat inside the furnace, causing the furnace to overheat and trip the main limit. A dirty evaporator coil is the #1 reason for a furnace to overheat and trip the main limit. 

• Improper Size of the Supply and Return Ducts

If the duct work is not properly sized for the system, this could cause the system to overheat and trip the main limit.

• Defective limit switch

This switch may wear out over time and stop working properly.  

 

The main limit is bypassed the same way as the rollout switch. By simply removing the leads from the main limit and connecting them to the Furnace Bypass Switch connectors. Once the connection is made, you can now activate the switch to bypass the limit. This will allow the furnace to operate.