Furnace High Limit Switch Keeps Tripping

Pulling off the plenum covers i was greeted with a nice airflow the ductwork looks clean and the filter inside has been changed.

Furnace high limit switch keeps tripping.

When the thermostat calls for heat the furnace burners ignite and begin heating the heat exchanger. Rollout switches and high temperature fuses are different from limit switches in that when they open they will not close again unless reset as in the case of the rollout switch or replaced as in the case of the high temperature fuse. One such safety device which also plays a role in the regular heating process is the furnace limit switch. For instance a high limit switch that s supposed to trip at 155f might begin tripping at 130f instead.

Function of the limit switch. They will be mounted near where the flame may rollout the front of the furnace due to venting blockages heat. My furnace is tripping the high limit. A common reason a furnace short cycles is because the heat exchanger the part that heats your air is getting too hot due to a lack of airflow.

If the heat exchanger highlighted above gets too hot the furnace shuts down. I pulled the blower door off and checked all vents to ensure they are open and it still short cycl. Once the heat exchanger overheats it trips the high limit switch which shuts the furnace down to protect it. I found that the hi temp limit switch was tripping the unit.

It is a downflow payne 90 have replaced the limit switch. One of the great things about modern furnaces is that they have built in protections against many of the things that can go wrong with your system. Some of these parts are shown on the left. Initially the air above the heat exchanger in the supply plenum is not warm enough to blow into the house so the limit switch keeps the blower off.

The limit switch plays an important role in every normal cycle of the furnace.