A limit switch is a device that's located on your furnace below the supply plenum. The limit switch on today’s furnace acts as a safety device. If the temperature inside the furnace gets too hot, the limit will open up, resulting in the burners shutting off.
There are multiple reasons for the limit to open:
One reason on initial start-up of the furnace is gas pressure. (See our Tech-Tip on setting gas pressure). The rating plate on the furnace will give you the proper settings for natural gas. If propane, the conversion sticker label (from the LP kit) will have the proper pressures listed.
The limit might trip after start-up due to a restrictive or dirty air filter. Some high performance air filters do not allow very much airflow through them and will cause a furnace to overheat due to a lack of air going across the heat exchanger.
Another reason could be a bypass humidifier. If your supply duct is restrictive due to being undersized or your supply registers are closed, the air will take the path of least resistance and that is through the humidifier, which will raise the return air temperature. Your installation instructions should tell you the maximum allowed return air temperature, which on most furnaces is 80 degrees.
Another reason for limit trip is the duct design not allowing for correct air flow. If you are lucky and it’s slightly undersized, you can increase the air flow to keep the furnace in its temperature range.
When calling for assistance you need to have checked the gas pressure and the temperature rise. Below are a couple of images checking rise: one is within range, and the other is not and will cause the limit to open.
When checking temperature rise, you need to know the range. This range can be found on the rating plate of the furnace. Typical is 40 to 70, but this is not true on all furnaces, so you must check your rating plate. The ideal setting is in the middle of the range; for a range of 40 to 70, the ideal spot is 55 degrees.