I need some help understanding the operation of a fan limit switch. What's the sequence of operation of this object. And how do one troubleshoot this part to determine if it's performing as it should.



Thanks!:confused:

Older furnaces had 2 limit switches. One turned the fan on when it was up to temperature, and one shut the gas off if the furnace overheated, most likely because the first one failed. Most of the fan switches are open when cold and close as they heat up.

Newer furnaces may use a thermister or an optical flame sensor and a timed delay to start the fan.

I have a Trane gas furnace that is about 11 years old at the time of this message, having been installed in 1998. I've had some issues with the limit sensor tripping off and today it finally quit all together . According to the schematic, the sensor should read zero ohms, or close to it, when cool or below the upper temp. limit, which in mine was 155 degrees. When the sensor detects temperature above the design limit, +/- some % depending on accuracy of the sensor, it will cause an open circuit which the controller board will detect as a fault because of the interruption of the 24v line. Once the sensor cools off, the bimetal spring in the main limit switch resets and then the 24v line is restored and the furnace can try to heat your home again, but will probably trip again until the issue causing the over temp is fixed. The cycle will repeat until you either fix the reason it trips or like my sensor, it breaks. In my furnace, the sensors are in series. Meaning that the sensor in the fan area and the one in the plenum as well as the fusible links (over temp sensors right by the manifold area, look like resistors) are all in the same line. Check the back of the panel on your heater to see if there is a schematic on the back. If you aren't sure what the symbols mean, you have two choices: get some training or get someone who knows what to do.