What is the main cause for tripping a primary gas limit switch?

Casey_7

I have a Lennox rooftop unit. The primary gas limit switch keeps tripping. Any suggestions as to what's wrong??

bill_71

primary gas limit switch?? not sure such an animal exists but if it is turning on and off by this switch then it sounds like a hi limit switch...airflow is the key for that issue( 90% of the time anyways). check your filters and belt..no airflow means the rooftop can't breath and gets too hot......of course the motor itself could not be working or the limit switch could be bad..pulleys tight and not "freewheeling"?

Unknown

Does that switch

have a reset buton on it? If so it is not necessarily the "primary Limit". What is its location, a picture would help?

joe p_3

rooftop

Lennox rooftop L-series diagnostics used along with the manual will point you to the location and function of the offending limit. Per Bill's suggestion, look for airflow problems including pre-filters (stung me a few times). Never bypass any limit. After you fix the problem, replace the limit switch as it may start giving false trips.