Where was this limit switch? If it was on the flue, then it's a symptom of a drafting problem when they fail. Basically the thing should rarely ever click, but

Where was this limit switch? If it was on the flue, then it's a symptom of a drafting problem when they fail. Basically the thing should rarely ever click, but if the flu isn't drafting well, then the snap disk will get triggered frequently. You'll never notice anything wrong because the heat still works fine. Then the thing fails and you fix it, but you only fixed a symptom and not the problem. Even so, I still think it's better to get in there and figure stuff like that out than to avoid it. "just let the professional do it and do whatever they suggest" only works if you have some way of finding a guy who is both knowledgeable and honest. Yeah there are obviously risks like being unaware of invisible unsuspected things like above, but life is risk regardless.