I have a ruud furnace that's about 4 yrs old. Recently it has been starting to short cycle. I have checked everything and even changed the filter but it still does the same thing. I have been able to take the door off and it works fine but when I put the door back, it shorts cycles again. What could be the problem?

Answer

It could be dirty and wants more air, but it also could just be drastically low temps making the unit run more to keep up. Mine does.