Preformed fall service on 3 ton 80% furnace changed filter,cleaned flame sensor ,ighter read 49 which is within manufacturer spec the induce draft motor capacitor was 4.0 of 4.0 uf lower wheel and evaporator where clean customer changes the filters every thirty days like clock work ðŸ˜ðŸ˜while in the attic I calculated the return to be undersized for the system installed which will lead to longer running times and unnecessary strain on the entire system heating and cooling modes recommend upsizing to a twenty inch duct this undersized duct is also the reason the filter is warped when you go to change it the blower is trying to bring back correct air but can not due to undersized diameter causing system to strain and suck harder to try to achieve correct return. Ran system through cycle the duct runs are also all just laying on the rafters cause an air flow restriction lowering the capacity of the system. The system is operating g correctly at this time system has a surge protector in