I changed mine about a month or two back and was sure I had closed the lid. Today I had it apart (for different reasons) but was shocked to find that the back of it was not closed up and therefore drawing in unfiltered air. I would never had known (until it was to late) if I didnt have the box out.

Worth checking.

Bob

 

Quite a common occurence.....

 

Esp a problem when putting a Green cotton one in, as they have to be cut to fit. If its not quite right you can miss the fact the lid isn't on.

MB