I have a question about furnace air filter placement for a fuel oil furnace. My furnace issues seem resolved at this moment (thank goodness...knock on wood), but I've noticed a change a technician made to the furnace filter placement. The air filter has always been attached to the outside of the blower compartment door - it is not a solid door but rather has a big vent in it. The air filter was attached there when we moved into this place, so we kept putting it there, and this has never caused any issues that I am aware of. When we had the preseason cleaning done, the tech replaced the filter and attached it instead to the inside door of the furnace "cabinet" (flimsy metal door that closes the furnace from view, as it is located in a hallway). There is an air vent at the bottom of that door and a rack to attach a filter behind it, and that is where he placed it. There is about half inch to three-quarter inch gap around the door that air gets pulled through. I can see dust building up along/in the doorframe. Sometimes the door gets bumped and is no longer secured shut, and the furnace will actually pull the door in when the blower is running, thus letting even more unfiltered air in. I try to make sure it is closed correctly, but from time to time it happens. Of course, the top fuel oil repairman is now out of town until after the new year, so I can't ask him which placement is correct/preferrable, and I don't know how knowledgable the other repairmen are, as they are not as experienced with my type of furnace. Any opinons on where the air filter should be placed? Happy Holidays!