Everyone uses the phrase furnace filters which got me thinking as I worked my way through an inspection. The filters remove dust from the air going through the furnace which has nothing that really needs to be protected. Yes, the fan motor will last longer if it is not dusty, but it’s not really there as a furnace filter.
It should probably be called an air conditioner filter. In most homes the furnace is coupled with an air conditioner and it’s coil (see photo) is a series of pipes with a fine metal grill that the air passes through. Without a filter this grill is fine enough to start trapping dust and during the summer months when the AC is in use the coil is wet with condensation which traps any dirt in the air.
Dirt on this coil reduces the efficiency of the AC system and can become a health issue if it becomes moldy. A dirty coil has to be professionally cleaned using an acid wash. Filter changing is a good thing, next time you change your furnace filter think about how happy you are making your air conditioner.