The HVAC filter in a forced air furnace system is normally inserted in a custom slot in the return air duct. This slot is most often right next to the furnace itself. Sometimes the slot will have a cover on it (meaning that the filter edge may not be visible while the cover is in place), other times there will be no cover. In still other cases the filter will be located in an attic air handling unit, for example when air conditioning has been added to a house that uses a boiler and radiators to heat the house. In the photo at right, the filter slot has a sheet metal cover on it that is directly beneath the number 1 inside the yellow rectangle.