A bag filtration element and system for its installation in a forced air furnace where the filtration element includes a channel element frame that receives rectangular bag frames fitted thereto that each received an open bag neck end fitted thereto, and the rectangular bag frames are fitted side by side into the channel element frame, whose upper end closes the frame into a rectangle, and, the bag ends are folded around the bag frames sides and connected. A rectangular opening is formed into the plenum of a furnace inlet air duct to extend, at an upward angle, across the rectangular opening, between opposing inner walls of the air duct, and a door is provides for positioning over the duct rectangular opening with the bag filtration element installed therein.