Flanders (4 Filters) 14 X 24 X 1 Precisionaire Nested Glass Air Filter. This is a standard grade furnace filter of the same type that has been protecting furnaces and central air conditioners around the world for over 50-years. The filtering medium is continuous filament spun glass. The frame is made with one continuous piece of chipboard to guard against corner separation. For peak equipment protection and energy efficiency these filters should be changed monthly in periods of high use. Buy a case and be set for the year.