34 and 36 which connect the central part 32 to the respective conduit parts 26 and 28 the central part 32 has a rectangular opening 38 for insertion and removal of the filter and a closure 40 which fits frictionally onto the central filter containing part 32 is provided to close the opening when the filter is in place various other forms of closures such as a hinged door can be used as alternatives to closure 40 the conduit part 28 is preferably formed from a length of flexible duct 42 typically having a circular cross section and an internal diameter of four inches the length of duct 42 is surrounded by a quantity of glass fiber insulating batting 44 which is in turn contained within a flexible tubular cover 46 conduit part 28 is similarly constructed the conduit parts are connected to tubular end portions 48 and 50 of the tapered transitions on the filter containing part 32 and to the supply and return ducts through adapters 52 and 54 resilient wire loops not shown can be used to clamp the ends of the flexible ducts to the tubular end portions and adapters and duct tape can be used to seal the connections in the operation of the apparatus the pressure difference across the filter unit will typically produce a flow in the range from about 50 70 cfm through the filter when the blower is operating depending on the static pressure within the duct system the loss in air flow through the building resulting from flow of air through the by pass path through the carbon filter is not.