or commercial building other arrangements are of course possible in conventional hvac systems for air can be returned to the hvac unit through two or more return ducts each having its own particulate filter and air can be supplied to the building interior through two or more supply ducts each being connected to receive air passed through the heat exchanger the heat exchanger can also be designed simply for the purpose of heating or for cooling but not both and in the case of a system designed both for heating and cooling the heating and cooling parts of the heat exchanger can be separate from each other in all cases however the system will have a heat exchanger a supply duct a return duct and a blower as shown in fig1 in addition to the conventional components described above the system includes a conduit 22 which provides a second air flow path that by passes the first air flow path composed of the return duct the supply duct the blower the heat exchanger and the building interior conduit 22 comprises a filter containing unit 24 a first conduit part 26 connecting the filter containing unit 24 to the supply duct 16 and a second conduit part 28 connecting the filter containing unit 24 to.