The General AC3 is a whole house air cleaner designed to be installed in the return air duct system of a home or office. It is typicaly mounted to the inlet air side of a furnace or air handler. The air filter media must be replaced every 6 months if you have heating and air conditioning and every 12 months if you have heating only. This unit also contains carbon media which must be replaced every 3 months for optimum performance.