The General AC22 is a whole house air cleaner designed to be installed in the return air duct system of a home or office. It is typically 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 air cleaner is also known as an Echelon model S1-TM11PAC20252 and a model L81-490. It uses the same parts as the General Aire AC22.