The forced warm air heating system uses a fan or blower to draw air from rooms through a return air duct way. The return air is passed through a furnace where it is heated. The heated air is then pushed into the plenum and distributed via ductwork to rooms as needed. Dampers within the ductwork are opened or closed to balance the heated air into the various rooms as desired. Furnaces are commonly set up to use fuel oil, natural gas or electricity in order to supply heat. Some systems incorporate a humidifier to lessen the dryness of the air. The moisture introduced into the system, however, can cause corrosion to the metal parts. The inspector with evaluate the overall condition of the furnace and control systems. Of all the major heating systems (Steam, Hot Water, Warm Air), warm air heating systems pose the greatest hazard. When the heat exchanger of a warm air furnace is cracked or leaking it can leak carbon monoxide gas into the living area. The heat exchanger is not visible for inspection.