Abstract: Air flows to the conditioned space through a cool air duct and a warm air duct which feed a common delivery duct. The inlet of the cool air duct is connected to a cool air supply duct of a parent system; and inlet of the warm air duct communicates with the return air plenum. Flow control components are air actuated and are controlled by actuators responding to a varying air pressure control signal, which is the output of a thermostat in the conditioned space producing a varying signal pressure signal related to varying temperatures. Dampers in the cool air duct and warm air duct provide for alternative air flow from either of these ducts to the delivery duct. The volume of air flow in the cool air duct is controlled by a modulating damper which responds to a selected signal pressure range. The air flow in the warm air duct is controlled by a fan and dampers, controlled through change of the signal pressure in either direction relative to a selected signal level.