Rule Based Control of Fan Coil Units

Fan coil units are widely used to realise a thermally comfortable indoor climate in rooms. The main components are the heating coil, the control valve and the fan. Hot water flows through the heating coil and heats the passing room air. In case cooling is required also a cooling coil is installed. This paper presents a new control system. This control system uses inexpensive open/closed valves for precise control of heat, released by the fan coil unit. It uses two cascade control loops to adjust the indoor temperature. The main control loop controls the indoor air temperature. The secondary loop controls the temperature of the (supply) air leaving the unit by opening or closing the valve. The setpoint of the secondary loop is adjusted by the main control loop. A rule based type of control for the secondary loop is presented. It is implemented in the control system of the fan coil unit and tested in a test room. Results show fast and accurate control of the supply air temperature. This control method is also applicable for other systems that need cheaper actuators for operation as well.