Control unit for an HVAC system comprising an economizer and method for operating such control unit

A control unit (22) for a heating, ventilation and air conditioning (HVAC) system (10) comprising an economizer (EC), which economizer (EC) is configured to introduce outdoor air (OA) into the HVAC system (10) for cooling and/or ventilation purposes in a manner controlled by said control unit (22), said control unit (22) comprising a base module (23) with:

• a control circuit (24), a man machine interface (25) connected to said control circuit (24), and first input/output (I/O) means (39, CR1, CR2, CR3) for connecting at least one sensor (32, . . . ,36) of said HVAC system to said control circuit (24) and for delivering at least one control signal from said control circuit (24) to control the operation of said economizer (EC), whereby said base module (23) is configured to optionally receive at least one extension module (26, 27, 28), which can be snapped on and electrically connected to said base module (23) for expanding the functionality of said control unit (22) and wherein said first I/O means (39, CR1, CR2, CR3) comprises connecting means (1Act, 2Act, 3Act, 5Act) for connecting an actuator (17), connecting means (Y1, Y2, W1) for connecting a thermostat (31), and at least one temperature connecting means or humidity connecting means.