1. An actuator ( 10 ) for HVAC systems ( 1 ), the actuator ( 10 ) comprising a network interface ( 20 ) for connecting the actuator ( 10 ) to a communication network ( 2 ), a bus interface ( 30 ) for connecting the actuator ( 10 ) to a sensor/actuator bus ( 3 ), a data store ( 12 ), and a processor ( 11 ) connected to the data store ( 12 ); wherein the actuator ( 10 ) further comprises: a loader ( 102 ) configured to receive via the communication network ( 2 ) and store in the data store a model definition ( 15 ) defining an HVAC control application; an element library ( 16 ) including a variety of stored model elements ( 4011 , 4021 ) comprising instructions for controlling the processor ( 11 ), and controller modules ( 401 , 402 , 403 ) configured in each case to control the sequential order of execution for associated model elements ( 4011 , 4021 ) referenced by the respective controller module ( 401 , 402 , 403 ), to propagate external data input to its associated model elements ( 4011 , 4021 ) prior to beginning the execution of the model element defined first in the sequential order, and to propagate data output from its associated model elements ( 4011 , 4021 ) to the outside of the respective controller module ( 401 , 402 , 403 ) at completion of the execution of the model element defined last in the sequential order; and a model interpreter ( 17 ) configured to implement based on the model definition ( 15 ) the control application for execution on the processor ( 11 ), by instantiating controller modules (D 1 F) and their associated model elements (A, B, C) as referenced by the model definition ( 15 ), assigning at least one instantiated model element to a device ( 4 ) attached to the sensor/actuator bus ( 3 ), and instantiating the controller modules (D, E) in each case as a different thread of execution, so that data is interchanged asynchronously between instantiated controller modules (D, E).