1. An actuator for HVAC systems, the actuator comprising: a network interface for connecting the actuator to a communication network, a bus interface for connecting the actuator to a sensor/actuator bus, a data store, and a processor connected to the data store; wherein the actuator further comprises: a loader configured to receive via the communication network and store in the data store a model definition defining an HVAC control application; an element library including a variety of stored model elements comprising instructions for controlling the, processor, and controller modules configured in each case to control the sequential order of execution for associated model elements referenced by the respective controller module, to propagate external data input to its associated model elements 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 to the outside of the respective controller module at completion of the execution of the model element defined last in the sequential order; and a model interpreter configured to implement based an the model definition the control application for execution on the processor, by instantiating controller modules and their associated model elements as referenced by the model definition, assigning at least one instantiated model element to a device attached to the sensor/actuator bus, and instantiating the controller modules in each case as a different thread of execution, wherein data is interchanged asynchronously between instantiated controller modules.