Modeling of HVAC Systems for Fault Diagnosis

Analytical model based fault diagnosis method is seldom adopted for heating, ventilating and air conditioning (HVAC) systems as it is rather difficult to establish and verify an explicit model for HVAC systems. On the other hand, a flexible graphically based software called TRNSYS has been widely accepted in the design and optimization of HVAC systems recently. It makes sense to use TRNSYS to establish and verify an analytical model for fault diagnosis of HVAC systems. In this paper, a TRNSYS model of a HVAC system in multi-zone buildings is firstly established on the basis of practical data. Then, a state-space model of HVAC system is given according to the thermal-mass balance and air flow balance equations, and the consistency between the TRNSYS model and the state-space model is demonstrated. Also, some typical faults in the HVAC system are considered and the consistency between the two models under the faulty cases is verified again. Further, the effects of the faults on energy consumption is analyzed. The results show that some faults such as pipe scaling can significantly increase energy consumption while having no obvious change in room temperature. Based on the established models, some promising and challenging research directions for fault diagnosis of HVAC systems are finally discussed.