Indoor unit fault detector for a multi-split VRF system in heating mode

A multi-split VRF system operates unsteadily most of time due to the constantly varying refrigerant flow rates of associated indoor units. VRF systems require a different approach from conventional techniques to detect faults, which have developed based on steady-state operations. In this paper, two fault detection techniques are proposed. Their advantage is that they do not require the test data to be preprocessed to obtain steady-state data. The first technique is applied to detect heat exchanger fouling by a state observer, and the other technique is used to detect valve sticking by temperature variance. These techniques were not chosen haphazardly but were derived from physical reasoning. Their validity was confirmed by test data. The methodology developed in this study can be applied similarly to other HVAC equipment that operates mostly in transient states.