System behavior of a R410A residential unitary split heat pump system was investigated. The system was operated in cooling mode and used a thermostatic expansion valve (TXV) as the refrigerant expansion device. Seven artificial faults were tested: compressor/reversing valve leakage, improper outdoor air flow, improper indoor air flow, liquid line restriction, refrigerant undercharge, refrigerant overcharge, and presence of non-condensable gas in the refrigerant. The no-fault test results were correlated to produce a reference model of 2nd order multivariate regressive polynomials. The reference model used three independent variables, outdoor air temperature, indoor air temperature, and indoor dew point temperature, to correlate all other heat pump features. Standard deviations of liquid line refrigerant subcooling and evaporator exit refrigerant superheat were used as the main indicators for a steady-state detector algorithm. From the no-fault reference model, heat pump feature residuals were derived. Since the system was controlled by a TXV, the system could adapt itself to considerable external variation. Thus faulty behavior was not as detectable as it would have been with an orifice expansion device equipped system. The distinctiveness of a fault depended on the TXV status. Heat exchanger faults' effects upon performance depend on the sizing of the heat exchanger. From the dynamic tests, the system showed that the most influential factor for dynamic behavior was the change of the evaporator refrigerant exit temperature.
Citation: NIST Interagency/Internal Report (NISTIR) - 7350
NIST Pub Series: NIST Interagency/Internal Report (NISTIR)
Pub Type: NIST PubsReport Number:
Heat pump, fault detection and diagnostics (FDD), multivariate polynomial model