Development of Temperature and Humidity-Based Indicators for Diagnosing Problems in Low Tonnage, Split System Air Conditioners

This paper presents results of a survey of the literature and identifies the most common degraded conditions associated with low-tonnage air conditioners. Other laboratory studies as well as marketed diagnostic systems are also summarized. A procedure for identification of useful, low-cost temperature-based indicators of degraded conditions has been developed at the Energy Systems Laboratory, Texas A&M University in College Station, TX under contract to Honeywell. This paper presents the methodology used to identify the temperature-based indicators for the most common degraded conditions gleaned from the literature. BACKGROUND It is common for automated monitoring and control systems to be applied to large scale HVAC systems found in large buildings. Many times these take the form of well-known building automation systems (BAS) and energy management and control systems (EMCS) with powerful microprocessorbased distributed control modules networked with central monitoring equipment such as PC's with large amounts of read-write memory available for data storage and manipulation. These systems have been shown to be useful for troubleshooting and the application of failure detection and diagnosis (FDD) techniques which help the operator to keep equipment operating at designed performance levels as well as maintain savings or keep operating costs at a minimum as a result of the improved maintenance. However, few applications to small-scale HVAC systems such as those found in residential and light commercial applications have been implemented due to the high cost of microprocessor-based equipment and on-board memory in past years. Recent improvements and technological advances have driven the cost of such resources down to a level that could justify application to the smaller units. This paper is a summary of a portion of the original work (Watt 1997) in which a split-system direct-expansion air conditioner was used to empirically determine temperature-based indicators that, together with one return-air humidity point, could detect performance deterioration resulting from equipment or system degradation. The air-conditioner test bench was equipped with the ability to use either a short-tube orifice (STO) or a thermal expansion valve (TXV). The degraded conditions studied include low evaporator airflow, highand lowcharge, and a blocked condenser coil. The empirical work was performed to identify degraded effects that fall outside of a simulationbased approach. It sought to utilize only low-cost temperature sensing means, although return-air humidity was also identified as an important factor for obtaining early detection over a wide range of operating conditions. Further, the original work sought to differentiate between a system that uses a short-tube orifice (STO), or fixed-orifice expansion, and a system that uses a thermal expansion valve (TXV), or variable-orifice expansion. The original work showed that low-cost temperature sensors can be used to detect the degraded conditions studied. However, it also showed that the temperature-based indicators of low evaporator airflow depend on three loading factors; outdoor-air temperature, return-air temperature, and return-air humidity. The original work did not include an investigation of the effect of the three loading factors on temperature-based indicators for improper charge or blocked condenser coil. In the literature, the return-air humidity sensing point has been neglected by previously published automated detection systems for failures and degraded conditions of low-tonnage air conditioners. Finally, while there were differences in the performance of a ST0 system and a TXV system relative to the degradations studied, a common set of indicators was identified that could detect the degraded conditions studied without regard to the expansion device.