No-Cost Air Conditioning System Diagnostics Using Fundamental Equations and Existing Controls (ST-16-C021)

Building operators can save significant amounts of time by using Direct Digital Control (DDC) system programming logic and by creating concise Building Monitoring System (BMS) screens to troubleshoot heating, ventilating, and air conditioning (HVAC) systems.  They can also drive down energy costs.  A mature energy efficiency utility has proposed the use of fundamental psychrometric formulas and parameters into DDC systems’ control logic to check sensor accuracy, improve equipment performance, and to verify valve and damper operation.  Relative humidity sensors are used to control humidifiers and dehumidification coils, and have been indirectly used to calculate enthalpy in economizers. The application of fundamental HVAC formulas into the DDC control logic identifies inaccuracies in relative humidity sensors, which, if left uncorrected can result in poor economizer performance, or unnecessary humidification and/or dehumidification. These problems frequently increase the cost of operation of the HVAC equipment. The utility also has applied outdoor air dew point calculations to check sensor performance and to optimize chiller and humidification system performance.  This paper will define dew point under several conditions, describe how it differs from relative humidity, and describe their effects on latent and sensible cooling and heating loads.  It will also present psychrometric facts and how they can be used to check the performance of HVAC systems.  For example, if the chilled water temperature is higher than the entering  air dew point temperature of a cooling coil, the coil will not remove any moisture from the entering air; therefore, the dew point will remain the same.  Programing this logic into the DDC can be done with simple “if” statements, and can be used to identify faulty sensors.  The paper will present a sample programming statement that can be modified to the facility’s DDC system programming language. The paper will also describe how, once the logic statement is entered into the DDC, it is important that the BMS screens are easy to evaluate and thus make it easy for operators to troubleshoot any system performance issues. An example of a fault detection screen for air handling unit economizers and an outdoor air weather station will also be included.