Airside Economizer- Comparing Different Control Strategies and Common Misconceptions

Abstract

Air-side economizer is broadly adopted in building HVAC design and operations. When the system is properly designed and the control sequence is properly implemented, air-side economizer provides significant energy savings. The design and operation basics of the air-side economizer are well understood and documented. However, some confusion and misconceptions do exist and are widely spread. When the economizer is not designed or implemented properly, an air handler cannot take the full advantage of "free cooling", and, in some cases, could even cause significant energy waste. This paper first introduces the fundamentals of the airside economizer and the typical control sequences. It goes on to discuss the determination of the activation temperature that enables or disables the dry-bulb temperature based economizer operation. The “best” activation temperatures that maximize the energy savings can be calculated based on weather data and are different from location to location. The activation temperatures for a few representative cities are presented. For drier weather regions, the activation temperatures are significantly higher than those for hot and humid weather regions. The second part of the paper discusses the benefits of the enthalpy-enabled economizer and points out some important misconceptions that could significantly impact the energy savings of the economizer operation. Specifically, it challenges the simplistic control strategy for the enthalpy-based economizer control that is commonly used in the industry. Some of the questions this paper tries to answer include: 1. What is the optimal activation temperature for a temperature-based economizer that provides the most energy savings? 2. How does enthalpy-based economizer compare with the temperature-based economizer in energy savings? 3. Does an economizer always save energy when the outside air enthalpy is below the return air enthalpy? 4. Is it necessary for the outside air enthalpy to be lower than the return air enthalpy to enable the economizer and save energy? 5. What happens if the economizer control fails? What are the potential penalties?