News & Ideas

July 12, 2014

Saving Energy with Air-Side Economizers

What is an economizer? The simplest definition: an economizer is a component of an HVAC system that uses cold outside conditions to provide “free-cooling” to a space. The goal of the economizer is to save energy and operational costs. 

An air-side economizer uses cold air, in lieu of mechanically conditioned air, to cool a space. When the conditions outside are favorable (winter for example), the HVAC system brings in a higher percentage of outdoor air to cool the space. This allows the compressors to be shut off and saves energy. 

Code requirements: The latest versions of the Energy Codes are increasing the requirements for economizers. Currently, in our region, the 2012 International Energy Conservation Code requires economizers on most systems over 2.75 tons (33,000 btu/hr). 

Considerations: 

  • Intake louver size: A system without an economizer cycle typically requires a louver sized for the ventilation air only, 20% of the total for example. A system with an economizer will require a louver sized for 100% of the system flow or 5 times the size.
  • Clearances: Due to the larger intake louver and associated ductwork, the system may require more room for installation. It will also require additional dampers and controls which also require additional space. On a small, office-grade 5 ton, split system, the dampers and associated plenum alone can double the size of the required mechanical closet.
  • Relief air: When a system is operating in economizer mode, it is introducing a large amount of outdoor air to a system. This can over-pressurize the facility and require an air relief system. On a larger rooftop unit, this relief fan can be integral to the unit but for smaller systems or indoor air-handlers, a separate relief fan and its associated exhaust louver is often required.

Water-side economizers are another option for water-cooled systems. They follow some of the same code requirements though their detail and considerations vary. We will discuss water-side economizers in more detail on a future article. 

For follow-up questions or more information, please contact: 

Scott Walthour, PE, LEED BD+C
Principal and Senior Mechanical Engineer
ARIUM AE
scottw@ariumae.com