Technology Overview: Economizers

August 27, 2007
 
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Economizer Digram
Economizer diagram
(click to enlarge)

Economizers reduce cooling energy consumption by allowing cool outside air into buildings, which reduces or eliminates the need for mechanical cooling.

How it works
Economizers provide 'free cooling' for HVAC systems by allowing more outdoor air to enter a space when temperature conditions justify it. The economizer works through use of a damper that opens and closes to regulate the amount of outside air introduced into the building. When the outdoor air temperature is too warm to help meet the building's cooling load, the damper closes to a minimum setting while allowing for minimum ventilation requirements.

How it saves energy
Economizers reduce cooling energy by using outside air to help meet the cooling load under appropriate conditions, thereby reducing or eliminating the need for mechanical cooling. An integrated economizer allows outside air to contribute as much cooling benefit as conditions allow, while mechanical cooling operates simultaneously to satisfy any difference in meeting the total cooling load.

Taking advantage of this 'free' cooling reduces energy costs by reducing the time the air conditioner compressor system operates, while maintaining comfort levels.

Design considerations
Except for special conditions, an economizer is required by California's Energy Efficiency Standards (Title 24) prescriptive compliance approach when an individual system's cooling capacity is greater than 75,000 Btu/h or the supply airflow is greater than 2,500 cfm. Title 24 requires integrated controls when following the prescriptive compliance approach. When using the per formance approach, nonintegrated controls are allowed if they comply.

When integrated economizer controls are used with a direct expansion (DX) cooling system, small cooling loads risk freezing the evaporator coil and compressor. Hot gas bypass, multiple compressors, or multi-stage compressors are sometimes specified to avoid this problem.

Title 24 requires Acceptance Testing for all economizers that are not factory installed, to verify that they perform according to the design intent. Regular maintenance is recommended, as a malfunctioning economizer can easily go unnoticed and cause substantial increases in heating and cooling energy consumption.
reduces or eliminates the need for mechanical cooling.

 
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