An economizer is a part of a building’s cooling system that uses cool outdoor air to cool the building instead of operating the air conditioning compressor.   An economizer is an optional feature to an HVAC air handler that draws in outdoor air and mixes it with return air from indoors when the air is cooler and dryer than indoor air.

Free Cooling

A controller in the economizer measures the temperature and humidity of the outdoor air. When the outdoor temperature is favorable, the control switches the air conditioning compressor off and cools the building with outdoor air instead. The economizer cools the building using far less energy compared to air conditioning.
Economizers typically operate at night when the outdoor air is cooler than the indoor air in a process known as “free cooling”. Economizers mix enough outdoor air into the indoor air in order to meet the thermostat setpoint without using the compressor. Fresh air that economizers bring in while they save cooling energy at night can count as ventilation. Therefore the ventilation system can run for fewer hours and avoid operating during the day’s peak electrical load.

Economizers for Ventilation

Many buildings use economizers as their ventilation system. Economizers don’t normally have heat recovery or energy recovery function, so ventilating with an economizer can has an energy penalty compared to a heat recovery ventilators. However, in California’s most mild climates ventilating with an economizer might be an acceptable option for a high-performance building. When buildings use an economizer for ventilation, its dampers are open a small amount while the HVAC system is heating, cooling, or operating on the fan-only option. The fan-only option is for when you need ventilation but not heating or cooling.
To run efficiently, the economizer would require very sophisticated control that tracks the amount of ventilation air delivered during the free cooling mode and during the ventilation mode. A controller similar to LBNL’s RIVEC controller could measure and regulate outdoor air into the building.

Economizers Frequently Malfunction

Economizers can also waste a lot of energy if there dampers get stuck open. Many economizers have installation flaws that cause them to under ventilate or over ventilating a building. There are a lot of energy savings available just from fixing economizers and installing smarter controls. Without regular maintenance, economizers are as likely to waste energy as to save energy.

Further information

HVAC Systems Field Guide

Turner, William J. N., Iain S. Walker, and Jordan Roux. Residential Pre-Cooling: Mechanical Cooling and Air-Side Economizers., 2015.
Turner, William J. N., and Iain S. Walker. Energy and IAQ Implications of Residential Ventilation Cooling., 2014.