Economizers and Effects on Architectural Design

September 25, 2015 by Chris Drake

During the sweltering summertime, I can’t help but daydream of cool autumn. I love to open the windows, turn on the ceiling fan, and turn off my A/C system. My energy bill seems to love it too! This idea is the basis for air economizer code requirements.

EconomizersA/C systems enter economizer mode when the temperature outside is around 55° and the thermostat inside calls for cooling because people, electronics, and appliances are generating heat. Air is pulled from outside through a louver, passes through the air handler, is supplied to the space, and is relieved through another louver. Yet the compressor, the largest energy consumer in an A/C system, is turned off. The air outside is used to deliver cheap cooling to a space in need.

Economizers are required for A/C systems with a capacity of about 3 tons under IECC and about 4.5 tons under ASHRAE 90.1. There are exceptions, but they usually do not apply to general office or amenity spaces. It is important for architects and mechanical designers to work together to satisfy the code requirement and minimize impact to aesthetics and budget.

There are additional components to an A/C system necessary to implement an economizer:

  • A larger outside air intake louver
  • A larger outside air duct
  • 2-3 motorized dampers OR a manufactured economizer box
  • A relief air duct
  • A relief air louver with a barometric relief damper
  • Possibly additional ceiling grilles

For planning purposes, the table below describes minimum sizes of louvers and ducts typical for different sizes of general office or amenity spaces requiring economizers:


Sq. Ft. Tons Duct Size Louver Size
900 3 18×14 30×18
1050 3.5 20×14 36×18
1200 4 22×14 36×18
1500 5 26×14 48×18
2250 7.5 36×14 66×18

*Assuming 18” of space above the ceiling available for ducts.

Depending on the floor plan, it can be challenging to coordinate ductwork, dampers, and louvers.  This coordination is crucial to satisfy economizer code requirements, maximize aesthetics, and minimize cost. Careful planning for louver locations at the beginning of a project will yield the best results.

With so many factors to consider in the selection of an HVAC system, such as economizers, knowledgeable consultants are of the utmost importance. At BIG RED DOG we have the experience and expertise to carefully evaluate system capabilities and limitations to determine the best option for your project. Contact our MEP team today to learn how we can put our expertise to work on your next project.

Written by Chris Drake

Chris Drake

Chris Drake is the Plumbing Discipline Leader at BIG RED DOG. He received a degree in Aeronautical Engineering from Rensselaer Polytechnic Institute in Troy, NY and is a registered P.E. His professional experience includes the design of plumbing and HVAC systems in the campus, institutional, sports, federal, and manufacturing markets. His is also experienced in design/build, construction administration, and is a member of ASHRAE and ASPE. Chris originally hails from the Finger Lakes region of upstate New York. Outside of work, Chris enjoys exploring Austin and the surrounding hill country, traveling, and supporting the Buffalo Bills.