Ventilation with Economizers

I just read an article in November’s ASHRAE Journal on outdoor air economizers1. It was excellent. It was also above the heads of most non-engineers. In this blog post, I want to describe the basics of economizers, their impact on indoor air quality, and briefly touch on their operation and control.

Introducing outdoor air ventilation into an office, school or home is a good thing. The down side, people quickly point out, is the cost of conditioning that outdoor air. As I write this, the temperature in Chicago is 9° F (-13° C). Introducing 9° air gets expensive because of the energy required to heat it. At extreme temperatures, most buildings will opt for only bringing in the code-required minimum ventilation. What about on more mild days?

Buildings2 have the opportunity to introduce more fresh air when outdoor conditions are favorable. On warmer winter days, outdoor air can be used in excess of code minimums to provide cooling.3 On a pleasant 55° day, the scenario may be such that the building brings in 100% outdoor air, recirculating no air. Those are conditions where energy engineers and IAQ consultants sing Kumbaya together. Energy engineers love the free cooling, IAQ consultants love all the extra ventilation.

Because ecomonizers are great for energy and IAQ, they are required by ASHRAE Standard 90.1 and are often adopted into building code.  Although you are required to have them installed, you’re not required to actually know how to use them. Therein lies the problem.

The outdoor air dampers are typically controlled by the computerized brains of the building: the building automation system (BAS). When conditions are present to bring in more than the code minimum ventilation, the BAS sends a signal to electric or pneumatic actuators that will open outdoor air dampers to the desired amount. Concurrently, the BAS will adjust other dampers to reduce the air being recirculated and increase the air being exhausted.

Here are the big questions…

  • What should the economizer setting be for every possible outdoor air and return air temperature & humidity?
  • When is it too warm, or too humid to operate? (What is the high limit control?)
  • Should its operation be based on outdoor temperature or outdoor enthalpy, which takes into affect humidity?

For precise answers to those questions, I suggest you read November’s ASHRAE Journal article, “Economizer High Limit Controls and Why Enthalpy Economizers Don’t Work” by Steven Taylor and C. Hwakong Cheng.  You’ll need to become a member of ASHRAE in order to access the article.  You won’t regret that membership.

Hopefully, you now understand the fundamentals of economizers and the ASHRAE article will make some sense.  For those of you who already knew the fundamentals, I strongly suggest you read the ASHRAE article.  It really rocked my economizer world and changed a long-held belief of mine.

  1. “Economizer High Limit Controls and Why Enthalpy Economizers Don’t Work” by Steven Taylor and C. Hwakong Cheng []
  2. Sorry most homes… you’re not bringing in outdoor air mechanically and can’t benefit from economizers. []
  3. I know it sounds strange, but buildings are typically cooling the core in the winter.  Lights, people, computers, elevators etc. give off a lot of heat []

4 thoughts on “Ventilation with Economizers

  1. When there is an outside pollutant source (i.e. diesel exhaust, re-roofing asphalt, etc..) that is impacting IAQ our HVAC engineers response is to close-off economizers. It’s been my experience that this is usually not very effective. Your thoughts.

    1. Ventilation is predicated upon outdoor air quality being better than indoor air quality. As you pointed out, that’s not always the case. Under unique circumstances such as asphalt work, outdoor air intakes should probably be closed. Assuming bathroom exhaust fans continue to run, the building will be under a negative pressure and the contaminants still come in, albeit at a much lower rate. You would need to shut down all outdoor air AND exhaust air to keep the building neutral. Post 9/11 there was a lot of talk on having controls to shut OA and RA in the case of an area-wide chemical or biological attack.


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    1. Mr. Mehta,

      You are very astute to see that in hot and humid climates, economizers won’t provide all the benefits reaped by other climate zones. In the article referenced, it shows the economizer savings in Wh/ft2/yr for different climate zones. A hot and humid climate (like Miami or Mumbai) may only get a benefit of 75 Wh/ft2/yr, whereas a more temperate climate like San Francisco will reap 575 Wh/ft2/yr. That’s a big difference!

      Another challenge faced in the Mumbai region is outdoor air pollution. When I was in Mumbai to give a presentation on indoor air quality, many attendees spoke of the problems outdoor air ventilation can introduce. ASHRAE’s ventilation rate procedure is based on outdoor air being better than indoor air. You may need to use their IAQ procedure, employing advanced air cleaning methods.


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