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.

  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 []

Expressions of Ventilation

Several expressions can be used when quantifying a building’s ventilation. You may talk to an on-site building engineer that expresses it in percent outdoor air. Or, you may talk to the mechanical engineer that designed the system, and he may express it as cubic feet per minute (cfm) per person. Heaven forbid you bump into a building scientist who likes to express ventilation in terms of air changes per hour!

In this post, I would like to describe these three common expressions for ventilation.


CO2 and Ventilation

Atmospheric CO2 Levels from Mauna Loa

Today I was teaching a Certified Indoor Environmentalist course in Connecticut when we measured carbon dioxide (CO2) levels in the meeting room.   When I read 2,850 parts per million (ppm) on my meter, I immediately assumed something was wrong with the device.   That was upsetting because it had just come back from factory calibration.  In such a well conditioned, modern meeting room, we couldn’t really have CO2 levels that high, could we? A typical meeting room ranges from say 800-1,200 ppm.

When taking the device outdoors, we were getting readings around 380 ppm as one would expect (see chart).  In other areas of the hotel, levels were more in line with expectations (around 1,000 ppm).  Our high levels were not a computer glitch, but an accurate reading of the CO2 levels in the space.

Here is a key point learned by the class today:

Ventilation VOCs

Reducing VOCs

Yesterday I received a call from a homeowner concerned about volatile organic compounds (VOCs).  After using an epoxy resin (and other VOC-laden building materials), the odors in the home were very strong and his wife stared showing signs of sensitivities.  Fast forward a year and now his wife has Multiple Chemical Sensitivity (although I prefer the term “toxicant-induced loss of tolerance or TILT).  They are trying to stay away from the home as much as possible, putting them in a very difficult position.

Here is my general advice:

#1. It’s always best to prevent the problem in the first place by using low VOC products.  These used to be very difficult to find, but now they everywhere.  You really have no excuse for using high emitting products.  For the homeowner’s situation, this advice didn’t help because he already installed all the materials and was unwilling to gut the place (I don’t blame him).