Ventilation: 'Airing' on the Side of Caution

by Moses D. F. Ling, PE, RA

 

This fall, I had the pleasure of attending a workshop at Woods Hole Research Center. The workshop participants were architects and graduate students involved in teaching environmental control systems. The focus of the workshop was to conduct case studies as a means to improve pedagogical delivery. By the second day, participants were engaged in proving a number of hypotheses and collecting data in this award winning building by William McDonough + Partners.

My hypothesis was that offices on the top floor of a building with an open core do not receive fresh air from the outside through an open window. The building operator had shut off the ventilation system assuming the occupants would open the windows for fresh air. At the end, I concluded that natural ventilation strategies need to be carefully considered in the design process. When properly incorporated into the building design, natural ventilation can be pleasing when the outdoor condition is favorable. Consider the following points of interest:

1. In a building where the culture promotes conservation, people will take the stairs. As the result, doors in stair enclosures will be propped open for convenience and create chimneys for updrafts through the building. (Hold-open devices are preferable to wood wedges if the enclosure is rated.)
2. Occupants, in general, do not open windows. They will work in rooms with elevated carbon dioxide levels without opening the windows.
3. Researchers and educators of architecture, who should know better, work in rooms with elevated carbon dioxide levels without opening windows.
4. Outdoor air is not as clean as one might expect. At this pristine Cape Cod location, outside air carbon dioxide levels were 500 ppm. The acceptable indoor threshold is 800 ppm. One might conclude that clean outdoor air potentially is more than 50% “contaminated”.
5. One occupant commented that the threat of rain causes her to keep the window closed. Overhangs and awning windows can allow windows to stay open during heavy downpours.
6. Occupants loved the visual connection to the outdoors through the windows. Perhaps the healthful effects are more psychological than physiological.

Through a studious day-long data collecting effort and blowing bubbles in the building, we concluded that air will exit through open windows on the top floor. This implies that the occupants in these top floor offices are receiving second hand air from the core of the building.

At the other end of the design process are the code minimum requirements. The International Mechanical Code mandates placement of outside air inlets. If the natural ventilation strategy is employed, operable windows become outside air inlets.

 

• 401.5 Locate outside air exhaust and intakes openings 10’ from lot lines. If the opening fronts on a public way or street, the distance is measured from the center line of the public way.
• 401.5.1 (a) Locate outdoor air intakes 10’ from any hazardous or noxious contaminants such as chimneys, plumbing vents, street, alleys, parking lots and loading docks.
• 401.5.1 (b) Where a source of contaminant is located within 10’ of the intake opening, the code mandates locating the intake opening a minimum of 2’ below the contaminant source. Careful consideration is recommended when invoking this rule. Two feet is the code minimum, good design relies on careful considerations by the designer. In the exhaust section of the code, exhausts are prohibited from locations where the discharge air can be readily drawn back into the building.

While operable windows are desirable (and I love them at my home), I don’t open them when it is hot, cold, or raining (now that I live in a house with few overhangs). If the control of the indoor environmental conditions is crucial, mechanically introducing treated outside air is the preferable option. The designers can centralize the intakes and locate the louvers strategically. An economizer cycle can be specified to introduce outside air directly when the conditions are favorable.