QUEBEC CITY - What is an extraordinary event? It's anything out of the ordinary, in particular something that could be dangerous. This includes fires, floods, chemical spills, or other types of wide-spread, toxic contamination. It could be an accident or a planned terrorist act.

What kinds of roles do residential HVAC systems play in protecting consumers from the problem, or making it worse indoors? That question was pondered at a forum of the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) at the group's annual meeting.

There is certainly increased public awareness about extraordinary events. Unfortunately, the moderator pointed out, the public is given conflicting information. In Houston, for example, when there is poor outdoor air, people are told to stay inside and run the HVAC. Other advice states that people should close their domiciles with duct tape and plastic, find a safe place, or evacuate.

(Note: Reporting of ASHRAE forums is limited to content; participants cannot be identified by name.)

"If you operate the HVAC," an attendee pointed out, "there is more infiltration due to duct leakage and pressurization. It could decrease particles, depending on the filtration."

What do public agencies recommend? How is the information disseminated to the public? Are there any reports of practical experience, or any research?

"There is no one-size-fits-all solution," because there is no one definite problem.

During the wildfires, people said they felt better inside their buildings, but that might have been a psychological effect. Fires and soot have particles smaller than 0.1-micron particles. This could dictate what kind of filters to recommend. As one participant said, "boulder catchers won't cut it."

The discussions shouldn't be limited to single-story structures, another participant pointed out. Apartment buildings and complexes can be affected by contaminants carried by plumes and stack effect. "These people in the cities are the ones we need to worry about," he said.

With the complexity of residential systems being proposed - a supplemental system to keep the location under positive pressure, and MERV 14 filtration for gaseous contaminant removal - contractors and engineers may be seeing a "bomb shelter mentality among clients."


Keeping a situation from going from bad to worse is the main concern. "There is a good chance that running outdoor air would bring in more particles that you don't want," said an engineer. Without pressurization, room-by-room pressurization would depend on the wind.

Evacuation probably would be preferable in some situations. However, what if people were to be overtaken by a plume while in their automobiles?

During the wildfires in the Pacific Northwest, agencies recommended the use of operated systems with prefilters that were replaced regularly. Another area went with static pressure and wound up with an inch of soot in the buildings, said an audience member. According to research, the conditions of indoor and outdoor air, including particle contents, equalize to each other after 84 minutes tops, with MERV 6 or 7 filters installed.

"The very fact that we're having this discussion means it's impossible to give a blanket recommendation," said a participant. The next steps would include identifying possible events and scenarios; what should be done in each case, before, during, and after the event.

Is there advice specific to single-family residences? "For some kinds, we can give very good advice, for other types we can't," said an engineer.

When should a homeowner run their system during an event, and when shouldn't they?

"We need to look at the housing stock," said an audience member. "Roughly half doesn't have central air." There also is very loose construction, and the possible need for getting people to safe places or decontamination sites.

Some days, the problems just look too big ... even for an HVAC engineer.

Publication date: 07/31/2006