I found Edward Liwerant's article on variable-air-volume- (VAV-) box selection in the February issue (“VAV-Box Selection, Code Conformance”) very useful and informative; however, I wanted to add a note of caution to the statement that, “Perhaps the best course of action is to oversize a VAV box.”
While I realize that undersizing any portion of an HVAC system can cause problems, so, too, can oversizing.
A VAV box is a control device, comparable to a control valve, and just as oversizing a control valve leads to a loss of controllability, so, too, does oversizing a VAV box. Some of the most difficult VAV control problems I've seen were caused by boxes that were so grossly oversized they achieved maximum design flow when the damper was open as little as 20 percent of its full stroke. In essence, the box was throwing away 80 percent of its normal control range and trying to do all of its modulation within the remaining 20 percent.
One of the symptoms of an oversized VAV box is the damper cycling open and closed at low flow set points, as Mr. Liwerant mentions later in the article. A cycling VAV box can maintain space temperature, but not without seriously shortening actuator life.
Steve Tom, PE, PhD
Mr. Tom is correct in urging caution regarding the oversizing of VAV boxes. However, the cycling on and off of VAV boxes to maintain space-temperature set point is a fact of life, regardless of the accuracy of box selection. I suggest a 20-percent oversizing limit for VAV boxes.
Edward Liwerant, PE
R.E. Lamb Inc.
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