VAV Systems

"Worst case" to me means all VAV zone dampers at minimum position. Is there some alternative?

I suppose you could make a case in certain spaces that the zone damper would not be at minimum when the space is fully occupied due to the heat gain from the people. I would explore that avenue if some spaces were troublesome at minimum damper position.

David - leaving the confusing (and varied) review comments aside, I think the critical terminology is 'worst case' under 'normal operating conditions'. So as I read it, that means the dampers at the minimum position they would be set for a given occupancy. In some instances, occupancy will be relatively fixed and its simply minimum damper settings. But for many systems, there is interplay between occupancy and damper settings (as Julia references in talking about heat gain). So worst case normal is a bit of a moving target that has to consider both variables in dampers and in occupancy. As for maximums, I'm pretty sure the template doesn't ask for measured airflows at maximum flow - it may ask for the design maximums but the only measurements required are the worst case normal we were talking about. Measuring maximum flow doesn't really tell anybody anything useful, unless the question is determining the ability of the system to deliver ASHRAE ventilation (making the ASHRAE vs 10cfm/person distinction). And I suspect the second reviewer comment was simply confirming that what you said was worst case normal really was 'worst case'. Its possible that the OA rates were so high as to make the reviewer wonder if that was really worst case. Perhaps not the most direct way to ask that question, but that's my theory. Hope this helps a little - I suspect other folks may have more technically savvy insights. . . Dan

To build on what Dan said, "worst case normal operating conditions" are typically encountered on peak heating days (for which you can also use the design heating day). In that case, recirculation is maximized to limit heat loss, so outside air damper position is minimized, while VAV boxes are typically also minimized. (Peak cooling day is not quite as extreme since usually more air is delivered to offset zone heat gains, meaning VAV boxes would be slightly more open.) So I would recommend simulating the peak cooling day for the air testing.

Worst case normal shouldn't be crowded and cold. For critical zones where design occupancy is <20 SF per person, I think the room will always be in cooling mode when fully occupied, so heating mode is not "normal" when most crowded. This changes several factors in AHU set-up in MZ 62, notably the air distribution factor "CSCRH", which gives a distribution to the breathing zone of only 0.8. In cooling distribution should be 1.0, thus calculating 25% more OA. Does this make sense, and would it be plausible to a reviewer? We are not up against this, but seeing how the calculator works, I see this as an issue, and a way to save energy while keeping people comfortable and meeting IAQp1. I have asked engineers what a cut off is for crowding and cooling, and none venture to guess so far.

Table 6.2 from the standard outlines the parameters for assigning the zone air distribution effectiveness (Ez) for a given system. For ceiling supplied ceiling return systems that provide warm air, assigning a factor of 1.0 or 0.8 depends on the temperature of the supply air and velocity of the supply air. Ceiling supply systems that only supply cool air will always have an Ez factor of 1.0.