How to ensure minimum outdoor air at zone level in VAV system 3

[Drazen]

Currently I am being pushed by prospective client to design VAV system for multi-zone space where there is number of conference rooms because "he has seen it working well".

I am concerned about how to ensure minimum outdoor air requirement in situation where zone peaks can occur in quire different times.

Say one of conference room's cooling load will significantly fall, while in other zones load will remain at peak. In that case system supply air will not change significantly while in one or two zones supply volume can fall to minimum.

Velocity sensor near outdoor air damper will not help me, I believe, because if system air volume will not change much, outdoor-to-total supply volume ratio will remain approximately the same, and if supply volume will be much reduced just in few zones, than these zones will not receive enough outdoor air.

Is there any way to resolve it other than demand-control-ventilation?

 

[ChasBean1]

317069, Hi, I wanted to get back to you. Basically, no – the AHU would continue to bring in the min. OA to offset the building exhaust plus 5-10%. If (for example) a big AHU serves a section of a building and there is a sun face with an unoccupied conference room… If the conference room gets hot from the sun beating on it, the VAV box (or FPT, etc.) serving that room might modulate toward maximum airflow set point to cool the zone. But it will not change the OA percentage at the AHU.

So if the same room is exposed to clouds and cold OA temperatures but has a packed conference, the VAV can also modulate toward maximum airflow when CO2 readings elevate. If that happens and it’s still too stuffy, the associated AHU slowly raises its minimum OA (subject to a mixed air low limit that prevents freeze trips) to accommodate the loaded conference room.

The correctly functioning CO2 control therefore can cost more money in extra OA conditioning, but has benefit in that it can provide adequate ventilation when typical minimum values (to offset building exhaust) cannot.

 

[urgross]

Another option would be to oversize the box supporting the high density population areas. Outside air damper would stay the same. The box would be controlled off of CO2. This would probably work best in conjunction with a fan coil unit or similar to Mr. Slim for temperature control.

 

[317069]

Chasbean.
to me, you descriped a DCV function, but Drazen does not want to use CO2 sensors at all.

 

[ChasBean1]

317069, sorry, just realized this ball was in my court with your last post. The systems should maintain the min. OA to offset exhaust. I'd recommend this for any system serving any space at any time. CO2 control could help with Drazen's percentage of OA concern. If CO2 control is not wanted and we instead design the minimum OA value based on what some space may need at some point, there will be more air conditioning costs when it usually would not be needed. A well designed system might have a 15% OA value to offset building exhaust. That value would be fixed. We would only increase if a space had a CO2 rise above a certain level, e.g., 850 ppm, and only after that zone's airflow increased to its max airflow set point.