Dear all,
I have a VAV system for my school, with each AHU feeding around 25 zones (classrooms and offices).
Each zone has a maximum primary air flow and an outdoor air intake flow (specified as per ASHRAE standard 62.1).
Taking the example of 1 zone:
The minimum ASHRAE required outdoor air intake flow is 800CFM, and the Design zone primary air flow is 2000CFM. If the cooling load in this zone drops, being a VAV, the supply primary air flow may get down to 1000CFM. However, since the fresh air and the recirculated air are being mixed in the AHU, the fresh outdoor air intake flow for out zone will drop down proportionally to 400CFM, which no longer complies with ASHRAE requirements!
How do you deal with this issue? Do you think that the fresh air minimum requirement must correspond to the minimum design primary air flow? In this way, if the zone load increased, the design primary air flow will induce a lot more fresh air than required by ASHRAE.
Thanks for your help!
Roger Chang
Principal, Energy and Engineering LeaderDLR Group | Westlake Reed Leskosky
LEEDuser Expert
398 thumbs up
October 14, 2010 - 8:31 am
This is one of the weak points of VAV systems. In order to guarantee proper ventilation at all conditions, the amount of outdoor air that needs to be brought in at the air-handling unit can be significantly higher than what's required if you just sum up the ventilation rate required for each zone. This is why decoupled ventilation systems are now preferred, especially for schools. A dedicated outdoor air system handles delivers only what is needed for ventilation to each zone, typically at neutral air temperature. Space sensible cooling and heating is provided by a recirculating VAV system (dual-duct), fan coil units, radiant panels, chilled beams, etc.