#1030 Exhaust VAV and Space Pressurization Control

Francis Mahony Mon 23 Jan 2023

Many labs and medical spaces served by VAVs have a complementary exhaust box with a modulating damper and flow setpoint. The point of this "exhaust vav" is to maintain space pressurization with some systems dynamically resetting the exhaust flow setpoint using a trim and response algorithm to do so.

I don't believe this system is currently covered. The closest I found is the exhaust equip proto which, while generic, doesn't seem to capture the sophistication of the system. Per the documentation, VAVs as they are currently defined, are for controlling the flow of "..supplied conditioned air from an AHU and use a damper to modulate the air flow to the zone..".

In my mind, the way to model this cleanly would be:

  • to define a zone with hvac zone points such as temperature, humidity, and space pressurization.
  • define a "supply vav" equip that keeps points relevant to supply flow and discharge temperature control.
  • define an "exhaust vav" equip that keeps points relevant to exhaust flow control.

The ramifications of such modeling includes:

  • an update to the vav definition to allow for boxes that either supply air to a space or remove air from a space.
  • equips conjucts such as discharge-vav and exhaust-vav. It would be understood that discharge is implicit when exhaust is not explictily provided.

It is already not-uncommon for one VAV to have multiple zone sensors (e.g. multiple HVAC zones) and utilize either an average or worst-case logic for control. This is modeled through placing an airRef tag on the space(s) and pointing it to the supply VAV.

The above would require a similar reference to identify the flow of air from the zone-space to the exhaust vav. The airRef tag is defined as "Air flows from the referent to this entity". Re-using that would imply that space is a provider of air and the exhaust VAV a consumer of air... which just seems wrong and doesn't jive with how things are currently setup. It seems a tag such as exhaustRef would be analogous to airRef, unless it's understood that the spaceRef of an exhausting equipment is also the space it is exhausting. However, this seems to muddy the waters on differentiating between the physical location of an equipment and its logical assoiction.


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