Thursday, July 25, 2013

VAV versus PIU, have you ever had to try and figure out the difference?

VAV versus PIU, .... WTF (Where's the Fan)?
In many commercial Air Conditioning Air Distribution systems, there is a strange phenomenon.  That is, what is a VAV and what is a PIU and why do many people call all of these one or the other?
Great questions indeed.Unlike your home, commercial and/or industrial duct systems work at substantially higher internal duct air pressure, this is called static pressure.  Static pressure allows the air to flow (move really) from a higher pressure within the duct system to a lower pressure area within the duct system.  This creates the air flow that we all love to feel when we are hot.
In the common fan unit, it can be called an Air Handling Unit (AHU) in many instances, but could located within a Roof Top Packaged Unit (RTU) or a built up air handler, this is where many components are assembled in the field to make an AHU, usually on rather large equipment, there is a fan of some variety that produces static pressure.  This fan(s) could be a typical fan as you would have in your furnace (Forward curve), a backward incline a fan axial, etc... Many types, all trying to do one thing, produce static pressure within the duct to allow the Air Distribution System to deliver air flow.
The Air Distribution System is made up of the fan, as we discussed above, the duct system as well as incorporates all Terminal Units.  These terminal units are the VAV's and PIU's that brought us here to begin with.
                          VAV = Variable Air Volume     PIU - Powered Induction Unit
Both of the devices incorporate a damper than automatically adjust the air flow when the building controls (#Tridium as an example) ask for cool air to be delivered into the space that particular VAV is serving.  These areas being served are referred to as Zones.  In most modern layouts and engineered buildings, the VAV's serve the interior parts of the building, the areas that typically do not require heat.  The heat in these areas is provided by lights, people, solar warmth, etc...
The PIU is an assembly that takes the same concept as the VAV, but once the controls tell the damper to close due to the space set point being satisfied, the PIU will in fact completely close the medium pressure air from the duct system then start a small internal fan that will recirculate return and/or plenum air to begin to temper the space if the temperature still continues to fall, once the fan moving this non-cooled air cannot suffice, the PIU can start to heat, many times this is a single or multi-stage electric heating element or possibly steam or heating water.  These PIU devices are typically located within the exterior zones of a building where heat is required to keep the occupants and or goods at a comfortable temperature.
Knowing a bit more about what is lurking above the ceiling tile may never wind up helping you in your day to day, but at least you will have a good idea of whom to call if things were to get hot and sticky in the office.
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DISCLAIMER:  Verbiage within our wacky Mechanical Industry can vary greatly, I have tried within the post to capture the general essence of what "most" people call "most" of the devices.  We try to get along, this is what happens when things go wrong!



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