Tag: VAV

If you work in large commercial office or mixed use buildings you almost certainly work with VAV systems on a regular basis. If you aren’t familiar with VAV this is a quick intro to the basics so that they won’t seem so overwhelming.

VAV stands for Variable Air Volume, this means that the AHU (Air Handling Unit) can vary amount of air output to suit the amount of air needed for the areas served.

A typical configuration for VAV would be to set the blower to  hit a fixed static pressure target (say 1 to 1.2 inches of water column pressure) and the cooling capacity varies to maintain around a 55° supply air temperature.

Each zone is served by at least one VAV box or more accurately a VAV terminal. The terminal is controlled by a temperature (Generally the BAS system) to open and close to provide more or less air to the zone. As VAV terminals open the static pressure in the supply duct decreases and the blower increases air volume to maintain the static. As the air volume is increased the system cooling capacity also needs to increase to maintain the supply air temperature.

This constant modulation requires variable refrigerant capacity in the form of multi stage or variable refrigerant compression and generally a blower fitted with a VFD (Variable Frequency Drive) although older VAV systems often used a adjustable vane at the blower inlet to vary the air volume.

One of my favorite YouTube channels is THE ENGINEERING MINDSET and the Video below covers VAV in more detail. You can click this LINK to watch it direct on YouTube

I heard a great presentation by Ron Auvil on VAV systems and it got me thinking…

Can you size a commercial system / perform a block load by the number of occupants?

Yes! 

No, just kidding that’s crazy talk. There is way more too it than that.

However, in a commercial environment, while the perimeter of the building is affected by heat loss/heat gain to the outdoors, the internal zones are “cooling only” zones with the primary load usually being PEOPLE.

This is where the 500 btus per hour comes in. On average a sedentary worker in a building will add 500 btus per hour to ALL areas of the building whether it is hot or cold outside. This creates an issue in the winter when the perimeter of a building requires heating and the center of the building requires cooling.

Now, keep in mind, a sleeping person generates heat more in the neighborhood of 260 btu/ hr so if it’s a REALLY boring job where workers dose off at their computers it may be less.

Add in the internal electrical loads from lights, computers and other equipment and you start to realize that EXTERNAL loads are only part of the equation, especially in large commercial buildings with many occupants. In fact, in a busy commercial space the internal loads generally far outweigh the heat entering from the outside (external load).

This is where the concept of thermal diversity comes in. On a cold day there may be a need for heat at the perimeter of the building to offset heat losses to the outside while still requiring cooling in the center of the building to offset the internal loads.

In a good commercial design you must have some method of dealing with the thermal diversity between internal and perimeter zones along with maintaining appropriate ventilation / outdoor air.

Food for thought.

— Bryan

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