VAV Box Types Fan Powered and Shut Off VAV Boxes - When attempting to name the different types of VAV boxes we find there are different names given to VAV boxes by different people and we find that no one is on the same page when VAV boxes are definitively defined.

We have fan-powered VAV and non-fan-powered VAV’s and we have series and parallel VAV’s and we have VAV boxes with a hot deck and a cold deck that mixes and one of the last projects I worked on some of the VAV’s were referred to as shut-off vavs. Variable Air Volume Boxes (VAV Boxes) regulates airflow into zones using a primary air source from a VAV air handler.

VAV Box Types Fan Powered and Shut Off

• Cool only single ducted series
• Fan-powered single duct series with reheat
• Fan-powered single ducted parallel with reheat - uses plenum air before using reheat for energy efficiency

Aside from the control aspect of VAV boxes, pneumatic, electric, electronic, or DDC, there are many different variations to VAV Boxes and we will attempt to define each type of VAV including offering a diagram of each type and matching it with a specific application. VAV boxes are simply an assembly of a damper, damper controls, and sometimes reheat such as electric heat strips or a hot water coil. VAV boxes get their primary air source from a central VAV air handler and then distribute air throughout a specific zone the VAV box controls.

Cool Only Single Duct Series (non-fan powered)

VAV Box Types Fan Powered and Shut Off VAV Boxes waiting to be Installed Cool Only Single Duct Series VAV (non-fan powered) – The most basic VAV box I have ever seen was simply a box with a damper inside it that was controlled by an actuator.

This was referred to as a cooling-only box and was connected to the primary duct and led to a zone within the floor of a multi-story building. It had no reheat nor did it have a way to mix plenum air with the conditioned air coming from the VAV air handler.

It had a DDC controller that controlled the damper along with a discharge air temperature sensor and a remote thermostat connected to the DDC controller. In the wintertime, this VAV box was always in the unoccupied mode.

It was assumed that the large office space would provide enough residual heat from the tenants and computers plus the heat generated from the VAV boxes in the outer zones that this particular zone would not need any heat. Of course, this assumption was incorrect and there were several people who complained about being cold in the winter time and the nearly unbearable working conditions.

The final recommendations from me were to change the box to allow reheat or add an electric duct heater after the VAV. They finally settled on changing the box to allow for electric reheat. In any case, I have always called this type of VAV box the Cool Only Single Duct VAV. If you have any other comments about this type of VAV box and you want to share your knowledge with others leave a comment below or email us using the contact form below.

Fan Powered Single Duct Series – VAV Box Types Fan Powered and Shut Off VAV Boxes

Fan Powered Single Duct Series VAV Box – The next type of VAV box I am familiar with is the fan-powered single duct series VAV box and this box to me makes perfect sense for use in an office environment (perhaps ideal for other places also). The damper for this VAV box will modulate within a set range depending on the zone temperature and the engineered airflow rate but this VAV is generally a good constant volume box for a particular zone.

The damper can modulate towards the closed position so you can mix primary air with plenum air to save on reheat in some cases. That results in energy savings because you are using residual heat from the people and machines locates in the zone to provide heat instead of electric or hot water reheat. If you have any other comments about this type of VAV box and you want to share your knowledge with others leave a comment below or email us using the contact form below.

Fan Powered Single Duct Parallel – VAV Box Types Fan Powered and Shut Off VAV Boxes

Fan Powered Single Duct Parallel VAV Box – This type of VAV box seems to be less prevalent to me but I have seen them. These VAV boxes are usually very big. The primary airstream usually flows through the VAV box unimpeded and when there is a call for heat the VAV box damper backs down to reduce the primary airflow while the fan increases the amount of flow from the plenum air which mixes with the primary air to warm it up and deliver it to the zone. In this regard, the fan is off in the cooling cycle and only on to draw warmer plenum air in to warm up the primary air.

The boxes come with back-draft dampers to prevent the primary air from reversing flow through the fan and into the plenum air. The backdraft dampers usually need attention from the maintenance staff from to time as they can make noise as parts begin to wear with time. The fan-powered single duct parallel VAV box also uses a filter where the plenum or recirculation air enters the box so the VAV filter is another part of regular maintenance for the maintenance staff.

I have seen these boxes with hot water reheat and electric reheat to provide heating to the zones so this box can best be used in the outer zones where the load can be the highest. If you have any other comments about this type of VAV box and you want to share your knowledge with others leave a comment below or email us using the contact form.

VAV Box Types Fan Powered and Shut Off VAV Boxes

Resource: The Fundamentals of HVAC Direct Digital Control