VVT vs VAV

[dxla]

Is a VAV system better than VVT system ? Which is more expensive ? What are the advantages/disadvantages? I have a new single level office building (offices, lockers, meeting rooms, computer room, lunch room. Total 6000 sq ft) to design in the west coast.

thanks in advance.

 

[walkes]

The VAV system if designed properly can be considered a "better" system depending on your goals. If you want controllability it is likely better, but it is generally more expensive. The VVT system in my experience is used more for packaged rooftop constant volume units where the user may want some space control but doesn't want to spend a lot of money.

With the VVT system the zones vote on what the RTU is doing (heat/cool or ventilate), so there may be times (depending on the similarity of zones) where the space has no airflow to avoid overheating or overcooling. If most of the zones on the RTU have similar exposures then the VVT system will likely not cause the RTU to "flip-flop" between settings.

With the VAV system your air handling unit will have some means of providing a variable air volume, with a relatively constant discharge air temperature (depending on the control strategy you may reset the discharge temperature to ensure higher air volumes in the zones). The VAV terminals will adjust from Max to minimum depending on space demand. Your system may have reheat or not (or perimeter radiation for heating).

In either case your computer room may have a separate cooling system to avoid high cooling demand on the system.

Eg.VVT Zone-All see

http://www.zoneall.ca/

 

[cme]

VVT is a piece of garbage and suffer occupant complaints therefrom.

On heating, you're stuck heating rooms requiring cooling.

 

[joken]

We have had very good success with Trane Tracker/Vari-trac VVT systems with roof top units. Properley balanced, I have consistant zone temperature variations of less than one degree total on an eight zone system. The system allows remote or local programing of setpoint limits, airflow etc. Very dependable. I do not work for Trane. Ken