Upgrading VAV Boxes It is not surprising when dealing with existing buildings to find a variety of VAV terminals.

Paul Ehrlich, Ira Goldschmidt & Angela Lewis Building Intelligence Group As published   October Issue - BAS Column

Variable Air Volume (VAV) terminal units have been widely used for space comfort conditioning over the last 30 plus years.  There are many variations on what you will find for boxes from simple shutoff boxes to those with reheat, and either series or parallel fan powered units.  VAV boxes provide a great solution for control of comfort with good efficiency, and reasonable initial cost.

It is not surprising when dealing with existing buildings to find a variety of VAV terminals.  The age, condition, and controls used on these terminals can vary dramatically – from near new and in great shape to completely failed.  Ideally, we like to see VAV boxes to be part of the Building Automation System (BAS) and to be equipped with pressure independent DDC controls.  This leads to the retrofit of boxes that have older pneumatic or electronic controls.  There are many benefits to retrofitting to new DDC controls including:

• Improved Operations:  It is easy to see the actual operating values (temperatures, air flow, setpoints), allowing improved response to “hot and cold” calls.
• Improved Box Efficiency:  A DDC controller allows for improved setpoint adjustments in occupied and unoccupied modes. They also can use different settings for minimum cooling and reheat, reducing unwanted sub-cooling.
• Improved System Efficiency:  A system of DDC boxes can be used for feedback for optimized fan operation including static pressure and discharge air reset strategies.

Still, installing DDC controls on existing boxes is not an inexpensive proposition.  Costs vary but are generally around $1,400 per zone, so we want to make sure we get the best value from this type of retrofit and that means that we need to have a box that is worth controlling.  In many cases, you may find that the existing box is in good shape and worth just putting on a new controller, but there are several pitfalls to watch out for:

• No Flow Measurement or Mechanical Flow Regulator:

o    Issue: It was not unusual for older pneumatic boxes to be pressure dependent and not have any sort of flow measurement ring, square, bar or cross installed or to have a mechanical volume regulator.  This makes pressure independent control impossible and doesn’t allow for measurement and control of flow, only damper position.
o    Solution:  A retrofit flow bar can easily be installed in the inlet to the box. If a mechanical regulator is installed, it can be removed.  While a flow bar is not as accurate as a true averaging element (such as a ring or cross), it will give an indication of flow and can be adequate.  Alternatively, a new box can be installed.

• Internal Pneumatic Actuator:
  

o    Issue:  Some older boxes have the pneumatic actuator as an integral part of the box.
o    Solution:  This generally will drive toward the use of a new box that is designed to have a shaft-mounted actuator.

• Binding Dampers, Broken Fans and Heaters:

[an error occurred while processing this directive] When approaching controls retrofits for existing VAV boxes, you really need to go into it with a systems perspective and have a plan to repair, or replace boxes as part of any controls upgrade.  The end result is well worthwhile – a working system that is easily supported and maintained.

Controls retrofit alternatives.  There are several products on the market that provide for a less expensive retrofit from pneumatic to DDC. These are typically a replacement pneumatic thermostat that has some wireless capability to allow for remote monitoring and setpoint adjustments.  While this is an attractive solution and provides many of the benefits of a full DDC retrofit, be aware that it does not allow for monitoring and control of the box volume and that the pneumatic volume regulator still remains in place above the ceiling.

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