Innovations in Comfort, Efficiency, and Safety Solutions.
Upgrading VAV Boxes
& Angela Lewis
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:
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:
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.
Some older boxes have the pneumatic actuator as an integral part of the
o Solution: This generally will drive toward the use of a new box that is designed to have a shaft-mounted actuator.
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
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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