Orion System Zoning - Manual (Page 5)

Orion Systems 5
Zoning Design Guide
General
Even though there are some similarities between zone
control systems and Variable Air Volume (VAV) sys-
tems, there are some major differences. In many cases
systems will be called VAV when in fact they are really
a zoning system or are referred to as a zoning system
when they are really a VAV system. Always make sure
that you do not try to adapt a zoning system to a VAV
design system. Understanding the differences will help
you to prevent misapplication of the Orion zoning sys-
tem. In the paragraphs that follow we will try to ex-
plain the differences, advantages and disadvantages of
each and explain their operation.
VAV Systems
These systems consist of an HVAC unit that is gener-
ally a cooling only unit and VAV terminal units located
in the downstream ductwork that are used to control
the amount of constant temperature air delivered to the
various building zones. Sometimes the HVAC unit may
have gas or electric heat, but it is typically sized and
applied for morning warm-up purposes. The HVAC unit
is designed to vary the volume of air that is supplied to
the duct system by using either inlet vanes or an elec-
tronic variable frequency drive. These devices modu-
late to control the air flow through the supply fan in
response to the static pressure in the duct system. VAV
systems typically use high velocity VAV terminal units
to distribute the air to the zones. As the various VAV
terminal units in the different zones open and close to
supply the constant temperature air to the spaces, the
HVAC unit varies the volume of constant temperature
air based on the static pressure in the ductwork. The
HVAC unit is designed to maintain a constant cold sup-
ply air temperature regardless of the air flow volume in
the system. The HVAC unit cycles it’s cooling stages
to maintain a constant predetermined supply air tem-
perature. It typically runs continuously based on a sched-
ule.
For perimeter zones requiring heat, reheat coils (elec-
tric or hot water) located in the terminal units are used
to supply heated air to the space. Many times fan pow-
ered terminal boxes are used and most of them incor-
porate electric or hot water heating coils to provide pe-
rimeter zone heating. In summary a true VAV system
uses a variable volume fan supplying constant tempera-
ture air to the system with variable volume terminal
units used to control the volume of constant tempera-
ture air delivered to the space. Generally these systems
use pressure independent damper control.
Zoning Systems Versus True VAV Systems
Orion Zoning Systems
The Orion zoning system is quite different in operation
and design from the VAV system previously discussed.
Air volume control of the zoning system can either be
achieved by utilizing a VFD drive to modulate the unit
fan speed or achieved by bypassing air from the HVAC
unit supply duct back into the HVAC unit return air
duct on the unit inlet. The supply fan VFD or the by-
pass air damper is controlled and modulated based on
the static pressure value sensed by a static pressure sen-
sor located in the supply air duct downstream of the
unit supply air discharge. The supply fan VFD modu-
lates the fan speed or the bypass damper modulates open
and closed based on the static pressure in the duct. The
temperature at the HVAC unit discharge varies in rela-
tion to the demand from the zones.
HVAC units used for the zoning system will typically
have both heating and cooling capabilities. The unit sup-
plies a variable volume of cold or hot air to the duct
system which is fed to the individual zones by modu-
lating zone dampers. Each zone controller relays its heat-
ing or cooling demand to the Polling Device installed
on it’s local communications loop. The Polling Device
determines the HVAC unit mode of operation (heating,
cooling or vent mode) depending on the demand from
the zone controllers and relays this to the HVAC unit
controller. The Polling Device utilizes a voting system
to determine the correct mode of operation. Each zone
controller determines (based on its heating and cooling
setpoints) whether or not to use the air being supplied
by the HVAC unit. For example, one of the zones is
calling for cooling when the temperature in the duct is
above the zones cooling setpoint. This zone will move
to its minimum cooling position to prevent warm air
being introduced into the space. With the zoning sys-
tem the zone dampers are generally pressure dependent.
Pressure independent operation is available but is not
very common. Reheat and/or fan powered terminal units
can be used but aren’t commonly part of the typical
zoning system.
Conclusion
Typically a VAV, HVAC unit and associated controls is
more expensive than a CAV (Constant Volume) unit
utilizing zoning system control, especially on smaller
HVAC units. Many times the system can be redesigned
to a zoning system with a significant cost savings and
equal or better performance and comfort than a VAV
system would provide. Orion Systems allow you the
option of which one is best for your application. Or if
desired, VAV and zoned HVAC systems can also be
mixed and matched on the same control system.