Optional Variable Air Volume (Vav) - York ZH078 Installation Manual

the assembly. Field installed Motorized Outdoor Damper
accessories include complete instructions for installation.
Economizer
The Economizer can be a factory installed option or a field
installed accessory. If factory installed, refer to the instructions
included with the outdoor air hood to complete the assembly.
Field installed Economizer accessories include complete
instructions for installation.
There are two Economizer options:
1. Down Flow application with barometric relief hood standard.
2. Horizontal Flow application that requires the purchase of a
barometric relief hood.
Power Exhaust
The Power Exhaust can be a factory installed option or a field
installed accessory. If factory installed, refer to the instructions
included with the outdoor air hood to complete the assembly.
Field installed Power Exhaust accessories include complete
instructions for installation.
The Power Exhaust factory installed option is for Down Flow
application only.
There are two field installed Power Exhaust accessories:
1. Down Flow application.
2. Horizontal Flow application that requires the purchase of a
barometric relief hood.
Rain Hood
All of the hood components, including the filters, the gasketing
and the hardware for assembling, are packaged and located
between the condenser coil section and the main unit cabinet, if
the unit has factory installed options. If field installed
accessories are being installed all parts necessary for the
installation comes in the accessory.

Optional Variable Air Volume (VAV)

A variable air volume (VAV) option using a variable frequency
drive (VFD) is available for applications requiring a constant
supply-duct static pressure. A differential pressure transducer is
used to monitor supply duct static pressure and return a speed
reference signal to the VFD to control the output of the indoor
blower motor.
Duct Static Pressure Transducer
A 0-5" WC pressure transducer, located in the control box
compartment, is used to sense static (gauge) pressure in the
supply air duct and convert this pressure measurement to a
proportional 0-5 VDC electrical output. Pressure-transmitting
plastic tubing (1/4" diameter) must be field supplied and
installed from the transducer to both the ductwork and to the
atmosphere. Connect the tubing from the 'HIGH' pressure tap of
Johnson Controls Ducted Systems
the transducer to a static pressure tap (field supplied) in the
supply duct located at a point where constant pressure is
expected. To prevent an unstable signal due to air turbulence,
there should be no obstructions, turns or VAV terminal boxes
up- or down-stream of the sensing tube location for at least a
distance of 6-10 times the duct diameter. Tubing must also be
run between the 'LOW' pressure tap of the transducer and
atmospheric pressure (outside of the unit).
Factory-installed VFD
Do not run plastic tubing in the supply or return air ducts
as air movement could cause erroneous pressure
measurements. If the tubing penetrates through the
bottom of the unit be sure openings are sealed to
prevent air and water leakage.
The factory-installed VFD is mounted in the blower access
compartment. The drive comes wired from the factory to include
both 3-phase power and control connections (run permit signal,
speed reference signal & fault signal). All required drive
parameters are pre-programmed at the factory, except in the
case of 208-volt applications, in which the parameter that
defines motor nameplate voltage must be changed to a value of
208.00 and the parameter that defines motor-rated current must
be changed to the appropriate value appearing on the motor's
nameplate. Refer to the enclosed drive material for instructions
on changing parameter settings.
For units also equipped with gas/electric heat, a terminal block
located in the unit's control box and connected to the "VAV
BOX" terminal, must be field wired to the building's VAV boxes
to ensure fully open dampers during heating operation.
Manual Bypass
An optional, factory-installed manual bypass switch available
with factory-installed VFD can be found in the Blower Motor
Access compartment and has the following three positions:
• DRIVE - routes power through the VFD for modulating
control of the indoor blower motor.
• LINE (or BYPASS) - routes power directly to the motor
which provides full-speed motor operation and complete
electrical isolation of the drive.
• TEST - routes power to the VFD but not to the motor to
allow for drive programming and/or diagnostics.
If a drive failure occurs, the unit does not automatically switch to
bypass mode. The LINE/DRIVE/TEST switch must be manually
switched to the LINE (BYPASS) position. If there is a call for the
fan, the indoor blower motor will run at full-speed while in the
bypass mode.
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