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Johnson Controls Ducted Systems
Motorized outdoor damper
The motorized outdoor damper 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 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 economizer 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 power exhaust 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
For factory-installed options, 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. For field-installed
accessories, all parts necessary for the installation come in the
accessory kit.
Duct static pressure transducer
A 0-5 in. 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 in. 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 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, make sure that
there are 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).
Economizer sequences
Several functions can drive the economizer, including: minimum
position, free cooling, economizer loading, and minimum
outdoor air supply.
Economizer minimum position
The economizer minimum position is set during occupied mode
when outside air is not suitable for free cooling. The position of
the damper is set proportionally between the Economizer
Minimum Position and the Economizer Minimum Position Low
Speed Fan setpoints, in relationship to the VFD output
percentage. On a constant volume single speed supply fan
system both setpoints should be set to the same value.
Free cooling
Four types of free cooling options are available: dry bulb
changeover, single enthalpy, dual enthalpy changeover, and
Auto.
Dry bulb changeover
For dry bulb economizer operation, the outside air is suitable for
free cooling if the outside air temperature is 1°F below the
Economizer OAT Enable setpoint
and
1°F below the Return Air
Temperature.
Free cooling is no longer available if the outside air temperature
rises above
either
the Economizer OAT Enable setpoint
or
the
return air temperature.
Single enthalpy changeover
For single enthalpy economizer operation, the outside air is
suitable for free cooling if the outside air enthalpy is at least 1
BTU/lb below the Economizer Outside Air Enthalpy setpoint
and
the outside air temperature is no greater than the RAT plus
9°F.
If the outside air temperature rises above the RAT plus 10°F,
free cooling is no longer available. The outside air temperature
must drop to no greater than RAT plus 9°F to enter free cooling
again.
Free cooling is no longer available if the outside air enthalpy
rises above the Economizer Outside Air Enthalpy setpoint.
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.
