Economizer Sequences; Dry Bulb Changeover; Single Enthalpy Changeover; Dual Enthalpy Changeover - York ZR Series Installation Manual

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, end return horizontal applications that include a
fresh air hood and exhaust hood with barometric relief.
2. Horizontal flow application (field-installed kit only) that
requires the purchase of a barometric relief hood.
Note: For the down flow, end return horizontal application,
you must keep the two side panels for the economizer
hood tops (See Figure 20).
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.

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.

Johnson Controls Ducted Systems

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.

Dual enthalpy changeover

For dual enthalpy economizer operation, the outside air
enthalpy must be lower than the return air enthalpy by 1 btu/lb
AND the outside air temperature is no greater than the RAT
plus 9°F.
Auto
The control determines the type of free cooling changeover
based on which sensors are present and reliable. Conditions
include:
• Return and outside air dry bulb = dry bulb changeover
• Return and outside air dry bulb and outside air humidity =
single enthalpy
• Return and outside air dry bulb and return and outside air
humidity = dual enthalpy
• If either the return or outside air dry bulb sensors are
unreliable, free cooling is not available

Free cooling operation

When the control determines that the outside air is suitable, the
first stage of cooling will always be free cooling.
Thermostat
In free cooling, with a thermostat input to Y1, the dampers
modulate to control the supply air temperature to the
Economizer setpoint +/- 1°F (default 55°F).
If the thermostat provides an input to Y2 and the parameter
Compressors Off in Free Cooling is turned OFF a compressor
output energizes. The economizer dampers continue to
modulate to control the supply air temperature to the
Economizer setpoint.
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