York ZD Series Installation Manual Download Page 36

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5188236-BIM-A-0316

Johnson Controls Unitary Products

Enthalpy Settings

When the OA temperature, enthalpy and dew point are below
the respective setpoints, the Outdoor Air can be used for
economizing. Fig. 29 shows the new single enthalpy
boundaries in the W7220. There are 5 boundaries (setpoints
ES1 through ES5), which are defined by dry bulb temperature,
enthalpy and dew point.

Refer to Table 13 for the ENTH CURVE setpoint values.

To use enthalpy the W7220 must have a C7400S Sylkbus
sensor for OA. The W7220 calculates the enthalpy and dew
point using the OA temperature and humidity input from the OA
sensor. When the OA temperature, OA humidity and OA dew
point are all below the selected boundary, the economizer sets
the economizing mode to YES, economizing is available.

When conditions are above the selected boundary, the
conditions are not good to economize and the mode is set to
NO.

Fig. 29 shows the 5 current boundaries. There is also a high
limit boundary for differential enthalpy. The high limit boundary
is ES1 when there are no stages of mechanical cooling
energized and HL when a compressor stage is energized.

Table 13 provides the values for each boundary limit.

Simplicity SE Economizer Sequences

Units with the Simplicity SE Control will have a factory option or
field kit with Johnson Controls Economizer.

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" set points, in relationship to the VFD output
percentage. On a constant volume single speed supply fan
system both set-points 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

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

Table 12: Single Enthalpy and Dual Enthalpy High Limit Curves.

Enthalpy

Curve

Temp.

Dry-Bulb (°F)

Temp.

Dewpoint (°F)