Options/Accessories; Economizer Sequences; Dry Bulb Changeover; Single Enthalpy Changeover - York ZF Series Installation Manual

5326601-XIM-A-0117
Each vent hood is installed by inserting the top flange of the
hood into the slotted opening in the access panel and securing
in place.
The products of combustion are discharged horizontally
through these two screened, hooded vent openings on the
upper gas heat access panel.
Vent Air
Outlet Hoods
Combustion Air
Intake Hood
Figure 18: Vent and Combustion Air Hood

Options/Accessories

Economizer/Motorized Outdoor Damper Rain Hood
The instruction for the optional economizer/motorized damper
rain hood can be found in the rain hood kit. Use these
instructions when field assembling an economizer rain hood
onto a unit. The outdoor and return air dampers, the damper
actuator, the damper linkage, the outdoor and return air divider
baffles, and all the control sensors are factory mounted as part
of the "Factory installed" economizer option.
Power Exhaust/Barometric Relief Damper and Rain Hood
The instructions for the power exhaust/barometric relief damper
and rain hood can be found in the rain hood kit. The exhaust
fan, all supporting brackets, angles, and the wiring are factory
installed as part of the power exhaust option.

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" 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
22
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
Slotted Openings in
Access Panel
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.
Gas Heat
Access Panels
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:

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).
• 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
Johnson Controls Unitary Products