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 (RAT).
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 the following:
• 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 is always 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.
If the supply air temperature cannot be maintained
within 5°F of the economizer setpoint, the first
stage compressor (C1) is turned on. The second
stage compressor (C2) is added as needed to keep
the supply air temperature within the 5°F of the
economizer setpoint.
Sensor
In free cooling, with a demand from the zone/return
sensor for the first stage of cooling, the dampers
modulate to control the supply air temperature to
the economizer setpoint +/- 1°F.
If the economizer output is at 100% and the SAT
is greater than the economizer setpoint + 1°F,
the control starts a 12-minute timer to energize a
compressor output.
If at any time the economizer output drops
below 100%, the timer stops and resets when the
economizer output returns to 100%.
When a compressor output is turned ON, the
economizer dampers continue to modulate
to control the supply air temperature to the
economizer setpoint.
At no time is a compressor output turned ON if the
economizer output is less than 100%, even if the
differential between zone (or return) temperature
and the current cooling setpoint is great enough to
demand more than one stage of cooling.
If the economizer output goes to minimum position
and the SAT is less than economizer setpoint -1°F,
the control starts a 12-minute timer to de-energize a
compressor output.
If at any time the economizer output goes above
the minimum position, the timer stops and resets
when the economizer output returns to minimum
position.
If the demand for cooling from the space/return
is satisfied, the economizer output modulates to
minimum position and the compressor outputs are
de-energized as long as their minimum run timers
have expired.
Installation Manual: YORK
®
Sun
™
Select YV28 to YV50
72
Johnson Controls Ducted Systems
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