5808011-JIM-A-0320
42 Johnson Controls Ducted Systems
Dry bulb
The control determines the type of free cooling changeover
based on which sensors are present and reliable. Conditions
include the following items:
• Return and operational outdoor air temperature = dry bulb
changeover
• If either the return or outside air dry bulb value is
unreliable, free cooling is not available.
Changeover methods
Dry bulb changeover
This section applies when the free cooling current mode is dry
bulb.
For dry bulb economizer operation, the outside air is suitable for
free cooling if the operational outdoor 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 operational outdoor air
temperature rises above either the economizer OAT enable
setpoint or the return air temperature.
Single enthalpy changeover
This section applies when the free cooling current mode is
single enthalpy.
For single enthalpy economizer operation, the outdoor air is
suitable for free cooling if the outdoor air enthalpy is at least 1
BTU/lb below the economizer outdoor air enthalpy setpoint and
the operational outdoor air temperature is no greater than the
RAT plus 9°F.
Free cooling is no longer available if the operational outdoor air
temperature rises above the RAT plus 10°F or the outdoor air
enthalpy rises above the economizer outside air enthalpy
setpoint.
Dual enthalpy changeover
This section applies when free cooling current mode is dual
enthalpy.
For dual enthalpy economizer operation, the outdoor air
enthalpy must be lower than the return air enthalpy by 1 BTU/lb
and the operational outdoor air temperature is no greater than
the RAT plus 9°F.
Free cooling is no longer available if the operational outdoor air
temperature rises above the RAT plus 10°F or the outdoor air
enthalpy rises above the return air enthalpy setpoint.
Cooling stages set to three for a two compressor unit
With a stage 1 cooling demand (Y1 input), the economizer
modulates to get the SAT to the upper SAT setpoint +/-5°F.
With a stage 2 cooling demand (Y2 input), the economizer
opens to 100% and the first compressor output energizes.
With a stage 3 cooling demand (Y3 input), a second
compressor output energizes.
When each cooling demand is removed, the compressor
outputs de-energize in reverse order without time delays. When
only a Y1 input remains, the economizer controls the SAT to the
upper SAT setpoint +/- 5°F.
Note: If the SAT limit for cooling enabled is turned on, the 20
minute timer reapplies when appropriate to re-energize
the compressor output.
Cooling stages set to two or more for multiple compressor
units
With a stage 1 cooling demand (Y1 input) the economizer
modulates to get the SAT to the upper SAT setpoint
+/- 0.5°F.
With a stage 2 cooling demand (Y2 input), the economizer
modulates to get the SAT to the lower SAT setpoint
+/-.0.5°F.
If the stage 2 cooling demand (Y2 input) remains on, the
economizer remains at 100% for 5 minutes and the SAT is
greater than the lower SAT setpoint + 5°F, the compressor
output energizes.
If the economizer position remains at 100% for another 5
minutes, the next available compressor turns on. This process
repeats every 5 minutes until all the compressors energize.
If the economizer position drops below 100% and does not
reach the minimum position then returns to 100% and remains
at 100% for 5 minutes, the next available compressor
energizes. If the economizer position remains at 100%, the
process repeats every 5 minutes until all the compressors
energize.
Any time the economizer remains at the minimum position for 5
consecutive minutes, the last energized compressor turns off. If
it remains at the minimum position, the compressors de-
energize every 5 minutes until all the compressor are off.
Y3 and Y4 inputs have no additional impact on economizer
operation.
Sensor
VAV unit sensor option A
The operating VAV SAT setpoint is determined by the reset
function not by the number of compressors operating.
When free cooling available and the SAT is above the operating
VAV SAT setpoint, the dampers modulate to control the
operating, upper or lower, SAT setpoint +/-0.5°F.
If the economizer output is at 100% for 5 consecutive minutes
and the operating space temperature is 0.6°F or greater than
the operating cooling setpoint, the control starts to energize
compressors. See for additional information.
As soon as the staged percent command begins to increase,
the economizer remains at 100%. If the SAT drops to less than
the operating VAV SAT setpoint +1.8F, the staged percent
command holds the current value.
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