5703827-XIM-A-0319
Johnson Controls Ducted Systems
45
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 is 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 compressors off in free cooling parameter 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 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 se/- 1°F.
If the economizer output is at 100%
and
the SAT is greater than the economizer se 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 does a compressor output turn 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.
