5567611-BIM-B-1218
Johnson Controls Ducted Systems
41
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.
CV option A thermostat sequence
When the control determines that the outdoor air is suitable, the
first stage of cooling is always free cooling.
If the parameter all compressors off in free cooling is on, free
cooling is used regardless of the number of cooling stages
demanded.
Cooling stages set to one for single compressor unit
With a stage 1 cooling demand (Y1 input), the economizer
modulates to get SAT to VAV cooling supply air temperature
upper se/-5°F. If the Y1 input remains on for 20 minutes,
the C1 output energizes and the economizer opens to 100%.
With stage 2 cooling demand (Y2 input) and the Y1 input has
been present less than 20 minutes, then the economizer opens
to 100%.
If the Y1 or Y2 has been present for more than 20 minutes, then
the C1 output energizes.
Cooling stages set to two for a two compressor unit
With a stage 1 cooling demand (Y1 input), the economizer
modulates to get the SAT to the VAV cooling supply air
temperature upper se/-5°F.
With a stage 2 cooling demand (Y2 input), the economizer
modulates to 100% and energizes one compressor output.
After 20 minutes, the second compressor output energizes.
When the Y2 stage 2 cooling demand is removed, all
compressor outputs are de-energized and the economizer
modulates to get the SAT to the upper SAT se/-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 four for a four compressor unit
With a stage 1 cooling demand (Y1 input), the economizer
modulates to get the SAT to the upper SAT se/-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.
With a stage 4 cooling demand (Y4 input), a third compressor
output energizes and 20 minutes after the Y4 input the fourth
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 se/- 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.
CV option B thermostat sequence
Cooling stages set to one for single compressor unit
With a stage 1 cooling demand (Y1 input), the economizer
modulates to get the SAT to the upper SAT se/-5°F.
If the stage 1 cooling demand (Y1 input) remains on for 20
minutes, the economizer remains at 100% for an additional 5
