351579-YTS-B-0608
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Johnson Controls Unitary Products
Turning on compressors #2 through #4:
If the economizer controller is saturated High (i.e. the
economizer is 100% open while one or more compressors are
running and the control can no longer maintain the active SAT
setpoint requested by the zone control),
• Temporarily override SAT setpoint to increase it by 5 °F
• When the SAT is within 0.5 °F of this temporary setpoint,
turn on the next compressor
• Resume SAT control at active setpoint
NOTE:
The pre-staging SAT increase must be done “proac-
tively”, by a temporary override of the active SAT set-
point, rather than by allowing the SAT to “drift” up by 5 °F
out of control. The time it would take for the SAT to “drift”
up could be fairly long, mostly dependent on the outdoor
air temperature change and in the meantime, the zone
temperature may go out of control as the additional
needed cooling is not being delivered.
NOTE:
The standard 5 minute delay before monitoring SAT
after a compressor is turned on, or off, and the check-
ing against a stored, predicted SAT temperature drop,
applies here also.
Turning compressors off:
A highest numbered running compressor is turned off when the
economizer controller is saturated Low.
This method of turning compressor off is considered better than
using the Excessive SAT Control - turning a compressor OFF
only if SAT reaches its specified trip point. If that method was
used and the Excessive SAT Control was not selected, there
would be no means for turning compressors off.
NOTE:
The compressors also will be turned off in CV systems
with zone thermostat, or zone sensor temperature
control, when the zone temperature control is satis-
fied. For the case of zone sensor, the compressor
destaging is described in section “CV Operation”,
paragraph “Control Algorithm for Cooling with the Sen-
sor Option, with Economizer”. For the case of zone
thermostat, the compressor destaging is described in
section “CV Operation”, paragraph “Thermostat Oper-
ation for Cooling with Y1 and Y2 inputs”, subpara-
graph “Economizer Mode”.
Compressor control when exiting Cooling Lockout on OAT:
A situation may arise when in Economizer Mode and one or
more compressors are required in addition to full available free
cooling in order to maintain the SAT setpoint, but Cooling
Lockout on OAT prevents the compressors use. This situation
may arise when the SAT setpoint is set very close to, or even
below the temperature set for Cooling Lockout on OAT - a
relatively unusual case. If the OAT then increases above the
lockout setting while the call for several compressors exists, the
compressors will turn on with a delay between compressors.
Economizer Loading Option
This is a programmable option. The user has the ability to turn
this function off. It is automatically disabled if the unit does not
use an economizer. The on/off programming choice is common
to both cooling and heating. The default setting is ON. This
programmable “Economizer Loading” function is used only
outside the normal Economizer operation.
During the Economizer operation, the “Loading” function is
always used and is an integral part of the Economizer control
algorithm.
Economizer loading option in cooling:
In cooling, this function causes changes in mixed air
temperature (as modulated by the economizer dampers) in
order to change SAT and keep it at SAT setpoint when only
compressor #1 is running. This makes a trade-off between
energy and compressor cycling and minimizes cycling of
compressor #1. The loading is done by the same type of control
algorithm (PI) as used in the normal Economizer operation.
The algorithm will be activated to do this function in following
conditions:
• Economizer is “not suitable” (i.e. we are not in a normal
Economizer mode)
• The programmable option “Economizer Loading to Control
SAT” is ON
• Only compressor #1 is running
The PI control algorithm in this case has a capability to
automatically change from direct to reverse acting in response
to difference between OAT and RAT. When OAT is less than
RAT, the algorithm is direct acting, for OAT
&
RAT, the algorithm
changes to reverse acting. This way, the “loading” of the DX coil
is correctly done with return, or outdoor air, as appropriate, and
there is no need to activate this “loading” function only at higher
outdoor air temperatures (e.g. OAT > 60 °F).
The algorithm controls SAT to its specified setpoint (when
applicable, e.g. in VAV operation), or, in control modes where no
SAT setpoint is specified (such as in Excessive SAT control state
in cooling), to a fixed temperature deadband of 50 °F to 55 °F.
NOTE:
As opposed to the PI algorithm used in economizer
control, the PI algorithm used here for economizer
loading function does not need to utilize the High satu-
ration state for any additional control functions. There-
fore no complications arise when switching between
direct and reverse acting modes.
Economizer loading option in heating:
In heating, this function uses additional outside air (as
modulated by the economizer dampers) in order to decrease
SAT when only the first stage of heating is running and keep the
SAT below the programmed “Economizer Loading Setpoint in
Heating”. This prevents heating stage from cycling on its
internal temperature limit safety switch (which is typically set
about 10 °F
above the Economizer Loading Setpoint). A need
