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102777-YIM-E-0206

Unitary Products Group

This is done in order to read and store the SAT temperature
differential (drop) due to turning on compressor #1, while
assuring that turning it on will not drop the SAT below com-
pressor #1 trip point (where the Excessive SAT control would
turn it off again). The same staging sequence is used for the
remaining compressors (see below). Note that the standard 5
minute delay before monitoring SAT after a compressor is
turned on, or off, applies here also.

Turning on compressors #2 through #4:

If the economizer controller is saturated High (i.e. the econo-
mizer is 100% open while one or more compressors are run-
ning 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

•

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 ”pro
actively”, by a temporary override of the active SAT
setpoint, 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 depen-
dent 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
checking against a stored, predicted SAT tempera-
ture 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 sys-
tems with zone thermostat, or zone sensor tempera-
ture control, when the zone temperature control is
satisfied. For the case of zone sensor, the compres-
sor destaging is described in section “CV Opera-
tion”, paragraph “Control Algorithm for Cooling with
the Sensor Option, with Economizer”. For the case
of zone thermostat, the compressor destaging is
described in section “CV Operation”, paragraph
“Thermostat Operation for Cooling with Y1 and Y2
inputs”, subparagraph “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 rel-
atively 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 compres-
sors.

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 temper-
ature (as modulated by the economizer dampers) in order to
change SAT and keep it at SAT setpoint when only compres-
sor #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 Con-
trol SAT” is ON

•

Only compressor #1 is running

The PI control algorithm in this case has a capability to auto-
matically 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 algo-

rithm changes to reverse acting. This way, the ”loading” of the
DX coil is correctly done with return, or outdoor air, as appro-
priate, 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 con-
trol state in cooling), to a fixed temperature deadband of 50

to 55