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43
then Lo Cool Mode mechanical stages = 2 and Hi Cool
Mode mechanical stages = 2.
5. If the economizer position is greater than Configuration
ECON
EC.MX – 5% for more than 8 minutes but
less than 11.5 minutes and mechanical cooling has started
then:
Lo Cool Mode mechanical stages = 2
Hi Cool Mode mechanical stages = 3
6. If the economizer position is greater than Configuration
ECON
EC.MX – 5% for more than 11.5 minutes and
mechanical cooling has started then:
Lo Cool Mode mechanical stages = 2
Hi Cool Mode mechanical stages = 3 (for 020 to 027
units only)
Hi Cool Mode mechanical stages = 4 (for 030 to 060
units only)
NOTE: If some compressors are not available due to being
faulted, the Hi Cool Mode number of compressors are affected
before the Lo Cool Mode number of compressors. For exam-
ple, if a 4 compressor unit has one compressor faulted, and the
economizer is not active, then an HVAC mode Hi Cool
requested number of compressors is changed from 4 to 3. If
another compressor faults, then both Lo Cool and Hi Cool
requested number of compressors are set to 2. In addition,
compressors cannot be brought on faster than one every
30 seconds. If the control needs to bring on 2 compressors at
once, the first compressor will come on followed by the second
compressor 30 seconds later.
Staging of compressors is shown in Tables 49-60.
EDT Low Override — There is an override if EDT drops too
low based on an alert limit that will lock out cooling. If the
supply air/evaporator discharge temperature (EDT) falls below
the alert limit (Configuration
ALLM
SA.L.O) cooling will
be inhibited. There is a 20-minute hold off on starting cooling
again once the following statement is true: EDT minus (Run
Status
COOL
SUMZ
ADD.R) has risen above SA.L.O.
The variable ADD.R is one of the SumZ cooling algorithm
control variables dedicated mainly for multi-stage control.
2-Stage Control and the Economizer — The 2-stage logic will
first check for the availability of the economizer. If free cooling
can be used, then the control will first attempt to use the free
cooling.
If no mechanical cooling is active, and the economizer is
active, the economizer will first attempt to control to a cooling
control point of either the supply air set point high (SA.HI) or
supply air set point low (SA.LO) plus any reset applied,
depending on whether High Cool or Low Cool mode is in ef-
fect, respectively.
If one stage of mechanical cooling is on, and the economiz-
er is active, then the economizer will attempt to control to 53 F.
Also If HVAC mode = LOW COOL, the second stage of
mechanical cooling will be locked out.
If the set point cannot be satisfied or the economizer is not
active, then cooling will be brought on one stage at a time
when the evaporator discharge temperature (EDT) is greater
the 1.5° F above the current cooling control point. A start-up
time delay of 10 minutes and steady state delay after a com-
pressor is energized of 5 minutes is enforced.
If both circuits of mechanical cooling are running, then the
economizer will attempt to control to 48 F. If the economizer is
active and the outside-air temperature (OAT) is less than the
cooling control point + 0.5 F, the compressors will be locked
off. When mechanical cooling is on, the control may also use
the economizer to trim the leaving-air temperature to prevent
unnecessary cycles of the compressor stages.
See the Economizer Integration with Mechanical Cooling
section on page 50 for more information on the holding off of
mechanical cooling as well as the economizer control point.
Table 49 — 2-Stage Sequence —
48/50AJ,AW020-027
Table 50 — 2-Stage Sequence —
48/50AJ,AW030-060
Table 51 — Staging Sequence without Hot Gas Bypass —
48/50AK,AY020-027 and Multi-Stage 48/50AJ,AW020-027
STAGE
SEQUENCE 1 SEQUENCE 2
01 201 2
Thermostat Inputs Thermostat Inputs
Y1 OPEN CLOSED CLOSED OPEN CLOSED CLOSED
Y2 OPEN OPEN CLOSED OPEN OPEN CLOSED
COMP Compressor Status Compressor Status
A1 OFF ON ON OFF OFF ON
A2 OFF OFF ON OFF ON ON
B1 OFF OFF ON OFF OFF ON
UNIT Unit Capacity Unit Capacity
020 0% 33% 100% 0% 33% 100%
025 0% 30% 100% 0% 30% 100%
027 0% 33% 100% 0% 33% 100%
STAGE
SEQUENCE 1 SEQUENCE 2
01 201 2
Thermostat Inputs Thermostat Inputs
Y1 OPEN CLOSED CLOSED OPEN CLOSED CLOSED
Y2 OPEN OPEN CLOSED OPEN OPEN CLOSED
COMP Compressor Status Compressor Status
A1 OFF ON ON OFF ON ON
A2 OFF ON ON OFF ON ON
B1 OFF OFF ON OFF OFF ON
B2 OFF OFF ON OFF OFF ON
UNIT Unit Capacity Unit Capacity
030 0% 45% 100% 0% 45% 100%
035,036 0% 48% 100% 0% 48% 100%
040,041 0% 43% 100% 0% 43% 100%
050 0% 45% 100% 0% 45% 100%
051,060 0% 50% 100% 0% 50% 100%
STAGE
SEQUENCE 1 SEQUENCE 2
0123 4 0123 4
COMP Compressor Status Compressor Status
A1 OFF ON ON OFF ON OFF OFF ON ON ON
A2 OFF OFF ON ON ON OFF ON ON OFF ON
B1 OFF OFF OFF ON ON OFF OFF OFF ON ON
UNIT Unit Capacity 48/50A Unit Capacity 48/50A
020 0% 33% 67% 67% 100% 0% 33% 67% 67% 100%
025 0% 30% 65% 70% 100% 0% 35% 65% 65% 100%
027 0% 33% 67% 67% 100% 0% 33% 67% 67% 100%