Specifications
36
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 effect,
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 for more information on the holding off of mechanical
cooling as well as the economizer control point.
Table 37 — 2-Stage Sequence —
48/50AJ,AW020-027
*Compressor start is delay 10 seconds from other compressor.
Table 38 — 2-Stage Sequence —
48/50AJ,AW030-060
*Compressor start is delay 10 seconds from other compressor.
Table 39 — Staging Sequence Without Hot Gas Bypass —
48/50AK,AY020-027 and Multi-Stage 48/50AJ,AW020-027
Table 40 — Staging Sequence With Hot Gas Bypass —
48/50AK,AY020-027 and Multi-Stage 48/50AJ,AW020-027
*With Minimum Load Valve ON.
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 Capacity Unit Capacity
20 0% 67% 100% 0% 67% 100%
25 0% 65% 100% 0% 65% 100%
27 0% 67% 100% 0% 67% 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 Capacity Unit Capacity
30 0% 45% 100% 0% 45% 100%
35 0% 48% 100% 0% 48% 100%
40 0% 43% 100% 0% 43% 100%
50 0% 45% 100% 0% 45% 100%
60 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 Capacity 48/50A Unit Capacity 48/50A
20 0% 33% 67% 67% 100% 0% 33% 67% 67% 100%
25 0% 30% 65% 70% 100% 0% 35% 65% 65% 100%
27 0% 33% 67% 67% 100% 0% 33% 67% 67% 100%
STAGE
SEQUENCE 1 SEQUENCE 2
01234 5 01234 5
COMP Compressor Status Compressor Status
A1 OFF ON* ON ON OFF ON OFF OFF OFF ON ON ON
A2 OFF OFF OFF ON ON ON OFF ON* ON ON OFF ON
B1 OFF OFF OFF OFF ON ON OFF OFF OFF OFF ON ON
Unit Capacity 48/50A Unit Capacity 48/50A
20 0% 18% 33% 67% 67% 100% 0% 18% 33% 67% 67% 100%
25 0% 17% 30% 65% 70% 100% 0% 22% 35% 65% 65% 100%
27 0% 21% 33% 67% 67% 100% 0% 21% 33% 67% 67% 100%