40
Staging of compressors is shown in Tables 38-45.
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 dis-
charge temperature (EDT) falls below the alert limit (Configu-
ration
ALLM
SA.L.O) cooling will be inhibited. There is a
20-minute hold off on starting cooling again once the follow-
ing 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.
Cooling Control and the Economizer
TRIM For SUMZ = NO
ECON
E.TRM = NO
Cooling control will first check for the availability of the econ-
omizer. 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 setpoint high (SA.HI) or supply air
setpoint 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 economizer is
active, then the economizer will attempt to control to 53°F.
Also If HVAC mode = LOW COOL, the second stage of me-
chanical cooling will be locked out.
If the setpoint cannot be satisfied or the economizer is not ac-
tive, 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 compressor 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 “ECONOMIZER INTEGRATION WITH MECHANI-
CAL COOLING” on page 50 for more information on the
holding off of mechanical cooling as well as the economizer
control point.
Table 38 — Capacity Control Staging Options —
48/50A020-027 Units VAV and Adaptive CV/SAV Staging Sequence with Variable Capacity Compressor
*On units with optional digital scroll compressor, compressor B1 modu-
lates from minimum to maximum capacity to provide increased stages.
Table 39 — 48/50A030-060 Units VAV and Adaptive CV/SAV Staging Sequence with
Variable Capacity Compressor
*With minimum load valve ON.
STAGE
0123
COMP Compressor Status
A1 OFF OFF ON ON
A2 OFF OFF OFF ON
B1* OFFONONON
UNIT Unit Capacity 48/50A
020 0% 20 to 40% 50 to 70% 80 to 100%
025 0% 17 to 33% 50 to 66% 83 to 100%
027 0% 17 to 33% 50 to 66% 83 to 100%
STAGE
SEQUENCE 1
01234
COMP Compressor Status
A1* OFFONONONON
A2 OFF OFF ON ON ON
B1 OFF OFF OFF ON ON
B2 OFF OFF OFF OFF ON
UNIT Unit Capacity 48/50A
030 0% 12.5% to 25% 37.5% to 50% 62.5% to 75% 87.5% to 100%
035 0% 9.8% to 19.6% 29.4% to 29.4% 59.8% to 69.6% 90.2% to 100%
040 0% 12.5% to 25% 37.5% to 50% 62.5% to 75% 87.5% to 100%
050 0% 12.5% to 25% 37.5% to 50% 62.5% to 75% 87.5% to 100%
060 0% 12.5% to 25% 37.5% to 50% 62.5% to 75% 87.5% to 100%
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