28
Power Exhaust —
To enable power exhaust, PE.EN
must be set to YES. If power exhaust is enabled, Power
Exhaust 1 will turn on when the economizer position is greater
than PE1.P. If power exhaust is enabled, Power Exhaust 2 will
turn on when the economizer position is greater than PE2.P.
There are small time delays to ensure that rapid cycling does
not occur.
Compressor Staging —
The electronic controls deter-
mine the number of cooling stages needed at any given time
as specified in the Cooling section. However, which specific
cooling stage operates at any given time depends on compres-
sor staging order and compressor status.
COMPRESSOR STAGING ORDER — The 48/50HG016-024
units have three stages of cooling (Circuits A, B and C).
The 48/50HG014 and 028 units have two stages of cooling (Cir-
cuits A and B). The position of the evaporators for Circuits A, B
and C are bottom, middle and top (see Fig. 4) for a 3-circuit unit.
The position of the evaporators is bottom and top for Circuits A
and B on a 2-circuit unit. Depending on conditions listed below,
circuits will either stage from the bottom evaporator to the top
evaporator (BtoT) or from the top evaporator to the bottom
evaporator (TtoB). When the unit is staging TtoB, the unit is
stated to be inverted.
If inverting is not allowed as determined by the configura-
tion (INV.E = NO), the unit will always stage BtoT. However,
if INV.E = YES, then the unit will invert if the outdoor temper-
ature as measured by OAT is greater than 80 F for 15 minutes
continuously. Inverting will continue until either INV.E is set to
NO or the OAT drops below 75 F for 1 minute.
COMPRESSOR STATUS — Having determined whether
staging will occur as either BtoT or TtoB, the unit controls will
determine the final order of compressor staging. If all circuits
are functioning properly, the compressor staging will always
follow either BtoT or TtoB. Therefore if the staging method is
BtoT, then Circuit A must turn On before Circuit B and
Circuit B before Circuit C. Likewise when removing stages,
the stages must be removed in the opposite order. In all cases,
the minimum on-time of 3 minutes (MRT.C) and the minimum
off-time of 5 minutes (MOT.C) must be honored. Therefore, if
the staging order is BtoT and Circuit A was shut down two
minutes previously, no circuits will turn on until the minimum
off-time for Circuit A has expired; no matter how many circuits
are requested.
The rules are different if a circuit has shutdown because of
an alarm such as low refrigerant pressure. When a circuit is un-
available because it is not functioning properly, it will be re-
placed immediately by another circuit. Therefore, if Circuit A
and B are operating because two circuits have been requested,
the staging is BtoT, and then Circuit A shuts down because of
low refrigerant pressure, the control will turn off Circuit A and
replace it with Circuit C. Later if Circuit A is allowed to restart
(see Alarm Handling on page 31) and BtoT is still active with
two requested stages, Circuit A will attempt to restart. If Circuit
A has run for 1 minute without sensing any problems, it will be
deemed acceptable, causing Circuit C to shutdown. Circuit C
will shut down because only two circuits are needed and
Circuit A and B are functioning properly.
Heating (48HG Units)
NOTE: THE 48HG units have 2 stages of heat.
For 48HG units, the heat type will automatically be set to 1
for gas units (HT.TY). Heat will not operate if the outdoor tem-
perature is greater than the heat lockout temp (HT.LO).
THERMOSTAT CONTROL — When the thermostat calls
for heating, the MBB senses that W1 is On and closes the
HTR.1 relay. When the relay is closed, it sends power to W on
the IGC (integrated gas unit controller) board. An LED (light-
emitting diode) on the IGC board will be on during normal
operation. A check is made to ensure that the rollout switch and
limit switch are closed. The induced-draft motor is then
energized. When speed is proven with the Hall Effect sensor on
the motor, the ignition activation period begins.
100
500
700
1100
INSIDE/OUTSIDE CO
2
DIFFERENTIAL
INSIDE CO
2
CONCENTRATION
AQ
DIFFERENTIAL
LOW (AQD.L)
AQ
DIFFERENTIAL
HIGH (AQD.H)
MINIMUM
IAQ
DAMPER
POSITION
(AQ.MP)
ECONOMIZER
MINIMUM
DAMPER
POSITION
(MIN. P)
INCREASING VENTILA
TION
VENTILATION FOR PEOPLE
VENTILATION FOR SOURCES
Fig. 12 — IAQ Control
202
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