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SIZE 044 UNITS — These units have 2 compressors and 1
unloader on compressor no. 1. See Fig. 42 for compressor
and condenser fan motor locations. The unit operating se-
quence is as follows:
Stage 1 Relays K1 and K3 are energized. Compressor
no. 1 starts with the unloader energized. Com-
pressor no. 1 is running at
1
⁄
2
capacity. The crank-
case heater on compressor no. 1 has been deen-
ergized, and the first stage condenser fan has been
energized. Outdoor (condenser) fan motor no. 1
(OFM1) has started.
Stage 2 Relay K1 is energized. Compressor no. 1 is fully
loaded.
Stage 3 Relays K1, K3, and K5 are energized. Compres-
sor no. 1 is running at
1
⁄
2
capacity, and compres-
sor no. 2 is running at full capacity. The crankcase
heater for compressor no. 2 is deenergized. The
second stage condenser fan has been energized.
Both OFM1 and OFM3 are operating.
Stage 4 Relays K1 and K5 are energized. Both compres-
sors are running fully loaded.
Size 044 units have one fan that can be controlled by the
processor. The other 2 are controlled by the compressors.
The OFM1 is energized by compressor no. 1, and OFM3 is
energized by compressor no. 2. The OFM2 is cycled by the
processor based on input from either circuit (thermistors T3
and T4).
SIZE 104 UNITS — These units have 4 compressors and
1 unloader on compressor no. 1. See Fig. 43 for compressor
and condenser fan motor locations. The unit operating se-
quence is as follows:
Stage 1 Relays K1 and K3 are energized. Compressor
no. 1 starts with unloader energized. Compressor
no. 1 runs at
2
⁄
3
capacity. The crankcase heater for
this compressor has been deenergized, and first stage
of condenser fans has been energized. Outdoor (con-
denser) fan motor no. 1 (OFM1) and oudoor fan
motor no. 3 (OFM3) have started.
Stage 2 Relay K1 is energized. Compressor no. 1 is fully
loaded.
Stage 3 Relays K1, K3, and K5 are energized. Compres-
sor no. 1 runs at
2
⁄
3
capacity and compressor
no. 2 is running at full capacity. The crankcase
heater for compressor no. 2 is deenergized. The
first stae of condenser fans on circuit 2 has been
energized. Fans OFM1, OFM2, OFM3, and OFM4
are operating.
Stage 4 Relays K1 and K5 are energized. Both compres-
sors no. 1 and no. 2 are running fully loaded.
Stage 5 Relays K1, K3, K5, and K5 are energized. Com-
pressor no. 1 runs at
2
⁄
3
capacity and compressors
no. 2 and no. 4 are running at full capacity. The
crankcase heater on compressor no. 4 is deener-
gized. Fans OFM1, OFM2, OFM3, and OFM4 are
operating.
Stage 6 Relays K1, K5, and K6 are energized. Compres-
sors no. 1, no. 2, and no. 4 are running fully loaded.
Stage 7 Relays K1, K2, K3, K5, and K6 are energized.
Compressor no. 1 runs at
2
⁄
3
capacity and com-
pressors no. 2, no. 3, and no. 4 are running at full
capacity. Fans OFM1, OFM2, OFM3, and OFM4
are operating. Crankcase heater for compressor
no. 3 is deenergized.
Stage 8 Relays K1, K2, K5, and K6 are energized. Com-
pressors no. 1, no. 2, no. 3, and no. 4 are running
fully loaded.
On Size 104 units, the first 4 condenser fans energize with
the compressors; circuit no. 1 compressors control OFM1
and OFM3, and circuit no. 2 compressors control OFM2 and
OFM4. The OFM5 and OFM6 are staged by the micro-
processor based on condensing temperature input from
either circuit’s T3 or T4 thermistor.
Head Pressure Control — All units have as standard
a basic head pressure control function which allows the units
to operate in cooling down to 45 F. If cooling is required at
outdoor ambient temperatures lower than 45 F, refer to ac-
cessory head pressure control literature for details.
Head pressure control is handled by the processor. The
processor attempts to maintain the head pressure by cycling
the condenser-fan motors. No condenser fans will be run-
ning without a call for mechanical cooling. Thermistors T3
and T4 provide the condensing temperature information to
the processor. These VAV rooftop units have dual refrigera-
tion circuits, and the higher circuit temperature will govern
unit operation. If the condensing temperature is above
133 F (236 psig), a condenser fan stage will be added. If the
condensing temperature is 78 F (142 psig) or less, the num-
ber of condenser fans operating will be decreased.After each
fan stage, the processor will wait one minute for the head
pressures to stabilize before changing again, unless ther-
mistor T3 or T4 senses a temperature greater than 125 F
(278 psig), in which case all condenser fans are started.
During start-up, if the outdoor ambient is above 70 F (as
sensed by thermistor T3 or T4), the first-stage, processor-
controlled fans are turned on to prevent excessive discharge
pressures.
034 AND 038 UNITS 044 AND 048 UNITS
LEGEND
OFM — Outdoor (Condenser) Fan Motor
Fig. 42 — Component Arrangement, 034-048 Units
36