Page 37
ZHA Sequence of Operation
Power:
1- Line voltage from unit disconnect energizes transform
er T1 and T18. T1 provides 24VAC power to terminal
strip TB1. TB1 provides 24VAC to the unit cooling,
heating and blower controls and thermostat. T18 pro
vides 24VAC to K1 and K2 relay coils and L1 and L2 re
versing valves.
Blower Operation:
2- Indoor thermostat terminal G energizes blower contac
tor K3 with 24VAC. N.O. K3 closes, energizing B3.
First Stage Cooling Demand (compressor B1)
3- First stage cooling demand energizes Y1 and G in the
thermostat. G energizes blower (see step 2)
4- Transformer T18 energizes reversing valves L1 and L2
via K58-1.
5- Y1 demand energizes K132 relay coil which closes
K132-1 N.O. contacts and routes 24VAC to S4 and S5
N.C. high pressure switches. Compressor contactor
K1 is energized.
6- K1 closes energizing compressor B1.
7- Y1 signal from CMC1 module energizes K10 relay coil.
K10-3 N.C. and K10-2 N.O. contacts close energizing
outdoor fan B4 and B5.
Second Stage Cooling Demand (compressor B2)
8- Second stage cooling demand energizes Y2.
9- Y2 demand energizes relay K133 relay coil which
closes K133-1 N.O. contacts. 24VAC is routed to S7
and S8 N.C. high pressure switches. Compressor con
tactor K2 is energized.
10- K2 closes energizing compressor B2.
First Stage Heat (compressors B1 and B2)
11- Heating demand energizes W1 in the thermostat.
12- W1 demand energizes K8 relay coil which closes K8-2
and K8-3 N.O. contacts and K132 and K133 coils.
24VAC is routed to K1 and K2 contactors
13- K1 and K2 close energizing compressor B1 and B2.
14- 24VAC from CMC1 module energizes K10 relay coil.
K10-3 N.O. contacts and K10-2 N.O. contacts close
energizing outdoor fans B4 and B5.
Second Stage Heat (electric heat):
15- Second stage heat demand energizes W2 in the ther
mostat.
16- See sequence of operation for electric heat.
Defrost Mode:
17- During heating operation, when outdoor coil drops to
35 ± 4° the defrost thermostat S6 or S9 closes initiating
defrost (after minimum run time of 30, 60 or 90 min
utes).
18- When defrost begins, the reversing valve L1 or L2 is
energized. Supplemental electric heat (W2) is ener
gized.
19- When L1 energizes, outdoor fan relay K10 and outdoor
fans B4 and B5 are de-energized.
20- Defrost terminates when the pressure switch for the cir
cuit S104 opens, or when 15 minutes has elapsed. The
defrost cycle is
not
terminated when thermostat de
mand ends.
