74
BOILER SEQUENCING
Sequencing is used in a multiple boiler (2-6) environment where a master boiler, during normal oper-
ation, is established to control the remaining slave boilers in order to achieve the desired setpoint as
set in the master boiler. Any boiler can be a master, and in a sequencing operation, only one boiler
can be the master at a time. In a sequencing operation, the slave boilers are allowed to come on line
and control their own firing rate to maintain a 'warm' state so they can be ready when called upon by
the master boiler. SENSOR 2 will be configured as STANDBY WATER and SETPOINT 2 will be
configured to use SENSOR 2 with the appropriate setpoints. The STANDBY WATER operation is
the mode of operation of the slave boiler when the master boiler is not demanding any help from the
slave to maintain the system pressure/temperature. This mode of operation is the default startup state
that the slaves will normally be in until they are commanded to be otherwise by the master boiler
control.
The master boiler controls the individual slave’s oper-
ation through an independent sequencing only com-
munications link. The independent communications
link is located on terminals P12.3 (SEQUENCING A)
and P12.4 (SEQUENCING B). Refer to the WIRING
CONNECTIONS section. A twisted shielded pair is
recommended such as Belden and each unit is daisy
chained to the next unit. That is, a cable is routed from
the first unit to the second unit (A connected to A | B
connected to B), from the second unit to the third unit
and from the third unit to the fourth unit. The drain
wires are connected to each other and at the first and
last unit the drain wire is connected to chassis or earth
ground.
Before the SEQUENCING sub menu is accessed the COMMUNICATION SETUP menu should be
accessed where each boiler is given a unique address. The address will be used to identify the master
and slaves. The baud rate for sequencing is unique and independent from the baud rate shown in the
COMMUNICATION SETUP sub menu.
A boiler is made a master through keypad setting, digital input or the modbus communication gate-
way. The system has to be instructed as to which method is selected through the SEQUENCING
SETUP menu. Below is the complete SEQUENCING sub menu with the assumption that 3 slaves
are available with addresses 3, 2 and 4 respectively.
<
S
E
Q
U
E
N
C
I
N
G
S
E
T
U
P
>
M
A
S
T
E
R
S
L
C
T
K
E
Y
P
A
D
<
S
L
A
V
E
S
A
V
A
I
L
A
B
L
E
3
1
S
T
S
L
A
V
E
O
N
3
2
N
D
S
L
A
V
E
O
N
2
3
R
D
S
L
A
V
E
O
N
4
S
L
A
V
E
O
N
R
A
T
E
8
0
%
S
L
A
V
E
3
O
N
D
L
Y
2
m
S
L
A
V
E
2
O
N
D
L
Y
3
m
S
L
A
V
E
4
O
N
D
L
Y
6
m
S
L
A
V
E
O
F
F
R
A
T
E
3
0
%
S
L
A
V
E
O
F
F
D
E
L
A
Y
2
m
P12
1. mbus A
2. mbus B
3. seq A
4. seq B
5. servo A
6. servo B
7. dis TX, ye
8. dis TY, bl
9. dis Rb, br
10. dis Ra, or
11. O2 A
12. O2 B
P12
}
