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BOILER CONTROL: INTERNAL WIRING & OPERATION
• With all the boilers operating, the system will
modulate the gas input to each of the boilers
to follow the system load. The control uses the
Stop Delay Time and the Stop Boiler
Differential values to determine when to drop
a boiler from the sequence.
C A S C A D E
S t o p
D e l a y
Figure 8.56: Cascade Stop Delay Time
Figure 8.57: Cascade Stop Boiler Differential
Table 8.12: Start Delay Time Range
Parameter
Minimum
Stop Delay Time
1 minute
Stop Boiler Differential
If the tank temperature exceeds the Tank
Setpoint + the Stop Boiler Differential for
longer than the Stop Delay Time, it will stop
one of the boilers.
• The control uses the Tank Setpoint + the
Offset Temperature as set in the User menu to
provide a Boiler Supply Setpoint. It will then
offset this setpoint up or down to meet the
tank temperature. The Calculated Setpoint
Max Offset Up (Figure 8.58) and Calculated
Setpoint Max Offset Down (Figure 8.59) are
used to limit this offset. Increasing either of
these parameters may cause the system to
react more quickly, but may result in frequent
cycling. It may be necessary to reduce the Max
Offset Up to prevent a lockout on boiler limit.
C A S C A D E
C a l c u l a t e d
M a x
o f f s e t
Figure 8.58: Calculated Setpoint Max Offset Up
C A S C A D E
C a l c u l a t e d
M a x
o f f s e t
Figure 8.59: Calculated Setpoint Max Offset Down
42
T i m e :
1 m i n ▼
Default
Maximum
1 minute
15 minutes
1°F
18°F
45°F
S e t p o i n t
u p :
1 8 ° F ▼
S e t p o i n t
d o w n :
9 ° F ▼
Table 8.13 shows the range of values for these
parameters.
Table 8.13: Calculated Setpoint Max Offset
Up/Down
Parameter
Calculated Setpoint
▲
Max Offset Up
Calculated Setpoint
Max Offset Down
• The control also uses the Next Boiler Stop
Rate to determine when to shed a boiler from
the cascade system. Figure 8.60 and Table
8.14 show the screen and the range of values.
C A S C A D E
N e x t
B o i l e r
S t o p
r a t e :
Figure 8.60: Next Boiler Stop Rate
Table 8.14: Next Boiler Stop Rate
Parameter
Next Boiler Stop Rate
• The Cascade Rotation Interval (Figure 8.61)
will change the frequency at which the control
will change the lead boiler in the system.
C A S C A D E
R o t a t i o n
Figure 8.61: Rotation Interval
• Table 8.15 shows how the Rotation Interval
affects the boiler rotation sequence.
Table 8.15: Rotation Interval (4 Boilers)
Days of
1 Day
Operation
▲
1
1-2-3-4
2
2-3-4-1
3
3-4-1-2
4
4-1-2-3
5
1-2-3-4
6
2-3-4-1
▲
7
3-4-1-2
8
4-1-2-3
9
1-2-3-4
10
2-3-4-1
11
3-4-1-2
12
4-1-2-3
Minimum
Default
Maximum
0°F
18°F
0°F
9°F
Minimum
Default
Maximum
5%
9%
I n t e r v a l
5 D a y s ▼
3 Days
5 days
1-2-3-4
1-2-3-4
1-2-3-4
1-2-3-4
1-2-3-4
1-2-3-4
2-3-4-1
1-2-3-4
2-3-4-1
1-2-3-4
2-3-4-1
2-3-4-1
3-4-1-2
2-3-4-1
3-4-1-2
2-3-4-1
3-4-1-2
2-3-4-1
4-1-2-3
2-3-4-1
4-1-2-3
3-4-1-2
4-1-2-3
3-4-1-2
36°F
36°F
▲
9 % ▼
40%
▲
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