56
b. Before starting the Lead boiler, the Master control
will first check to be sure that the System
temperature is lower than the System Setpoint +
Stop Boiler Differential
.
c. After a Trial For Ignition (TFI), the lead boiler will
modulate its input rate to meet the heat demand.
d. Any time that a boiler is started the Cascade Start
Delay Time will be initiated and no boiler will be
allowed to start before this time elapses. This is to
allow time for the system to stabilize before
bringing on an additional boiler.
e. After the delay time has elapsed, one of the
following conditions must be met before starting
the next boiler:
• The System Temperature must be lower than
the Set Point minus the Cascade Start Boiler
Differential temperature, or
• The input rate of all boilers operating must be
higher than the Next Boiler Start Rate
.
f. After operation of the second boiler is initiated,
the Cascade Start Delay Time
must elapse before
bringing on additional boilers. Again, in order to
initiate operation of additional boilers one of the
same conditions must be met.
g. When the boiler system approaches its Setpoint,
the boilers will reduce input rates at approximately
equal values. When the input rates of all of the
operating boilers fall below the Next Boiler Stop
Rate, the master control will shut down the last
boiler that started after the Cascade Stop Delay
Time has elapsed.
h. The Calculated Setpoint Max Offset Up
and
Calculated Setpoint Max Offset Down
are applied
to individual boilers in multiple boiler cascade
operation.
• These temperature offset values are used to
change the response of individual boilers to
the system setpoint.
• Increasing these values will cause the system
to react more quickly, but may result in
frequent cycling.
• Decreasing these values will cause the system
to react more slowly to achieve the setpoint.
• By increasing the Calculated Setpoint Max
Offset Up value, the Master boiler will offset
individual boiler setpoints temperature by a
larger amount in order to achieve the system
setpoint.
• By increasing the Calculated Setpoint Max
Offset Down value, the boilers will decrease
the boiler setpoints by a larger amount to
achieve the system setpoint.
i. The Rotation Interval
is the frequency at which the
Master Boiler will change the Lead boiler in the
sequence.
• For example, if the Rotation Interval
is set
to 4 days and there are 4 boilers operating
together, the following chart shows the
operating sequence over the next 25 days.
6. Multiple Boilers – Boost & Warm Weather
Shutdown (WWSD):
a. To use the Boost function with multiple boilers, the
parameters on the master boiler are used. Boost
parameters set on dependent boilers have no
effect on the system operation.
b. Similarly, the WWSD Temperature set on the
master boiler will prevent any boilers from
operating for a CH demand when the outdoor
temperature is above this value.
7. Multiple Boilers – Ramp Delay:
a. The ramp delay feature can be set on each
individual boiler.
b. However, it is important to note that since the
dependent boilers treat DHW demands as a CH
call, the ramp delay will take effect even when the
DHW tank calls.
8. Multiple Boilers – Anti-Cycling:
a. The Anti-Cycling feature is active on all boilers
unless it is disabled.
b. This will prevent dependent boilers from operating
within the prescribed time limit. However, it will
only prevent the boiler from cycling on its own
limit. Repetitive calls from the master boiler will
result in cycling.
9. Multiple Boilers – DHW Operation:
a. Since the DHW tank sensor or thermostat is
connected only to the Master boiler, this is the
only boiler that will receive the DHW demand.
b. The master control will start boilers as necessary
to meet the demand.
c. Boilers with a boiler address of 2 or higher will
automatically be switched to DHW Mode 0 (No
DHW).
Table 8.34: Rotation Interval Sequence
Days of Operation Start/Stop Sequence
1-5 1-2-3-4
6-10 2-3-4-1
11-15 3-4-1-2
16-20 4-1-2-3
21-25 1-2-3-4
BOILER CONTROL: OPERATION
To Next Page
Loading...
