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Installation & Operation Manual
57
11 Operating information (continued)
Cascade (continued)
If the water temperature at the system supply sensor is less
than the set point + the turn-off offset - the off-on
differential, then the control will initiate a call for heat on
the Cascade (see the Knight XL Service Manual for an
explanation of the offset and differential). The Leader will
energize the lead boiler on the Cascade. For a new startup
this will be the Leader boiler.
The boiler will fire at its ignition speed and will then
modulate its firing rate to maintain the set point. If the first
boiler reaches 100% of its firing rate, the Leader will
calculate at what point the second boiler could fire at 20%
of its firing rate. At this point, the Leader will fire the second
boiler on the Cascade. For a new startup, this would be the
first Member boiler. The boiler will fire at its ignition speed
and will then modulate its firing rate to maintain the set
point.
If the set point still cannot be met, the Leader will continue
firing more Members until either the heat demand is met or
all boilers on the Cascade are firing. As the heat demand
decreases, the last boiler on will modulate down to 20% of
its firing rate. Once the demand for that boiler is zero, it will
shut down. As the heat demand decreases further, the
second to last boiler will modulate down and shut off. This
will continue until the demand is satisfied and all boilers are
shut off.
DHW, Night Setback, and Ramp Delay operation with
cascade
For DHW operation any boiler(s) in the Cascade can be selected
to provide heat for a DHW call. Select a boiler to be designated
as the DHW boiler. Connect the DHW thermostat to the
terminals on the Low Voltage Connection Board marked for the
DHW Thermostat. When the boiler receives a DHW call, the
Leader control will take that boiler out of the Cascade sequence.
If another boiler is available, the Leader will start it up to take its
place.
The DHW boiler will adjust its set point to the programmed
DHW set point and will adjust its firing rate to maintain this.
Once the DHW call has been satisfied, the Leader control will
place that boiler back into the Cascade sequence.
Switching of the boiler between DHW operation and SH
operation when there is a call for both does not occur. The boiler
will provide heat for the DHW demand until it is satisfied.
Night Setback operation of the boilers within the Cascade is
available. Programming of the Night Setback will be done
through the Leader boiler. Refer to the Knight XL Service
manual for information regarding Night Setback.
Ramp Delay operation of the boilers as described in the Knight
XL Service Manual is not active when the boilers are part of a
Cascade system.
Sequence of the cascade
To equalize the run time of all boilers on the Cascade, the
firing sequence will automatically be changed at set
intervals.
For the first 24 hours after initializing the Cascade, the
sequence will be changed every hour. After that the
sequence will be changed once every 24 hours. The
switching on/off sequence will be as follows:
DAY SWITCHING ON SEQUENCE
Day 1 L-M1-M2-M3-M4-M5-M6-M7
Day 1 + 1 hour M1-M2-M3-M4-M5-M6-M7-L
Day 1 + 2 hours M2-M3-M4-M5-M6-M7-L-M1