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10 Operating information (continued)
49
Installation & Operation Manual
Cascade
When multiple boilers are installed, they can be wired together
in a cascade sequence. A maximum of eight boilers can be
controlled from a single control. In this application one boiler
would be designated as the Leader control and all others would
be designated as Member controls.
Once the Leader boiler receives a call for heat from the Enable
input, the 0 - 10 VDC input, or ModBus, the control will
determine what the set point will be. If outdoor air reset is
desired, connect the outdoor air sensor to the terminals on
the Low Voltage Connection Board on the Leader boiler. The
set point will be calculated based on the programmed reset
curve parameters. See the Crest Service Manual to program
the reset curve. If outdoor air reset is not desired, do not
connect the outdoor air sensor. A fixed temperature set point
can be programmed into the control, or provided through the
ModBus interface.
If the water temperature at the controlling sensor is less than
the set point + the turn-off offset - the off-on differential,
the control will initiate a call for heat on the Cascade (see
the Crest 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 80% of its firing rate, the Leader will calculate the rate
at which the first and second boiler should fire to meet the
load. 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 then both
the Leader and the Member boilers will modulate together to
a firing rate to maintain set point.
If the set point cannot be met, the Leader will continue firing
additional Members until either the heat demand is met or
all boilers on the Cascade are firing. As additional Member
boilers are fired, the running boilers in the Cascade will
modulate together to meet load.
As the heat demand decreases, the boilers firing in the Cascade
will modulate down together until they reach a minimum of
10% of its firing rate, where the last boiler will shut down
and the remaining boilers will modulate together to meet
load. As the heat demand decreases further, the second to last
boiler will shut down. This will continue until the demand is
satisfied and all boilers are shut off.
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.
HW operation with cascade
For HW operation any boiler(s) in the Cascade can be selected
to provide heat for a HW call. Select a boiler to be designated
as the HW boiler. Connect the HW thermostat or sensor to
the terminals on the Low Voltage Connection Board marked
for the HW Thermostat or sensor. When the boiler receives
a HW 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 HW boiler will adjust its set point to the programmed
HW set point and will adjust its firing rate to maintain this.
Once the HW call has been satisfied, the Leader control will
place that boiler back into the Cascade sequence.
Night Setback and Ramp Delay operations with
cascade
Night Setback and Ramp Delay operation of the boilers within
the Cascade are available. Programming will be done through
the Leader boiler. Refer to the Crest Service Manual for
information regarding Night Setback and Ramp Delay.