Installation & Operation Manual
10 Operating information (continued)
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
The two (2) types of Cascade control are listed below:
Efficiency optimization
Efficiency optimization is chosen when the application is such that overall heating efficiency is most important. The Cascade will fire the boilers in a way that is the most efficient. This allows more boilers to fire at one time, at a lower rate, thus more efficient. This Cascade is also required when cascading boilers in a common vent application.
Lead/Lag
The Lead/Lag method is chosen when the application requires firing as few boilers at a time as possible. This method will not bring on another boiler until the current number of boilers cannot meet the demand. This Cascade is NOT allowed in a common vent application.
The parameters for both types of Cascade are adjustable. Reference the Crest Service Manual for a more detailed description of these adjustments.
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.
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