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8. BOILER CONTROL: OPERATION
A. THE CAST 92 BOILER CONTROL
1. Control Overview:
The Cast 92 control monitors boiler temperature and
limit circuit inputs, modulating boiler firing rate to
meet demand. The control uses microprocessor
electronics, watching time-average response from the
system to anticipate how much heat the system needs.
Coupled with the five-to-one turndown of the Cast 92
boiler, this results in maximum possible condensing-
mode operation. The Cast 92 boiler will provide
unmatched seasonal efficiency.
2. Indoor Air Reset (IAR):
The unique approach to boiler output regulation is its
Indoor Air Reset function. The control monitors the
demand from up to 8 different zones. Watching the
demand duration and response to supply temperature,
averaging over time, the control anticipates system
needs. It sets boiler maximum firing rate and adjusts
supply water temperature to fine-tune boiler heat
output. All that is required to enable IAR is to connect
thermostat circuit wires to the IAR inputs. The control
can also be configured for outdoor reset operation,
but IAR provides response based on system behavior
rather than just looking at outdoor temperature.
3. The Cast 92 Control Platform:
Cast 92 controls are designed to provide an integrated
boiler management system on every boiler. The
platform provides multiple levels of selectivity. Cast 92
electronics can be operated as a simple single-boiler
control, while still providing intelligent regulation of
boiler firing rate to match system demand. With a few
key strokes on the key pad, the Cast 92 control can
operate as a sophisticated multiple-boiler controller,
using simple RJ45 cable interfacing between units.The
control can even accept external control commands
from building managements systems (Modbus
standard, with optional bridge for BACnet or
LonWorks) or 20-milliamp analog input from an
external controller.
The control method used by the Cast 92 control is
based on digital communications, which eliminates
the need for analog control signals. Analog signal
inputs are supported, but a higher level of control
precision, repeatability and feedback is gained with
digital communications.
BOILER CONTROL: OPERATION
The Cast 92 control can be versatile, providing for
operation in multiple ways:
• Operation as a stand-alone boiler.
• Operation as a boiler in a boiler network, using
the on-board protocol.
• Operation as a member boiler in a boiler
management system.
• Operation as a member of a remotely-controlled
boiler network (20-milliamp regulation).
• Setpoint can be determined by the control or by a
20-milliamp input signal.
4. PID Response:
The Cast 92 control uses proportional-integral-
derivative calculations to determine the response to
boiler water temperature changes. This means it not
only looks at how far away the water temperature is
from the setpoint temperature, but how fast the
temperature is changing and how it has responded
over time. This ensures the boiler won't make sudden
unnecessary changes in firing rate.
5. Multiple Boiler Operation:
The Cast 92 control easily interfaces with other Cast
92 controls. Multiple boiler operation only requires
RJ45 cables daisy-chained from boiler to boiler and a
few key strokes setting up control behavior. The
master boiler is automatically selected by connecting a
sensor lead to its HEADER sensor terminals. The
control recognizes the sensor and configures the boiler
as the master. Other boilers only need to have an
address assigned.
Among the advanced design features of the Cast 92
control is the MOD-MAX setting. This limits the firing
rate of all boilers to a pre-set maximum (50% by
default). This means all of the boilers will be run at a
very efficient level until all boilers are on. Only then
can firing rate increase above this setting. Boiler
rotation can be first-on/first-off, first-on/last-off, or true
rotation (the control monitors the total on time of all
boilers, and rotates their usage so the total on time is
the same for all).
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