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Installation & Operation Manual
54
General
How the boiler operates
The Knight XL uses an advanced stainless steel heat exchanger
and electronic control module that allows fully condensing
operation. The blower pulls in air and pushes flue products
out of the boiler through the heat exchanger and flue piping.
The control module regulates blower speed to control the
boiler firing rate. The gas valve senses the amount of air
flowing into the boiler and allows only the right amount of
gas to flow.
How the control module operates
The SMART SYSTEM control module receives input from
boiler sensors and external inputs. The control module
activates and controls the blower and gas valve to regulate
heat input and switches the boiler, Domestic Hot Water
(DHW), and system pumps on and off as needed. The user
programs the module to meet system needs by adjusting
control parameters. These parameters set operating
temperatures and boiler operating modes. Boiler operation
can be based on boiler outlet water temperature, boiler inlet
water temperature, or system temperature, depending on the
parameter setting.
Control inputs and outputs
Room thermostat / zone control
This input tells the boiler to provide water for space heating.
Domestic Hot Water (DHW) tank thermostat
This input tells the boiler to provide water for heating an
indirect DHW tank.
0 - 10V input (set point or power)
The Knight XL can be controlled by a Building Management
System (BMS) using a 0 - 10 VDC signal. The control can be
configured by the installer to use this signal to either control
set point or firing rate.
DHW priority
The SMART SYSTEM control module allows connection of a
DHW thermostat to the low voltage connection board. When
the DHW thermostat calls for heat, the module activates the
DHW pump, shuts down the boiler pump, and immediately
sets the target outlet water temperature to 180°F (82.2°C).
This provides automatic priority heat allocation to the
indirect water heater for maximum response and recovery.
The DHW pump continues for 30 seconds after the heating
cycle to deliver the most possible heat.
DHW / space heating (SH) cycling
If a DHW call for heat is received while a space heating call is in
progress, the control will start the DHW pump and shut the
boiler pump off. The system pump will remain on. If the space
heating call is still active while the DHW call is in operation, the
control will wait for 30 minutes (time adjustable by installer)
then it will switch back to the space heating demand. The
control will switch back and forth until one of the heat demands
end. This function does not apply to cascade systems.
Programmable controlling sensor
The control module is programmed to use the outlet sensor as
the control sensor by default. If a system supply sensor is
connected, the control automatically uses it as the control
sensor. The control sensor can be changed by the installer to the
inlet sensor. If the inlet sensor is chosen as the controlling
sensor, it is recommended that the system supply sensor be
installed.
Anti-cycling
After a space heating demand has been satisfied, the control will
delay the next space heating call for a set time period (time is
adjustable by the installer). The time delay will be bypassed if
the inlet water temperature drops too far during the delay.
Boiler, system, and DHW pump control
When a space heating call for heat starts and no DHW call is on,
the system and boiler pumps are turned on. As long as the space
heating call for heat is on, the system pump will remain on. If a
DHW call for heat is on, the boiler pump will wait to turn on
until just before the DHW pump turns off. After the space
heating call for heat ends, both pumps will run for an additional
period of time.
When a DHW call for heat starts, the DHW pump is turned on.
If a space heating call for heat was on, the boiler pump will turn
off a few seconds after the DHW pump turns on.
Temperature control
Modulation
The Knight XL is capable of modulating its firing rate from a
minimum of 20% to a maximum of 100%. The firing rate is
dictated by the call for heat (i.e., space heating or domestic hot
water), the heating load, ramp delay (if enabled), and various
other temperature limitations.
11 Operating information