Operating Information; General - Cadet 40-120 Installation And Service Manual

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Operating information

General

How the boiler operates
The Cadet heating boiler 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 boiler control module receives inputs from boiler sensors
and external devices. The control module activates and
controls the blower and gas valve to regulate heat input and
switches the boiler and Domestic Hot Water (DHW) 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 or system supply temperature.
Control inputs and outputs
Room thermostat
The room thermostat input tells the boiler to provide heat for
space heating.
DHW priority
The boiler control allows the connection of a DHW thermostat
or tank sensor to the low voltage connection board. On
standard boilers, when a tank sensor is connected, the tank
thermostat input is ignored. On combi boilers, the DHW
flow switch is connected to the tank thermostat input.
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. If the space heating call is still active
while the DHW call is in operation, the control will wait
for 30 minutes, then it will switch back to the space heating
demand. The control will switch back and forth until one of
the heat demands end.
Anti-cycling
After the burner turns off, the control will delay the next
burner cycle for a set time period. The time delay will be
bypassed if the inlet water temperature drops too far during
the delay.
Installation & Service Manual
Boiler pump control
The boiler pump will run whenever the burner is firing, unless
the boiler is heating the DHW tank. The boiler pump will
run during Freeze Protection Mode as well. It will continue
to run for a short time after the burner turns off or the Freeze
Protection Mode ends.
Temperature control
Modulation
The Cadet heating boiler 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, and various other
temperature limitations.
Gradient limiting
If during operation of the boiler the outlet water temperature
is rising too quickly, the control will reduce the firing rate to its
lowest setting.
Outdoor air reset
With the outdoor air sensor connected, the control module
will calculate the set point space heating demand based on the
programmed reset curve. The installer can change the slope
of the reset curve by two (2) adjustable parameters. The user
can limit the maximum set point for the system using the space
heating set point.
Flame current support
To prevent nuisance shutdowns when the boiler is firing at low
rates, the control will increase the firing rate when the flame
signal drops too low.
Protection features
Outlet temperature, flue temperature, and temperature
rise limiting
The outlet temperature is monitored by the boiler outlet
temperature sensor. When the outlet temperature exceeds
185°F, the unit will reduce the maximum fan speed. If the outlet
water temperature exceeds 195°F the control will shut the unit
down until it cools off.
The control module monitors the flue temperature by a sensor
located in the flue exhaust. If the flue temperature exceeds
215°F the control will reduce the maximum fan speed. If
the flue temperature exceeds 240°F the control will shut the
unit down. The unit will restart automatically once the flue
temperature drops 25°F and the minimum off time has expired.
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