sequences and safety features associated with a gas ignition Pump (MCB - 120VAC - required on systems that do not have an
control system. The system continuously performs various external pump): Provides electrical power to directly operate a pump
diagnostic tests to verify proper appliance and control or the coil of an externally connected contactor.
operation. Should an unsafe condition occur, the control will
Blower (MCB - 120VAC - required): Variable speed blowers utilize
the high blower output only.
shut down the burner and display a red service light as well
as indicate the cause of the error on the display. The operating
programs for the system are stored in permanent memory
Igniter (MCB - 120VAC - required): Provides power to operate
inside the microcontrollers. User-selectable operating
the HSI igniters.
parameters and a history of detected faults are stored in
rewritable memory in the microcontrollers. A loss of power
does not affect either of the memories.
Gas Valve (MCB - 24VAC - required): Provides power to activate
the gas valve. The gas valve cannot be activated when the ECO
contacts are open.
INPUTS TO MCB
Low Water Cut Off (MCB - 24VAC - optional): Directly connected to
Temperature probes (MCB - outlet and either inlet or tank are
required): The MCB accepts analog temperature inputs from
up to three sensors (inlet, outlet, and remote).
MCB AND PDB INDICATOR LAMPS AND FUSES
A green LED is mounted on the PDB to indicate when line voltage
is applied (the PDB also contains a yellow and red LED, and a
test/run jumper that are used during installation to verify proper
power connections). A red LED on the MCB is used to indicate
when the 24VAC input fuse has blown.
ECO input (MCB - required): The ECO (energy cut off) is a hi-
limit switch located inside the outlet probe. It is a normally
closed switch that opens if the probe is exposed to a
temperature higher than the trip point. Once tripped, the control
system needs to be manually reset.
Yellow LEDs are located near the microcontrollers on the MCB.
Thermostat input (MCB - optional): This input is set up to work with
an externally connected thermostat that provides a contact closure. These LEDs are “heartbeat indicators,” and blink approximately
If this input is closed and everything else is in the proper state, a “call twice per second to indicate that the microcontrollers are running.
for heat” condition will be initiated. These leads should be shorted
together when a thermostat is not being used. If it is desired that the
of the system should be set higher than the temperature that the
thermostat is controlling. This will allow the thermostat to control the
boiler. When the thermostat closes, a call for heat will be generated
until the thermostat determines that the required temperature has
The MCB has two jumpers. JP1 on the MCB is used to terminate
the external communications line. It is normally left off and installed
when the external cable is very long.
The EMC modulation system operates with Silicon Carbide Igniters.
Blocked Flue (MCB - required): Normally closed switch that opens
if the flue becomes blocked during operation.
Blower Prover (MCB - required): Normally open switch that closes
when the air pressure produced by the blower is above the set point.
The EMC modulation controller has four modes of operation:
Initialization, Standby, Running/Modulation, and Service. The
internal MCB micros control these modes through a sequence of
Low Gas (MCB - required): Normally closed switch that opens
when the gas pressure falls below the trip level. This input is
enabled/disabled by a dip switch on the MCB
When power is applied to the system, it enters the initialization
mode and the following automatic functions are performed:
Hi Gas (MCB - required): Normally closed switch that opens if the
gas pressure exceeds a set value. This input is enabled/disabled
by a dip switch on the MCB.
A. O. Smith opening screen is displayed.
The systemgoesthroughacalibration indicatedbythegreen
running LEDblinking andthenstayingon; next theredservice
LED and yellow standby LEDs come on, next the service and
runnings LEDs blink ON and OFF.
Low Water Cut Off (MCB - optional): Normally open switch that
closes when water reaches preset level. This input is enabled/
disabled by a dip switch on the MCB.
Stored values are recalled from memory.
Configuration dipswitches are read.
Pending faults are recalled
Micros on all boards start running (indicated by a flashing
Yellow LED near each micro)
WATER FLOW SENSOR:
Flow (MCB - required): Normally open switch that closes when
flow exceeds a set value.
Input sensors are read
Flame (MCB - required): Returns a signal to the microprocessor if
flame is detected on the burner. If the flame rod is missing or
shorted, the flame will not be detected.
After initialization is complete (approximately 10 seconds) the
system turns the green LED off and goes to the standby mode
(yellow "Standby" LED on), unless a previously stored fault
has been recalled, which will send the system into the service
model (red "Service" LED on). In standby mode the display
shows the temperature screen and in fault mode the current
error screen is displayed.
OUTPUTS FROM MCB
RELAY CONTACT OUTPUT:
Alarm (MCB - 24VAC - optional): Provides electrical power to
operate an external alarm. This can be an audio device (i.e.,
Sonalert), a visual device (lamp), or any other device that will 4. The system then compares the temperature read from the
operate with the voltage and current level provided.
controlling probe (inlet or tank) to the setpoint temperature. If