A.O. Smith VB/VW- 1000 - Manual (Page 38)

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INPUTS TO MCB
TEMPERATURE SENSORS:
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 tank).
MRHL (Manual Reset HIGH LIMIT): The MRHL 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.
Thermostat input (MCB - optional): This input is set up to work with
an externally connected thermostat that provides a contact closure. If
this input is closed and everything else is in the proper state, a “call 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 thermostat
control the temperature of the boiler, the operating setpoint 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 been reached.
AIR PRESSURE SENSORS (OPEN CONDITION INDICATES FAULT):
Blocked Inlet (MCB - required) Normally closed switch that
activates if the air inlet is blocked 3/4 or more during operation.
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.
GAS PRESSURE SENSORS (OPEN CONDITION INDICATES FAULT):
Low Gas (MCB - required): Normally open switch that closes when
the gas pressure rises above the trip level. This input is enabled/
disabled by a dip switch on the MCB
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.
WATER LEVEL SENSOR (OPEN CONDITION INDICATES FAULT):
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.
WATER FLOW SENSOR:
Flow (MCB - required): Normally open switch that closes when
flow exceeds a set value.
FLAME SENSOR:
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.
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
operate with the voltage and current level provided.
Pump (MCB - 120VAC - required on systems that do not have
an external pump): Provides electrical power to directly operate a
pump or the coil of an externally connected contactor.
Blower (MCB - 120VAC - required): Variable speed blowers utilize
the high blower output only.
Igniter (MCB - 120VAC - required): Provides power to operate
the HSI igniters.
Gas Valve (MCB - 24VAC - required): Provides power to activate
the gas valve. The gas valve cannot be activated when the MRHL
contacts are open.
Low Water Cut Off (MCB - 24VAC - optional): Directly connected to
the 24VAC line to provide power to operate an external LWCO device.
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.
Yellow LEDs are located near the microcontrollers on the MCB.
These LEDs are “heartbeat indicators,” and blink approximately
twice per second to indicate that the microcontrollers are
running.
MCB JUMPERS
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.
IGNITERS
The EMC modulation system operates with Silicon Carbide Igniters
MCB - Ten Position Dipswitch:
Dipswitch configurations are READ ONLY ON POWER UP. These switches are only to be set at the factory or by authorized trained
personnel! Once set the boiler will operate according to the chosen options. If a switch is changed, power must be cycled before the
change will take effect. The status of all dipswitches can be observed on the system status screen on the UIM.
TABLE 13. - MCB/FCB Dipswitches:
Dipswitch Function Switch Position
Hot Water Boilers Hydronic Heating Boiler
Switch 1: Selection of the type of boiler application: On = VW Off = VB
Switch 2: Trials for ignition: On = 3 Off = 1
Switch 3: IRI Gas Valve Not Available:
Switch 4: Controlling Probe: On = Tank (Remote) Off = Inlet
Switch 5: Powered Venter: On = Yes Off = No
Switch 6: Low Water Cut Off: (LWCO) On = Yes Off = No
Switch 7: Low Gas Pressure On = Yes Off = No
Switch 8: Modulation: On