A.O. Smith VB/VW- 1000, VB/VW- 750 instruction manual Inputs to MCB

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.

AirPressureSensors(openconditionindicatesfault):

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.

GasPressureSensors(openconditionindicatesfault):

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

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