A.O. Smith 100 Series Service Handbook page 23

MCB - SECTION B
See Figure 12 on page 19 for location of this section on the MCB.
• J4 Socket (Outputs/Inputs)
• Pin 1 - Power Accessory 24 VAC
• Pin 2 - Power Accessory Prover
• Pin 3 - Power Accessory 24 VAC RTN
• Pin 4 - 24 VAC line low water cut off (optional)
• Pin 5 - Low water cut off proving signal
• Pin 6 - 24 VAC line low water cut off (optional)
• † Pin 7 - 24 VAC alarm bell circuit (optional)
• † Pin 8 - 24 VAC alarm bell circuit (optional)
• Pin 9 - Spare - not used
• Pin 10 - Spare - not used
• J11 Socket (Enable/Disable Circuit)
• ‡ Pin 1 - 24 VAC to dry control contacts
• ‡ Pin 2 - 24 VAC to dry control contacts
• Pin 3 - Spare - not used
• J19 Socket
• Pin 1 - 120 VAC - Gas Valve solenoid
• Pin 2 - 120 VAC - Gas Valve solenoid
† J4 Socket Pins 7 & 8 provide 24 VAC for an optional alarm bell that will sound whenever the Boiler Control
System declares a fault condition. This is a switched 24 VAC control circuit with a maximum amp rating of 1
AC amp @ 24 VAC.
‡ The enable/disable circuit is provided for use with external supervisory controls. Field wring is installed
between this circuit and a set of "dry contacts" on the external control, see Figure 1 on page 6. This is a
switching circuit only: Do not apply any external voltage or connect any load (IE: relay coil) to this circuit. This
will permanently damage the MCB circuit board and is not covered under the limited warranty.
5 4 3
J4
10 9 8 7 6
J11
1 2
10 9 8 7 6
J5
5 4 3 2 1
J17
J19
Figure 14
Servicing should only be performed by a Qualified Service Agent
2 1
3
B
4 1
5 2
6 3
1 2
• J5 Socket (Inputs)
• Pin 1 - 24 VAC Flow Switch
• Pin 2 - 24 VAC Flow Switch
• Pin 3 - 24 VAC Low Gas Pressure Switch
• Pin 4 - 24 VAC Low Gas Pressure Switch
• Pin 5 - 24 VAC Blocked Flue (Exhaust) Switch
• Pin 6 - 24 VAC Blocked Flue (Exhaust) Switch
• Pin 7 - Spare - Not used
• Pin 8 - Spare - Not used
• Pin 9 - Spare - Not used
• Pin 10 - Spare - Not used
• J17 Socket (Blower Prover/Hi Gas Press Switch)
• Pin 1 - 24 VAC Blower Prover Switch
• Pin 2 - 24 VAC Blower Prover Switch
• Pin 3 - 24 VAC High gas press switch (optional)
• Pin 4 - 24 VAC High gas press switch (optional)
• Pin 5 - 24 VAC Blocked Intake Air Switch
• Pin 6 - 24 VAC Blocked Intake Air Switch
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