A.O. Smith TC-099 User Manual

Page 29

VF BOILER SERVICE MANUAL

AOS WPC - Tech Training

28 of 72

Servicing should only be performed by a Qualified Service Agent

MCB - SECTION B

The upper middle section of the MCB contains the following sockets/components:

†

J4 Socket (Outputs/Inputs)

• Pin 1 - Spare - not used
• Pin 2 - Spare - not used
• Pin 3 - Spare - not used
• 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 line alarm bell circuit (optional)
• Pin 8 - 24 VAC line alarm bell circuit (optional)
• Pin 9 - Spare - not used
• Pin 10 - Spare - not used

J11 Socket (Enable/Disable or Tstat 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 - 24 VAC - Gas valve solenoid
• Pin 2 - 24 VAC - Gas valve solenoid

†. J4 Socket Pins 1-8 provide 24 VAC circuits for optional components and equipment. These are switched

24 VAC control circuits with a maximum amp rating of 1 amp. When these optional components are used the
MCB must be configured to recognize and enable the components by setting the SW1 dip switches
accordingly. See page 30.

2
1

1
2
3
4
5

6
7
8
9
10

1
2
3
4
5

3
2
1

6
7
8
9

1 2 3

4 5 6

J4 Outputs J11 Tstat

J5 Inputs

J17 BlwPrv/Hi Gas

J19 Gas Valve

The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.

J5 Socket (Inputs)

• Pin 1 - 24 VAC Flow switch
• Pin 2 - 24 VAC Flow switch
• Pin 3 - 24 VAC Low gas press switch
• Pin 4 - 24 VAC Low gas press switch
• Pin 5 - 24 VAC Blocked flue (exhaust) switch
• Pin 6 - 24 VAC Blocked flue (exhaust) switch
• Pin 7 - 24 VAC IRI prover switch - N/A on VF boilers
• Pin 8 - 24 VAC IRI prover switch - N/A on VF boilers
• Pin 9 - Spare - Not used
• Pin 10 - Spare - Not used

J17 Socket (Blower Prover / Hi Gas Press Switch)

• Pin 1 - 24 VAC High Blower Prover switch
• Pin 2 - 24 VAC High 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 Low Blower Prover switch
• Pin 6 - 24 VAC Low Blower Prover switch