A.O. Smith 100 Series, Service Handbook

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Servicing should only be performed by a Qualified Service Agent
MODULATING FIRE OPERATION
XB boilers & XWH water heaters are modulating systems. The control system modulates the firing rate of the burner to match system
load by controlling the speed of the combustion blower.
They do not have a gas orifice. The combustion blower "pulls" fuel gas from the outlet of the 24 VAC gas valve (when energized) into
a venturi that is connected to the inlet of the Combustion Blower. The firing rate is directly proportional to the speed of the combustion
blower motor. As the blower speed increases, the pressure inside the venturi falls creating a stronger vacuum which pulls more fuel
gas into the blower/burner assembly which increases the firing rate.
The control system controls the speed of the combustion blower in response to system temperature. As the system temperature falls
blower speed is increased to provide more heating capacity. The control system sends digital speed instructions to the electronic
speed control which is part of the combustion blower assembly.
COMBUSTION BLOWER
The combustion blower is an assembly that includes the blower motor, housing and an integral electronic speed control. The power
junction box sends 120 VAC and an earth ground from its J3 socket to the 120 VAC 3 pin socket on the combustion blower assembly
to power the electronic speed control. The control board sends a PWM (Pulse Width Modulation) signal, an instruction to start, stop
and control the blower motor speed from the J2 socket to the 5 pin wiring socket on the combustion blower assembly. Four wires from
the J2 socket on the control board carry this instruction to the 5 pin wiring socket on the combustion blower assembly, see Figure 4
below.
XB/XWH MODELS BLOWER PART NUMBERS
1000/2000 EBM RG-175
1300/1700/2600/3400 EBM G1G170-AB05-20
Service Note:
The 5 pin PWM signal plug must remain plugged in to the 5 pin socket on the blower assembly at all times. Disconnecting this plug
will cause the combustion blower to run at maximum speed continuously. This may cause rough starts, rough operation and control
system lock out. If the electronic speed control is functioning properly combustion blower speed should noticeably reduce during the
operating state. If blower speed reduction does not occur during the operating state ensure the 5 pin plug from the control board is
securely plugged into the matching 5 pin socket on the blower assembly and that the J2 plug is securely plugged into the J2 socket
on the control board. Perform a close visual inspection of the pins inside the plugs and sockets at the combustion blower and the
control board, replace any worn or damaged wiring harnesses as necessary.
TABLE 1. COMBUSTION BLOWERS
FIGURE 4. COMBUSTION BLOWER ASSEMBLY
OPERATION AND SERVICE