Operation And Service; How It Works; Combustion Blower; Modulation - A.O. Smith BTH 120 Service Handbook

OPERATION AND SERVICE

HOW IT WORKS

This section of the Service Handbook will cover operation, common service procedures and water heater
construction. The water heater covered in this Service Handbook has a helical shaped coil heat exchanger
that is submerged in the storage tank. These water heaters use a top mounted down fired radial design
Burner. This is a forced draft burner; hot burning gases are forced through the heat exchanger under pressure
and exit through the exhaust/vent connection located at the bottom of the water heater.
Starting at the top air and fuel gas are drawn in by the Combustion Blower and Venturi. Flue gases and are
forced through the helical shaped heat exchanger by the Combustion Blower and out through the exhaust/
vent outlet.

COMBUSTION BLOWER

The Combustion Blower is an assembly that includes the blower housing, blower motor and an electronic
speed control. The Combustion Blower is controlled by the CCB (Central Control Board) and is powered by
a 3 pin wiring plug receiving 120 VAC from the Junction Box.
The CCB sends a signal from the J5 socket to a 5 pin wiring socket on the BTH 120-250 blower and VFD on
the BTH 300-500 top cover control assembly. The signal is an electronic instruction to start, stop and control
blower speed.
The Combustion Blower runs at higher speeds during the Pre/Post Purge operating states and runs at a
lower speed during the Trial For Ignition operating state. The Igniter Status icon is displayed on the UIM
during the Trial For Ignition operating state.

MODULATION

The water heaters covered by this Service Handbook are capable of modulating their firing rate. The CCB
monitors the water temperature in the tank and regulates the firing rate to achieve the target temperature
setpoint. The firing rate is dictated by the hot water draw, proximity to the tank temperature setpoint, and
various other temperature limitations. Periodically, when the heater is in modulation mode, the CCB will
increase the blower speed for a short period of time to clear out any condensation that may have accumulated
in the heat exchanger then decreases the blower speed back to the modulating firing rate required to maintain
the desired tank temperature setpoint. This ramping up and down of the blower speed is considered normal
operation of the water heater.
38 Servicing should only be performed by a Qualified Service Technician