Bradford White 20A Service Manual download pdf (Page 6)

Thermostat calls for heat.

The control board sends 24 volts from damper terminal #2 on the control plug

to the flue damper.

Flue damper begins to rotate open. Once the flue damper is fully open, the damper end switch closes and 24 volts

is allowed to continue through damper to damper pin terminal #5.

Once pilot flame is proven, sparking will stop.

Page 6

Sequence of Operation

rial for ignition (three 90 second ignition trials, with 65 second pauses

between trials).

Control Board simultaneously sends:

1. 24 volts from control pin terminal #8, to “MV/PV”

terminal of gas valve (common terminal).

2. 24 volts from control pin terminal #2, to “PV”

terminal of gas valve to establish

gas flow at pilot.

3. Low current high voltage from “spark”

terminal, to generate spark at the pilot and

ignite pilot gas flow.

4. Pilot flame proving signal (measured in

micro-amps). from the “sense” terminal, to

prove pilot flame.

Once sparking stops, 24 volts is sent from control pin

terminal #5 on control board, to “MV” terminal on gas valve

to establish main burner gas flow. Main burners ignite from

the pilot flame.

The control board constantly monitors pilot flame through

the flame sensor rod. If pilot flame is lost, pilot and main

burners are shut down. After a 65 second inter-purge

period, the control will attempt to re-light the pilot beginning

at sequence 3 above.

Main burner fires until the thermostat is satisfied. The control

board interrupts 24 volts through the damper and the gas valve

circuit. Pilot and main burners are turned off.

Flue damper rotates to the closed position.

WIRING DIAGRAM

LOCKOUT CONDITION

Control board will go into “Soft Lockout” if the pilot cannot be lit

after 3 ignition trials. The water heater display indicates a lockout

condition by showing an error code number (62 or 63) with

“Service Needed” in the display window. Refer to error codes in

the diagnostic section of this Service Manual. In a “Soft Lockout”

condition, the control will wait for 60 minutes and then make 3

more attempts to light the pilot and establish the main burners.

Soft lockout reset is accomplished by depressing the lower right

button under “Reset” for 3 seconds.

If the water heater should reach 200 degrees F, then the high limit

control will shut off the burners and the water heater will go into a

“Hard Lockout”. Error code 65 will be shown in the water heater

display. The control can only be reset in the “service mode”,

which is detailed in the next section of this Service Manual.

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Bradford White 20A Service Manual Page 6

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