A.O. Smith BTH 120 Service And Parts Manual page 14

If ECO is closed low
gas pressure switch
Pressure Switch Information
BTH 120 and 250
Only
Low Supply Gas
Pressure
Switch NC 4.3"
w.c. gas
press. to close
Display Failure Code
Note: If the lower temperature probe is defective or unplugged, display will show upper tank
temperature. Heater may fire but stack.
If upper high limit/tank temp. thermistor is defective or disconnected, cycling power off then on
displays will be: 00, 120, 12, water temp at lower probe, flashing "04" code.
A. O. Smith Water Products Company
©
Irving, Texas 2000
BTH Sequence of Operation
Call for heat (Temp.
is closed
(120 & 250
models only)
Blocked Inlet-NC
Opens .75" wc
Neg. Press
Blocked Outlet-NC
Opens at 2.5"
reduces BTH 250
Positive Press
Blower Prover
Closes at .45"
Positive Press
Gas Valve Opens
Blower to Full RPM
NO
Flame Rod Does
Not Prove Flame
30 Second
Purge
Retry-Max
3 times
Power Off
Power On
setting less diff.)
Blower Prepurge
20 Seconds
Draft Prover
Switches to
Proper State
HSI is Energized
Weighted Average
Blower speed
Tank Termpature
only
Igniter
warm-up period
BTH-250 only
7.5 Sec Trial
for ignition
Flame Rod Senses
Flame .7 microamp
Tank Water Heated
Gas Valve Closes
30 Sec Post Purge
13
Display Sequence
Display "00"
If reset is pushed
and released while
Supply Volts
this is displayed, the
"120"
board will do an
operational self
Temperature
Offset 12%
Pushing the reset at
this time will display
tank temp. setting.
Push once and hold
Bottom vs Top to increase setting.
Push twice and hold
to decrease settings
YES
minimum
to Set Point
Training Department
check.
110°-180°
Service Handbook