A.O. Smith GPDH Owner's Manual page 14

WATER HEATING IGNITION SEQUENCE
(Make sure gas and electric power are connected properly)
1. The ignition control module is powered and monitors the
system, waiting for a call for heat from the thermostat.
2. The thermostat calls for heat by reading a resistance value
within a given range directly proportional to water temperature.
3. The control Module:
a. Checks the pressure switch for an open circuit.
b. Energizes the blower.
c. Checks the pressure switch for a closed circuit to prove
draft.
d. Sends line voltage to the hot surface igniter with a
20 second warm up period.
e. Opens the gas valve and checks the sensing rod for
flame.
4. The burner heats the water to the desired thermostat setting.
a. The resistance in the thermostat rises to the value
selected by the temperature setting.
b. The control module closes the gas valve and 5 seconds
later, removes power from the blower.
5. Cycle is completed.
FIGURE 29.
CONTROL SEQUENCE - HOT SURFACE DIRECT IGNITION
Pre-purge
HSI Warm-up
Ignition Activation Period
Flame Recognition Period
Ignition Trial
Interpurge
Post-purge
Retries
Reset from Lockout
Flame Sensing (Nominal)
HSI Off/Run Mode
HIGH ALTITUDE INSTALLATION
INSTALLATIONS ABOVE 2000 (610 m) FEET REQUIRE
REPLACEMENT OF THE BURNER ORIFICE IN ACCORDANCE
WITH THE NATIONAL FUEL GAS CODE (ANSIZ223.1/ NFPA 54).
FAILURE TO REPLACE THE ORIFICE WILL RESULT IN IMPROPER
AND INEFFICIENT OPERATION OF THE APPLIANCE, PRODUCING
CARBON MONOXIDE GAS IN EXCESS OF SAFE LIMITS, WHICH
COULD RESULT IN EXCESS OF SAFE LIMITS, WHICH COULD
RESULT IN SERIOUS PERSONAL INJURY OR DEATH.
A.O. Smith builds models specifically for high altitude service.
Please check the rating plate before making changes.
VENTING MANUAL RESET SWITCH
There is a venting manual reset switch located on the venting
hood. If the switch is activated, it will not reset itself. Before
resetting this reset switch, check for flue blockage and propriety
of the venting systems. To reset, press the red button.
DISCONNECT FROM ELECTRICAL SUPPLY BEFORE
SERVICING UNIT.
FOR YOUR SAFETY, WATER HEATER SERVICE SHOULD BE
PERFORMED ONLY BY A QUALIFIED SERVICE TECHNICIAN.
READ THE GENERAL SAFETY INFORMATION SECTION FIRST.
14
5 seconds.
20 seconds.
4 seconds.
1 second.
4 seconds.
5 seconds.
5 seconds.
2
1 hr.
4.0µ DC
WARNING
FIGURE 30.
MAINTENANCE
WARNING