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Part # MCOSM06 Rev 1 (11/03/08)
G77x INTERMITTENT PILOT CONTROLS
Application Requirements
The following are the application requirements of the G77x
control.
• TheG77xcanbeusedongas-redequipmentwitha
maximum ring rate of 117kW (400,000 BTU/Hr). Any
application over 117kW (40,000 BTU/Hr) must have written
approval in advance from the Johnson Controls Heating
Products Engineering Department.
• AllG77xapplicationsmustusearedundantgasvalve
system where the pilot and main valve seats are in series
and opened in sequence for intermittent pilot ignition.
Operating Mode Denitions
The following denitions describe operating conditions.
• Prepurge: Initial time delay between thermostat contact
closure and trial for ignition.
• Trial for Ignition: Period during which the pilot valve and
spark are activated, attempting to ignite gas at the pilot
burner. The control attempts to prove ame at the pilot
burner within the trial-for-ignition time.
• Run: Pilot and main gas valves remain energized and spark
is turned o after successful ignition.
• 100% Shuto: For controls with automatic recycle, pilot
gas did not ignite within the trial-for-ignition time. The
control de-energizes the spark circuit and pilot valve.
• Automatic Recycle: If shuto occurs, the control delays for
a specic recycle delay period before beginning another
trial for ignition (models with recycle only).
• Interpurge: Period between trials for ignition when both
the gas valve and spark are deactivated to allow unburned
gas to escape before the next trial. Interpurge occurs
between unsuccessful trials on a multi-trial control or after
a ameout (if the control has an interpurge).
• Ignition Lockout: Pilot gas did not ignite within the nal
trial-for-ignition time. Open thermostat contacts for 30
seconds, then close to restart the sequence of operation.
(Models with an optional LED will ash the LED to indicate
ignition lockout.)
• Flameout: Loss of proven ame.
• Hard Lockout: The control detected a fault. Open
thermostat contacts for 30 seconds, then close to restart
the sequence of operation. (Models with an optional LED
will turn o the LED to indicate a hard lockout.)
Sequence of Operation
Figure 2 illustrates the sequence of operation of the G77x
control.
The control is energized on a call for heat from the system
thermostat. (Models with an optional LED will turn on the
LED [steady on] until the call for heat is satised.) The vent
damper, if used, is energized and when fully opened, also
energizes the ignition control. If the control is equipped with
the optional prepurge, the appliance prepurge fan or relay
is also energized through the thermostat contacts. In the
prepurge mode, the control will delay for the time selected
(e.g., 15 seconds) before simultaneously opening the pilot
valve and supplying a continual spark at the pilot burner. If
prepurge is not selected, the pilot valve is opened and spark
initiated within one second after the call for heat.
Under normal conditions, the pilot burner gas ignites within
the trial-for-ignition time (e.g., 8 seconds), the pilot ame
sensor detects pilot ame and signals the control to energize
the main valve. The main gas valve will not be energized until
the ame sensor detects the presence of pilot ame. The
spark will continue until ame is sensed at the pilot burner or
the trial-for-ignition time has elapsed, whichever occurs rst.
If the pilot ame is not sensed before the end of the trial-for-
ignition time, the control may:
• proceedtointerpurge,followedbyanothertrial(three
trial models whose rst two trials produced no ame).
• proceedtoalockout(noautomaticrecyclemodelswhose
nal trial produced no ame).
• proceedtoarecycledelayperiodfollowedbyanothertrial
(automatic recycle models whose nal trial produced no
ame).