Garland 200 - Manual (Page 32)

Page 32 Part # MCOSM06 Rev 1 (11/03/08)
G77x INTERMITTENT PILOT CONTROLS
Application Requirements
The following are the application requirements of the G77x
control.
• The G77x can be used on gas-fired equipment with a
maximum firing 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.
• All G77x applications must use a redundant gas valve
system where the pilot and main valve seats are in series
and opened in sequence for intermittent pilot ignition.
Operating Mode Definitions
The following definitions 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 flame at the pilot
burner within the trial-for-ignition time.
• Run: Pilot and main gas valves remain energized and spark
is turned off after successful ignition.
• 100% Shutoff: 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 shutoff occurs, the control delays for
a specific 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 flameout (if the control has an interpurge).
• Ignition Lockout: Pilot gas did not ignite within the final
trial-for-ignition time. Open thermostat contacts for 30
seconds, then close to restart the sequence of operation.
(Models with an optional LED will flash the LED to indicate
ignition lockout.)
• Flameout: Loss of proven flame.
• 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 off 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 satisfied.) 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 flame
sensor detects pilot flame and signals the control to energize
the main valve. The main gas valve will not be energized until
the flame sensor detects the presence of pilot flame. The
spark will continue until flame is sensed at the pilot burner or
the trial-for-ignition time has elapsed, whichever occurs first.
If the pilot flame is not sensed before the end of the trial-for-
ignition time, the control may:
• proceed to interpurge, followed by another trial (three
trial models whose first two trials produced no flame).
• proceed to a lockout (no automatic recycle models whose
final trial produced no flame).
• proceed to a recycle delay period followed by another trial
(automatic recycle models whose final trial produced no
flame).