The G77x family of controls are designed for indirect ignition of various gas types, including natural gas, Liquefied Petroleum (LP) gas, manufactured gas, mixed gas, or LP gas-air mixtures. These controls are suitable for gas-fired equipment with a maximum firing rate of 117 kW (400,000 Btu/hr). For applications exceeding this rate, prior written approval from the Johnson Controls Heating Products Engineering Department is required. All G77x applications must utilize a redundant gas valve system where the pilot and main valve seats are in series and open sequentially for intermittent pilot ignition.
Important Technical Specifications:
- Ignition Type: Indirect
- Ignition Source: High voltage spark, capacitive discharge
- High Voltage Cable Maximum Length: 1,220 mm (48 in.)
- Flame Sense Cable Maximum Length: 1,220 mm (48 in.)
- Flame Detection Means: Flame rectification
- Flame Detection Types: Integral and remote
- Minimum Flame Current: 0.15 microampere
- Flame Failure Response Time: 0.8 second maximum (1.5 seconds maximum for G776_G models only)
- Spark Gap: 2.5 mm (0.1 in.) nominal
- Ignition Trials: 1 or 3
- Ignition Trial Times: 8, 25, 50, 85, or 120 seconds (60 Hz operation; timings increase 20% for 50 Hz operation)
- Prepurge Times: 0, 4, 15, or 30 seconds (60 Hz operation; timings increase 20% for 50 Hz operation)
- Interpurge: 0, 15, or 30 seconds (60 Hz operation; timings increase 20% for 50 Hz operation)
- Automatic Recycle Delay Period: No recycle (G770, G777); 5 minutes (G775, G779); 60 minutes (G776, G778) (60 Hz operation; timings increase 20% for 50 Hz operation)
- Power Requirements: Control: 24 VAC, 60 Hz, nominal; Operation Current: 0.2A nominal + valves
- Contact Rating: Main Valve: 2A continuous, 5A inrush; Pilot Valve: 2A continuous, 5A inrush
- Ambient Operating and Storage Temperature: -40 to 70°C (-40 to 160°F) for standard models; -40 to 80°C (-40 to 175°F) for high temperature models
- Humidity: 95% RH non-condensing
- Wiring Connections: Spark: Spike, Rajah, or 6.35 mm (1/4 in.) male spade; Control: 6.35 mm (1/4 in.) male spade, 4.76 mm (3/16 in.) male spade optional
- Types of Gas: Natural, Liquefied Petroleum (LP), manufactured, mixed, or LP gas-air mixture
- Agency Listing: CSA (AGA/CGA) Certificate Number 164933-1073441; Australian Gas Certificate Number 4524 (G770 models only)
- Specification Standards: ANSI Standard Z21.20, Z21.20a, Z21.20b; CAN/CSA-C22.2 No. 199
Usage Features (Operating Mode Definitions):
- 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 this time.
- Run: Pilot and main gas valves remain energized, and spark is turned off after successful ignition.
- Shutoff: Occurs if pilot gas does not ignite within the trial-for-ignition time. The control de-energizes the spark circuit and pilot valve, then enters the automatic recycle period.
- 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. To restart, open thermostat contacts for 30 seconds, then close them. Models with an optional Light-Emitting Diode (LED) will flash to indicate ignition lockout.
- Flameout: Loss of proven flame.
- Hard Lockout: The control detected a fault. To restart, open thermostat contacts for 30 seconds, then close them. Models with an optional LED will turn off to indicate a hard lockout.
Sequence of Operation:
The control is energized on a call for heat from the system thermostat. Models with an optional LED will turn on (steady) until the call for heat is satisfied. If a vent damper is used, it is energized, and when fully opened, it energizes the ignition control. If equipped with optional prepurge, the appliance prepurge fan or relay is also energized through the thermostat contacts. In prepurge mode, the control delays for the selected time (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 opens, and spark initiates 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 energize until the flame sensor detects the pilot flame. The spark continues until flame is sensed at the pilot burner or the trial-for-ignition period elapses, whichever occurs first. If the pilot flame is not sensed before the end of the trial-for-ignition period, the control may:
- Proceed to interpurge, followed by another trial (for three-trial models whose first two trials produced no flame).
- Proceed to a lockout (for no automatic recycle models whose final trial produced no flame).
- Proceed to a recycle delay period followed by another trial (for automatic recycle models whose final trial produced no flame).
If the pilot burner flame extinguishes during the run state (flameout), the control de-energizes the pilot and main gas valve for the interpurge period. After this period, another trial for ignition is initiated. If the flameout cycle repeats for a total of 16 times (pilot burner flame established then lost), the control will:
- Enter the lockout mode if it is a G770/G777. The thermostat contacts must be opened for 30 seconds and then closed to escape the lockout condition.
- Enter the shutoff mode and recycle period (5-minute recycle for G775/G779 and 60-minute recycle for G776/G778) before beginning another trial for ignition, starting with prepurge.
Rollout Switch Function:
The rollout switch is wired into the G77x ignition control at terminal R1. This normally closed switch detects flames rolling out of the combustion chamber. If rollout occurs, the switch contacts open, and the G77x immediately enters a lockout condition, closing the main and pilot valves to prevent system function. To escape lockout, the thermostat contacts must be opened for 30 seconds, then closed. Once the rollout switch contacts return to the closed position, the G77x will restart its operating sequence when the thermostat contacts close.
Maintenance Features:
- Installation and Wiring: These instructions are a guide for qualified personnel. Follow all instructions in this bulletin and on the appliance. Limit repairs, adjustments, and servicing to operations listed in this bulletin or on the appliance.
- Location/Mounting: Choose a location that provides the shortest, direct cable route to the spark electrode, pilot burner/igniter-sensor assembly. Ensure easy access to terminals for wiring and servicing. The control can be mounted in any position on a grounded metal surface using metal screws or bolts through the mounting holes. Avoid mounting where it will be exposed to direct infrared radiation from the main burner or temperatures exceeding the maximum product temperature rating.
- Pilot Burner/Igniter-Sensor Installation: Instructions are typically provided by the appliance manufacturer. If not, mount the pilot burner/igniter-sensor securely to the main burner with metal screws, approximately 9.52 mm (3/8 in.) above and 6.35 mm (1/4 in.) away from the center of the nearest main burner port. Ensure the pilot burner flame is properly positioned, receives ample air, and is not exposed to drafts, the full force of main burner ignition, or falling scale. Ensure main burner flames do not impinge on any part of the pilot burner.
- Wiring Precautions: Connect the high voltage cable to the spark transformer terminal and spark electrode (pilot burner assembly) before applying power. Ensure the ground wire is attached to the pilot burner and control ground terminal strip. Locate all limit and operating controls in series with the thermostat terminal (THS 2) on the ignition control. Use a National Electrical Code (NEC) Class 2 transformer to provide 24 VAC under maximum load. The high voltage spark transformer cable must not be in continuous contact with a metal surface; use standoff insulators. Separate the flame sensor wire and high voltage spark transformer cable by a minimum of 6.35 mm (1/4 in.) and do not wrap them around any pipe, other wiring, or accessories.
- Checkout and Startup Procedure: Before leaving the installation, perform a shutoff test to ensure all components function properly and there are no gas leaks. This involves turning off gas and thermostat, powering on, verifying shutoff sequence, turning off thermostat, turning on gas and purging lines, checking for gas leaks, verifying successful ignition and run condition, rechecking for gas leaks, and verifying successful ignition multiple times.
- Thermostat Heat Anticipator Settings: The anticipator setting should equal the ignition system current draw plus that of the pilot and main valve. Measure the actual current draw of the heating system at the thermostat location using an AC ammeter or a clamp-on ammeter with a 10-turn multiplier.
- Replacement Procedure: Shut off power and gas. Label all wires before disconnecting. Disconnect power supply, thermostat lead, sensing probe lead (if applicable), high voltage cable, pilot valve, and main valve leads. Disconnect ground wires. Remove and discard the old control. Mount the new control using the same holes. Refer to installation and wiring sections for electrical connections. If replacing a control with a Rajah or male spade connection, modify the existing ignition cable by cutting off the crimp-on connector and connecting it to the Spike transformer. Perform the checkout and startup procedure after replacement.
- Troubleshooting (Service Checkout Procedures): If the system does not function properly, perform preliminary checks: verify tight connections, correct wiring, gas inlet pressure, power, thermostat call for heat, LED status (flashing for recycle/lockout, off for no power/fault). Faults include flame present for more than 30 seconds at call for heat or a defective control. Identify the failure condition (no spark, spark present but pilot doesn't light, pilot lights but main burner doesn't come on) and use the respective flowcharts for troubleshooting. Perform checkout and startup procedures after any servicing.
- Repairs and Replacement: G77x controls are not field repairable. Use only an exact or factory-recommended replacement control. Other accessories (flame sensors, electrode assemblies, pilot assemblies, and leads) can be obtained through the original equipment manufacturer or a Johnson Controls distributor.
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