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7.3.2. Identifying the Components and Operating Characteristics of Electronic Ignition Systems.
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7.3.2 Identifying the Components and Operating Characteristics of Electronic Ignition Systems Electronic ignition systems apply the flame rectification principle outlined in module 7.3.1. Service technicians must understand the operation of electronic ignition systems to successfully service mid and high-efficiency furnaces and boilers. In this module, you will learn to: • The purpose and function of electronic ignition systems • Types of electronic ignition systems • How to purge air and gas/air mixtures before placing electronic system-equipped appliances into service
Identifying the purpose and function of electronic ignition systems The purpose of electronic ignition systems is to provide a verifiable ignition source to establish the main burner flame and to monitor the flame throughout the burner operating cycle. Development of electronic ignition systems has been an important step in improving gas appliance efficiency by eliminating the need for standing pilots which consume gas without contributing directly to the productive heat transfer process.
Identifying Types of Electronic Ignition Systems Three types of electronic ignition systems will be discussed in this section: • Intermittent Spark Ignition (ISI) • Direct Spark Ignition (DSI) • Hot Surface Ignition (HSI)
In general, each of these systems is made up of the following components that provide these functions: • an ignition module that responds to a signal from the gas appliance thermostat provides voltage to • an ignition device which serves to start gas combustion; • a flame sensing device which produces a flame safety signal sent back to the ignition module, and • a gas burner valve which is energized to open by a signal from the ignition module. • A lockout device is also part of the system and serves to shut down the appliance if the ignition of the gas cannot be confirmed within a predetermined time.
Always read and follow manufacturer’s instructions for service and installation of system components. • Never substitute a component or part that is not a manufacturer’s designated replacement part for the particular appliance being serviced. • Electronic ignition modules are not field repairable. • Use only factory supplied wiring components specified for use with the particular electronic ignition system being serviced.
Intermittent Spark Ignition Systems Intermittent spark ignition systems use a pilot burner as the ignition device. The pilot burner is not a standing pilot, and therefore is only ignited when the thermostat calls for heat. Identifying Components of an Intermittent Spark Ignition System Figure 1. Typical Ignition Wiring Diagram
High Voltage and Risk of Electrical Shock—DO NOT TOUCH OUTPUT TERMINAL. Disconnect power supply before servicing or replacing any connection to an ignition control module. EXPLOSION HAZARD—Turn off gas supply at appliance shutoff valve before servicing. Ignition Control Module
Figure 2. Ignition Control Module The wiring terminal connections on the module shown are located at the lower edge of the module. Ignition modules respond to the call for heat signal from the thermostat and generate the ignition spark to the pilot assembly and thereafter send power to the pilot valve. The ignition module receives a flame proved signal at the “sense” terminal when the pilot flame is established.
Spark Ignition Pilot Assembly The design of pilot assemblies will vary from manufacturer to manufacturer, but each design provides for a spark electrode, a pilot orifice, burner and hood assembly, and a target electrode for flame rectification. The assembly shown in Figure 3 uses a flame sensor rod as part of the flame rectification circuit. Figure 3. Ignition Pilot Assembly Many ignition control modules provide a “pre purge time” before delivering the spark and gas to the pilot ignition assembly. With these systems there is a short delay between the thermostat call for heat and the time of pilot spark and gas delivery to the pilot assembly.
Figure 4. ISI Control Valve Gas Valve Gas valves used in ISI systems are combination valves. One valve is required for the pilot gas supply—the second valve is required for main burner supply. Each valve must be wired to the proper terminal on the ignition control module.
Operating Sequence Resetting The System After A Lockout. To reactivate the system, on most electronic ignition systems, turn the thermostat to its lowest position for ten seconds and then return it to the desired thermostat setting. Always read and follow manufacturer’s instructions.
Direct Spark Ignition Systems (DSI) Direct Spark Ignition Systems light the main burner without the use of a pilot burner; a spark igniter electrode is placed near the main burner and is the ignition device for the system. Figure 5. Diagram of a Direct Spark Ignition System
When the burner flame is established, a flame proved signal produced by flame rectification is required to maintain the main burner gas flow, or on high-low burner designs, to open the second stage gas valve for high burner operation. Figure 6. DSI Ignition Control Module Gas control valves in DSI systems do not have the pilot burner valve used in ISI systems. DSI control valves may be single main burner valves or combination valves that provide multiple burner control or 2-stage burner control in the case of high/low systems.
Hot Surface Ignition Systems (HSI) Hot Surface Ignition differs from the spark systems primarily due to its ignition device. Figure 7 illustrates a silicon carbide igniter used in HSI systems Figure 7. Hot Surface Igniter The hot element will rapidly heat to temperatures in excess of 2500° F when supplied its operating voltage. This high temperature provides a dependable ignition source which is especially useful in outdoor and roof top furnace applications where lockouts may be a problem for ISI and DSI systems.
Figure 8b. Diagnostic LED Panel Ignition Module Figure 8a. HSI Ignition Control Module The unit is equipped with an LED indicator lamp which signals the system component that is faulty by flashing a trouble code in the event of a component failure.
Gas Control ValvesHSI control valves must be matched to the ignition system, as is the case of the other electronic ignition systems. The control valve used with a particular appliance will be specified by the manufacturer to provide the proper gas volume and the burner design such as a single rate burner or a two-stage high/low burner system.
EXPLOSION HAZARD—Turn off gas supply at appliance shut off valve before servicing. Operating Sequence
Purging air and gas/air mixtures before placing electronic ignition system-equipped appliances into service A word of caution is in order here: Because the appliance may have to be cycled through several lockout and ignition attempts (especially due to the low volume of gas that flows through the pilot burner assembly on ISI systems) technicians are sometimes tempted to vent air and gas/air mixtures near the appliances to speed up the purging of air from the gas distribution lines. This practice may result in injury or property damage. NFPA 54 and other gas safety codes state that the open end of piping systems being purged of gas/air mixtures and gas must not be discharged into confined spaces or areas where ignition sources are present unless precautions are taken to perform this operation in a safe manner by ventilation of the space, control of purging rate, and elimination of all hazardous conditions.
Time to See If You Got the Key Points of This Module… • Complete the Review on pages 11 - 13. • See if you are ready for the Certification Exam by checking off the performance criteria on page 14.
