Auto reignition

Auto reignition is a control mechanism used in gas burners and gas-fired appliances to monitor whether the burner flame is present and to automatically reactivate the ignition source if the flame goes out. The system is designed for both safety and convenience—preventing unburnt gas accumulation and automatically maintaining operation without manual relighting.^[1]

Overview

When a gas burner is operating, continual sparking or heating of an igniter is unnecessary once ignition is established. An auto-reignition system senses the presence of flame and disables the igniter under normal operation. If the flame is extinguished while gas continues to flow—due to a gust of wind, vibration, or fuel interruption—the controller automatically restarts ignition to relight the burner.^[2]

Auto-reignition systems therefore combine flame detection (or supervision) and automatic ignition control. In many designs, the same electrode is used for both ignition and flame sensing.

Flame detection

To determine whether the burner flame is present, auto-reignition systems use one of several flame detection methods, which are also common in flame supervision devices (FSDs):

• Flame rectification / conductance – The flame acts as a weak conductor that allows a small DC current to flow between the ignition electrode and ground. If current is detected, the control system confirms that the flame is present.^[3] This principle is used in many direct spark ignition systems for cooktops and ovens.^[4]

• Thermocouple / thermopile sensing – A thermocouple positioned in the flame produces a small voltage when heated. If the flame goes out, the voltage drops and the safety circuit cuts off gas flow. This method is common in older pilot systems.^[5]

• Optical (UV/IR) sensors – Some industrial or large heating systems use ultraviolet or infrared sensors to detect the characteristic light of a flame.^[6]

Control logic

Once flame presence is verified, the ignition controller transitions between several modes:

• Ignition mode: Activates the spark generator or hot surface igniter (HSI) when gas flow begins.
• Flame sensing mode: After successful ignition, sparking stops and the system monitors flame continuity.
• Re-ignition mode: If flame failure occurs while gas is flowing, ignition restarts automatically until flame re-establishes.

Some controllers maintain ignition for several seconds after initial flame detection as a proof period.^[2] More complex systems, such as the Robertshaw 780 series direct spark modules, integrate separate flame-sense and ignition circuits to manage this behavior.^[7]

Safety features

If repeated ignition attempts fail, most auto-reignition systems shut off the gas supply and enter a "lock-out" state until manually reset, ensuring that unburnt gas does not accumulate.^[8] Gas appliance standards typically require automatic shut-off within a defined time if flame failure persists—for example, within 2½ minutes for pilot gas supplies.^[9]

Applications

Auto-reignition technology is used in a wide range of gas-fired equipment:

• Domestic cookers, hobs and ovens – Many modern appliances use electronic ignition systems with automatic reignition to relight burners if the flame is blown out.^[1]
• Gas fireplaces and log sets – Auto-reignition maintains continuous flame operation and safety in decorative gas fires.^[10]
• Outdoor grills and fire pits – Some ignition controllers automatically reignite flames lost to wind or rain.^[11]
• Industrial burners – Larger combustion systems employ auto-reignition as part of an integrated burner management system for safety and efficiency.^[12]

Advantages and limitations

Advantages

• Enhances safety by relighting flames automatically and preventing unburned gas release.
• Provides convenience by maintaining operation without user intervention.
• Reduces igniter wear and energy use by disabling ignition once flame is established.

Limitations

• Adds electronic complexity and cost compared with manual or pilot ignition systems.
• Can mis-detect under certain flame conditions (e.g. fouled electrode, weak flame).
• Requires electrical power and may not function during power outages.

See also

• Flame supervision device
• Ignition system
• Thermocouple