Light back occurs about the appliance burner and is one of several conditions that can give rise to the emission of carbon monoxide at high levels. If not recognised or attended to such emissions can cause serious illness or, in extreme circumstances, death.

Light back occurs when the speed of the gas/air mixture is reduced in relation to the flame speed so as to cause the flame to burn behind the burner port. The flame can then travel back to the injector. The emission of carbon monoxide at high levels can result, even with an extremely low gas flow rate.

Light back may be temporary, may be difficult or impossible to reproduce and may occur with or without any design or manufacturing fault with the appliance.

Light back may occur

  • With a variety of appliances, when the injector or aeration port is partially blocked (in some cases from the webs or bodies of spiders about the injector and aeration port); or
  • With radiant heaters, from damage to the burner plaque or cement holding the plaque in place; or
  • When the appliance user has inadvertently failed to turn gas off completely (This is believed to have been the case in a double fatality with an LPG cooker in a caravan and a “near miss” event with a cabinet heater in a bedroom).

The possibility of light back should be considered wherever otherwise unexplained carbon monoxide poisoning has occurred or is suspected (e.g. from reported illness or unusual smell associated with appliance use) with a gas appliance.

The possibility of light back should be considered wherever any of the following conditions exist: 

  • Disappearance of the flame from the burner or burner plaque, possibly preceded by flaring about the burner, has been observed or reported; or
  • Except where the gas flow rate is exceptionally low, a roaring noise has been observed or reported about the burner.
  • When gas flow to the burner has been significantly reduced (e.g. with the burner control left on slightly or the gas supply valve not completely closed or passing gas).
  • Where there is evidence of carbon monoxide emission from the appliance even after gas has been reportedly turned off.
  • Where there is any otherwise unexplained evidence of carbon monoxide emission from the area of a gas appliance.

In such cases:

  • The ratio of carbon monoxide to carbon dioxide and the level of carbon monoxide should be measured in the products of combustion to detect abnormal combustion (Due to the sometimes temporary or transient nature of light back, however, such measurements will not necessarily provide evidence of light back).
  • The area around and behind the burner should be inspected for sooting or heat-induced colouration change.
  • The area of the injector and aeration port should be checked for actual or potential sources of partial blockage.
  • Checks should be carried out for leakage about the injector.
  • The appliance manufacturer or manufacturer’s agent should be consulted and the appliance checked to ensure the injector is of the correct size and that the appliance otherwise complies with the manufacturer’s specifications.

Where the appliance is known or suspected to have emitted high levels of carbon monoxide (by light back or any other means), it should not be returned to service unless the cause of the emission has been positively identified and eliminated.