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Gas Safety

Exam revision materials for Plumbing Level 2&3 and ACS Gas Safety test

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Safety Devices and Controls

Flame Supervision Devices FSD

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Safety Devices and Controls

Air/Gas ratio valves

Appliances Controls

Flame Supervision Devices FSD

Gas Controls

Gas Meter Controls

Safety Cut-off Devices

Spare Part Identification


Open-vented System


Gas Appliances

Ventilation Sizing

Unsafe Situations

Dealing with Unsafe Situations

Emergency Notices and Warning Labels

Gas Leak




Tightness Test

Gas Performance

Gas Rate

Heat Input

Gas Supply

Gas Meters

Gas Pipework

Gas Fittings and Pipes

Gas Supply


New Installation

Pipe Protection

Pipework Sizing

Chimney and Flue


Flue Construction

Flue Inspection and Testing

Flue Terminals

Flue Types


Carbon Monoxide Poisoning

Combustion Process

Natural Gas

Portable Combustion Gas Flue Analysers


Components and Safety Devices

Thermal FSD

Bi-metallic Strip
bi-metal strip flame supervision device 
  • Found on older cookers and water heaters (over sink heaters)
  • Relies on different expansion rates of metals
  • Takes 90 seconds to close the valve
  • Doesn't shut of the pilot
Liquid Expansion FSD
Liquid expansion flame supervision device
  • Found on modern cookers and water heaters (circulators)
  • Found on older room heaters, central heating boilers and water heaters
  • If the flued is lost from the device the liquid expansion FSD is designed to fail to safe
  • They are faster acting than bimetal strips but are slower than the other alternatives
  • Usually fitted on a cooker in conjunction with a liquid expansion mechanical thermostat
  • Will close the valve to the oven if there is no flame
  • Delay is usually 30 seconds
Thermoelectric FSD (thermocouple):
Thermo-electric flame supervision device
  • Found on the majority of open flued and room sealed central heating boilers, water heaters and modern gas fires
  • Utilises a chrome-nickel alloy and copper to produce between 15 to 30mV
  • Delay is usually 20 seconds before thermo-electric gas valve will close down
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Electronic FSD

Flame Conductance FFD
Flame conductance circuit
  • Very rarely used now
  • Utilizes D.C. current
  • The major fault - is a built up a carbon soot between the electrode and the burner
Flame Rectification FFD
Flame rectification circuit
  • Commonly found on fanned flue appliances with full automatic ignition
  • Utilizes AC current
  • The flame completes an electric circuit using its ions
  • This system fails safe in case any build up of carbon etc.
Testing Electronic Devices
Testing a flame rectification circuit
  • Test the reaction time of the device - turn off the gas when the boiler is lit
    • re-igniting or 'lock-out' will occur
  • 'Lock-out' will occur :
    • Flame lift - will cause the break in the circuit
    • Accidental earthing of the probe
    • Incorrect gas/air mix
  • Connect ammeter in series in accordance with manufacturer's instructions to indicate the right amperage
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Vitiation Sensing Device (VSD)

Also known as: Atmosphere Sensing Device (ASD); Oxygen Depletion Sensor (OSD); Oxy gas Pilot

Opeation of Vitiation Sensing Device

  • When CO2 level in combustion air to the pilot burner reaches 1.5 to 2% or 100ppm (0.01%) of CO the VSD will shut down the appliance
  • Regulations 26(9) - checks and tests to prevent incomplete combustion
  • Supplied as a complete spare unit - have no serviceable components
  • Used in flueless space or water heaters, gas fires/back-boiler units
  • Precise position on the burner is important
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