Thermal FSD

Bi-metallic Strip

• 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

• 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):

• 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

• 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

• 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

• 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

• 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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