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Gas Safety

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

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Portable Combustion Gas Flue Analysers

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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

Electronic Gas Flue Analyser

  • Should confirm to BS EN 50379-3 (BS7927:1998)
  • Should measure:
    • O2 oxygen
    • CO carbon monoxide in parts per million (ppm)
    • CO2 carbon dioxide
    • CO/CO2 ratio
    • Temperature
    • NO Nitrogen monoxide
    • Pressure
    • Excess air
    • Combustion efficiency
  • Most analysers do not measure the absolute levels of CO and O2
  • Analysers are not designed for continuous measurement
  • Should not be used to identify leakage of combustion products
  • Not all analysers can record the maximum CO reading
  • Accuracy
    • Readings at or below 20ppm - ±3ppm
    • Readings above 20ppm - ±5%
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Gas Analyser Preparation and Use

  • A small bottle of calibration gas can be used to check the sensor
  • Analyser reading can be affected by:
    • cooking activities
    • cleaning fluids
    • polishes and paints
  • Multi-hole sample probe for use on gas cookers
  • "Zero" procedure - O2 reading in outdoor air is 20.9%
  • Do not smoke before or during the test
  • Preparation for testing:
    • Position sampling probe 2m above the floor level and 1m away from appliances
    • Record the level of CO over 15min period
      • there shouldn't be any rise of CO level
  • Testing
    • With only one appliance on measure the CO level
    • If at any time the CO reading exceeds 30ppm (except when testing cookers):
      • stop the test
      • turn off all appliances
      • open all windows and doors to ventilate the room
      • and vacate the room under test
    • Flueless Cookers Testing
      • Ensure that ventilation is unobstructed
      • Put 2 saucepan (with 1 liter of water; 160 to 220mm base) on 2 largest pan-supports
      • Place the grill pan in its highest position under the grill
      • Light the grill, oven (at mid-range) and 2 hotplate burners at maximum
        • Turn hotplates down to simmer when water boils
        • Turn the grill off after 30min
      • Record CO levels at 1min intervals:
        • Reading doesn't exceed 30ppm - pass
        • Reading doesn't exceed 30ppm for 20min and falling not exceeding 90ppm - pass
        • Reading exceeds 90ppm - fail
    • Flueless Water Heaters
      • Light the appliance at maximum rate for 5 minutes recording the CO levels at 1 min intervals
    • Flueless Space Heaters
      • Light the appliance at maximum rate for 30 minutes recording the CO levels at 1 min intervals
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Combustion Performance (CO/CO2 ratio) Action Levels

Appliance type CO/CO2 ratio
Back boiler unit Boiler unit 0.008
In combination with fire 0.020
Central heating boiler 0.008
Circulator 0.010
Flued space heater Fire Convector Heater 0.020
Flueless space heater Fire Convector Heater 0.001
Flueless cabinet heater (LPG) 0.004
Water heater - flued and flueless 0.020
Warm air heater 0.008
Flueless cookers Cooker oven 0.008
Cooker hob Assess flame picture
Cooker grill (CE marked) 0.010
Cooker grill (non CE marked) 0.020
Range oven (flued) 0.020
Refrigerator (LPG) 0.007
Tumble drier Flued 0.010
Flueless 0.001
Gas lights (LPG) 0.020
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