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Gas Safety

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

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Chimney and Flue

Flue Types

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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

Flue Categories

  • A, B, C - flueless, open flues, room sealed
  • 1st digit - denotes method of arranging discharge of combustion
    • 1 - with draught diverter
    • 2 - with out draught diverter
  • 2nd digit - denotes flue draught method
    • 1 - natural draught
    • 2 - fan downstream of heat exchanger
    • 3 - fan upstream of heat exchanger
    • 4 - fan downstream of draught diverter
  • Category A

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Open-flued Systems

Open-flued systems
  • Category B
  • Balanced Compartments
    • Self-closing and draught proof door
    • Switch fitted to the door that turns off the appliance if the door is opened
    • Door shall not open into a bathroom
    • If over 12.7kW shall not open into bedroom
    • 'Keep closed' notice on the door
    • No other ventilation into compartment
  • 4 parts of an open flued appliance
    Parts of an open flue
    • Primary Flue
      • Part of the appliance
      • Creates the initial flue pull to clear the products of combustion
    • Draught Diverter
      • Diverts any down draught from the burner
      • Allows dilution of flue products
      • Breaks any excessive pull on the flue
      • Usually part of the appliance
    • Secondary Flue
      • Horizontal runs must be avoided
      • Bend minimum 45°
      • Keep internal where possible
      • Minimum 600mm vertical rise to the first bend
      • Cross sectional area of flue must be at least equal to that of the appliance
    • Terminal
      • Stop birds nesting or leaves entering the flue
      • Minimise down draught
      • Help the flue gases discharge
      • Terminals are not required for chimneys with a flue size greater than 170mm if no nesting will occur
  • Vertex Flue
    • Air supply taken from the roof space
    • Draught Diverter min height is 300mm above the loft insulation
    • The flue above the Draught Diverter must run for 600mm before any bend
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Room-sealed Flue Systems

Room-sealed Flue Systems
  • Category C
  • Balanced (natural draught) flue
  • Fan-assisted flues
    • negative pressure within the casing
    • positive pressure within the casing
  • Termination options
    • Vertical room-sealed flue termination
      Vertical room-sealed flue termination
    • SE/U Duct room-sealed flue termination
      SE/U Duct room-sealed flue termination
      • C2 and C4 appliances (room sealed) may be installed
      • All air inlets should be labelled
      • All appliances should be labelled that flue system is shared
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Shared Flues

  • Each appliance shall have:
    • draught diverter
    • flame failure device
    • atmospheric sensing device
    • flue must be the correct size
    • chimney must have access for inspection and maintenance
    • appliances labelled that they share a flue

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