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Gas Safety

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

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Components and Safety Devices

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Code of Practice



Gas Supply

Gas Meters

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 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

Diverter Valve Working Principle

Diverter Valves in Gas Combi Boliers

Most combi gas boilers have a complicated hydraulic circuit with a circulator pump and a diverter valve. There are two types of diverter valves. The first is a mechanically operated valve powered by a motored diverter head (just like a normal y plan system). Either switches to hot water or heating depends on what is being called for. The second one found on the SPA combis is a hydraulic three way valve. These have diaphragms in that are connected to push rods which operate microswitches to fire the boiler up. They work off a pressure differental valve in hot water mode and in heating mode its the pump.

  • The valve piston moves inside the 3-way valve cartridge in the axial direction. The piston is operated by the lever arm (shaft), which is operated by the actuator.
  • Due to the special principle of the valve, the shaft seal is not dynamically loaded by axial movement but statically loaded by a tilt motion. This prevents deposits at the shaft and extended wear.
  • The actuator is usually a hydraulic diaphragmconnected to the HW
    supply system. Opening a tap causes a drop in pressure on one side of
    the diaphragm, which is pushed by the pressure on the other side,
    operating 3-way valve. There are electric diverter valves (e.g.,
    Honeywell) but they're far too slow for this application.

diverter valve operation diagram



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