The group of inlet controls that condition the cold supply to an unvented cylinder and what each one does.
An unvented cylinder is fed from the cold mains through a group of controls on the inlet, and G3 expects you to know the parts of that cold-water control set and what each does.
A line strainer is usually first, catching grit and debris from the supply so the finer controls downstream are not damaged or held open by dirt.
A pressure reducing valve sets and holds a steady working pressure for the cylinder, so the unit sees the pressure it is designed for rather than a fluctuating mains pressure that could be too high.
A check valve, a single check valve, prevents backflow, stopping heated or stored water being pushed back into the cold main, which is both a wholesomeness and a control requirement.
An expansion device deals with the water that expands as it is heated. Because the check valve stops expansion pushing back into the main, the system needs somewhere for that extra volume to go, provided by an expansion vessel or a built-in air gap, with an expansion relief valve as a safeguard on the cold side if pressure rises too far.
For revision, picture the order from the main inwards: strainer, then pressure control, then backflow protection, then the expansion provision, each doing a distinct job. The exact components, arrangement and settings come from the current standard and the manufacturer instructions for the specific unit, and the temperature-side safety controls are separate from this cold-water set.
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