The cold-water control set on an unvented cylinder

Start at the cold main and follow the inlet-control group towards the unvented cylinder. Each device prepares or protects the supply before it reaches the vessel.

1. Line strainer: catches grit and debris so the finer controls downstream are not damaged or held open by dirt.
2. Pressure-reducing valve: limits a varying mains supply to the regulated inlet pressure specified for the cylinder and control set. Read that operating setting from the manufacturer’s data; it is distinct from the cylinder’s maximum design pressure and the relief-valve settings.
3. Check valve: a single check valve holds heated, expanded water on the cylinder side, so it cannot warm the cold supply, reach the main or push warm water to the cold taps.
4. Expansion vessel and expansion relief valve: with the route back to the main closed, the vessel, or an internal air gap, takes up the expansion. The expansion relief valve is the pressure backstop if the vessel fails or the pressure-reducing valve lets pressure climb.

Why the order matters

Each control sits where it can do its job: pressure control protects the parts downstream, the check valve guards the supply, and the expansion connection sits where it can take up and relieve expansion. Rearranged, the same parts can trap expansion or bypass protection, even though every component is present. Adding a non-return valve to a supply closes the route that expansion used to take, so review the expansion provision whenever one is fitted.

Two relief valves, two jobs

• The expansion relief valve responds to pressure. It does not sense temperature.
• The temperature relief valve, in the T&P valve on the cylinder, responds to dangerously high water temperature.

They guard against different failures and are not interchangeable. If the expansion relief valve weeps, check both the vessel charge and the pressure-reducing valve. Temperature itself is controlled by the thermostat and the non-self-resetting cut-out, not by any part of the cold-water set. Follow the manufacturer's arrangement and settings for the specific unit.

Safe delivery temperature

A cylinder must be stored hot enough for water hygiene, while very hot water delivered to taps can scald. Set the storage control thermostat appropriately, then limit the temperature people receive with thermostatic mixing valves at outlets used by children, older people or anyone less able to react. Lowering storage temperature alone is not the solution.

Some stores, such as heat stores, solar cylinders and solid-fuel systems with no control between the heat source and the vessel, can hold water above 80°C in normal use. Where that can happen, fit an in-line tempering valve at the vessel outlet so the supply to the hot water distribution pipework does not exceed 60°C. A bath's supply in a new dwelling is separately limited to 48°C by a blending valve.