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Multiblocs are designed to protect unvented cylinders by controlling the mains supply pressure that goes to the tank, and are comprised of a pressure reducing valve, a pressure relief valve, and an internal check valve. The expansion vessel connection on the manifold is screwed into a ¾” female socket or a ¾” female to compression adaptor, if the vessel is going to be installed with copper pipe.

Relief Valves

Pressure Relief Valves (PRVs) also known as Safety Release Valves, prevent the build-up of system pressure, commonly seen in domestic unvented cylinders.
All manufacturers will advise that all safety control devices should be annually maintained and serviced.
Relief Valves

Expansion Vessel

Potable expansion vessels are built using different materials than heating expansion vessels to ensure that the water is contaminant free and safe for human consumption. When fitted to an unvented cylinder, the internal pressure of the vessel is set to match that of the pressure reducing valve expansion vessels which are a vital component in all sealed heating systems and unvented cylinders.
Expansion Vessel

Temperature Pressure Relief Valve

A Temperature Presssuere Relief Valve (TPRV) is designed to protect against over pressurisation of hot water, which can occur within an unvented hot water cylinder. The valves should be fitted to the upper portion of a hot water cylinder. If a TPRV fails, the system should not be used until this part is replaced.
Relief Valves

Immersion Heaters

An immersion heater is an electric water heater that sits inside a hot-water cylinder. They can be made of copper, incoloy or titanium and are available in varying element lengths and boss thread sizes. In hard water areas, it's recommended to use an incoloy or titanium element. Immersion heaters can either be used as the primary water heater, or as a backup water heater if the cylinder is being heated indirectly by a boiler.
Immersion Heaters

Controls & Thermostats

Where hot water is stored in a cylinder and heated by a separate boiler, a dual control thermostat is required.The dual cylinder thermostat combines both the controller and limit thermostats.The thermostats are designed to restrict the water temperature to around 65°c and have an integrated reset button. Older style unvented cylinders would have a single control thermostat and a separate cut out thermostat (Prior to 2010 regulations).
Controls & Thermostats

Further advice on controls

Plug In-Style Thermostat
As stainless-steel unvented cylinders are becoming more commonplace, the factory fitted immersions can vary, most of them will use a rod style thermostat with a different colour head, and there are some subtle differences between them:
RTS 3 – Brown Cap
Dual function rod / stem thermostat, with an adjustable trip and safety cut-out. Both functional and safety cut-out are bi-polar with bimetallic disc.
Unival - Blue Cap
Unipolar stemmed type thermostat, for single fuse voltage bipolar bimetallic safety device with manual reset.
TSR - White Cap
Direct plug-in thermostat with 1-pole temperature regulation and 2-pole safety limiter.
TSE – Green Cap
Direct plug-in rod combination thermostat with 1-pole temperature regulation and 2-pole safety limiter.
All of these thermostats are designed to work with unvented cylinders, but the TSR versions are better suited to cylinders with a built-in air gap, as they are less prone to being influenced by any ambient heat build-up and also the temperature is read in the stem triggering a reaction in the head making them more accurate when used with this type of cylinder.
Cylinder spares suppliers

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