They use inlet pressures ranging from 1.5 bar to 3.5 bar, and are available in both indirect and direct form. They can be ‘bubble-top' or have an external expansion vessel to contain the expansion volume of water on heat up. There are also direct oil and gas fired units.

They can be heated by an electric immersion heater, or natural gas, LPG or oil fired boilers, but not with solid fuel boilers as these cannot be controlled sufficiently.

Building Regulation G3 requires that anyone installing an unvented hot water cylinder must demonstrate that they are competent.

Ideally, the mains water pressure at peak times should be a minimum of 2 bar dynamic, although all unvented systems will operate at pressures as low as 1 bar dynamic. The size of the incoming main will normally need to be 22mm copper or 25mm MDPE in order to give adequate flow rate.

The simple answer is no. Safety discharge pipework (sizes, materials and orientation etc) as detailed by Approved Document G3 of the Building Regulations. Any deviation must be authorised by the Building Inspectorate, so this is not something on which manufacturers can exercise variation.

Definitely not. Under no circumstances should an unvented cylinder be used with a solid fuel boiler because the primary heat source is not immediately controllable.

No you don't, because the two circuits are totally independent. If you aren't using a sealed system, then the boiler circuit must be pumped to the cylinder primaries and be both temperature and safety overheat controlled. Apart from that there are no restrictions, although the use of system boiler will enable a completely dry loft space.

Yes, to comply with Building Regulation G3 they must be supplied with a complete package of controls - either factory fitted, or supplied as an unvented kit - and they must be installed in strict accordance with the relevant manufacturer's instructions.

It is mandatory to fit the two-port valve and this must be wired through the energy cut-out. However, temperature control can be achieved via a three-port valve if one is already installed, but with new installations a second two-port valve should be used to serve the heating circuit. Both the control thermostat and energy cut-out should be wired to the cylinder two-port valve.

Current guidance notes do not cover the connection of a solar thermal circuit to an unvented storage vessel (cylinder). However, if guidance is sought for compliance with current regulations the fundamental principle is to provide a failsafe means of shutting off the solar input to the heat exchanger if the cylinder temperature should rise above the set temperature of the cylinder's energy cut out. (See Note 1). As with all unvented hot water systems, notification of intention to install should be given to your local building control. Option A. A non self resetting mechanical shut-off should be installed on the solar primary flow to the cylinder. The mechanical shut-off should be suitable for use with a solar primary circuit (i.e. high temperature and glycol resistant). The mechanical shut-off should be integrated electrically with the cylinder energy cut out/s and if necessary the solar circuit temperature control, please refer to the solar controller manufacturer for further information. Option B, Where the solar controller and hydraulic system demonstrate that by no lesser means the requirement in Option A is satisfied by other means; certification by an approvals body is required to demonstrate that in the event of the stored water going over temperature, the heat input to the cylinder is isolated by physical means and is non self resetting. These systems should be clearly identified with reference to the approvals body. (See Note 2) Note 1 : Whilst most solar cylinders use a coil type heat exchanger other options such as external plate to plate devices , external annulars or ‘tank in tank' systems may be used but the same control options always apply. Note 2 : Current approved bodies include the British Board of Agrement (BBA) , WRc-NSF Limited, or KIWA

The unit can be installed virtually anywhere within the building which reduces costs and allows greater flexibility of design. The superior performance ensures high flow rates and rapid filling of baths. There is less pipework because the cold water storage cistern is eliminated. Balanced hot and cold water pressures allow use of modern showers and taps.

Apart from the advantages referred to before, noise is reduced because there is no filling of the cold water storage cistern, and the risk of freezing or bursting of pipes in roofspaces is eliminated. The roof space is freed for use for storage or conversion. Risk of contamination of the water via the storage cistern is eliminated. The unvented cylinder has a white goods' finish, which is aesthetically pleasing.

All unvented cylinders must be supplied either complete, or with a kit of components comprising all the safety and other items with which the cylinder was tested and certified. These must be used to comply with the certificate. Most cylinders are supplied with some loose components. Depending on the exact type of cylinder, these may consist of:- *Expansion vessel, pressure reducing valve / filter, expansion relief valve / non-return valve, indirect thermostat, energy cut-out, two-port valve (spring return), immersion heater with control / safety thermostat. Other components, which you may need to supply, are: - *Programmer, secondary circuit pump and control components.

These controls fall into two groups - functional controls and safety controls.

They are required to protect the mains water supply from contamination and the unvented unit from over pressure, and consist of the following:- Line strainer which filters debris from the water supply to reduce the risk of damage to downstream controls. Pressure reducing valve which reduces the mains water pressure to the specified cold working pressure of the unit. Single check valve which prevents contamination of the mains water supply from backflow and crossflow between hot and cold distribution pipes. Expansion valve which protects the unit from over pressure caused by the failure of the pressure reducing valve, failure of expansion vessel or loss of internal air bubble.

The UVHWSS is mains fed via an inlet control group pre-set to the relevant manufacturer's specified pressures. As hot water is drawn from the unit it is replenished from the mains supply. As the water expands on heating, the volume of expansion is contained either in an expansion vessel or, in the case of bubble top units, an internal air bubble is generated within the unit as the system is commissioned. The traditional low pressure system is fed from a cold water storage cistern usually located in the roofspace. The hot water cylinder has an open vent which prevents the pressure increasing beyond the pressure derived from the static head created by the cold water cistern.

Safety controls are required to protect the householder and prevent the temperature of the water in the unvented system exceeding 99°C. They usually comprise the following:- Control thermostat which is usually set between 60 and 65°C. Energy cut out device with manual re-set usually set between 85 and 89°C. Temperature and Pressure Relief Valve set to 90 - 95°C.

They are manufactured from either copper, glass-lined steel or stainless steel, are covered with thermal insulation material and finished with a white easy clean finish.

There is no absolute minimum pressure required for an unvented cylinder. If it contains water, it will get heated. However, the lower the incoming pressure (while running) the lower the flow to the taps etc. Most manufacturers would recommend a minimum operating pressure of 1.5 Bar (21lb / in²) for reasonable performance. It should also be noted that standing pressure is no indication of whether or not an unvented cylinder will perform satisfactorily as the pressure may decrease dramatically with water flow.

When water is heated its volume increases, and because water cannot be compressed unless it is allowed to expand, its pressure will rise enormously. An expansion relief valve would relieve this situation by automatically opening at 6 Bar pressure and so allow water to escape from the system. However, UK water regulations/byelaws prohibit the regular waste of water if this is the only pressure relief device and so the increase in volume is taken up by the use of an expansion vessel (or expansion volume in the case of 'bubble-top' tanks). Either system allows the incompressible water to compress air until an equalisation of pressure occurs.