At What Pressures Will an Unvented Hot Water Cylinder Operate

HWA UNVENTED FAQs

What is an Unvented System? It is a hot water system where the water is held in a cylinder with no open vent to atmosphere. As there is no cold feed tank or vent pipe, the system does require safety controls and features to ensure the water temperature can’t exceed 99 °C and to control the pressure in the system. They are inherently safe and have a number of safety devices fitted to them. At what pressures will an unvented hot water cylinder operate? They use inlet pressures typically range from 1.5 bar to 3.5 bar, and are available in both indirect and direct form. A pressure control valve is supplied to control inlet pressure. They can be ‘bubble-top' with an internal expansion space 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. Can a UVHWSS be used with any fuel? They can be heated by an electric immersion heater, or natural gas, LPG, solar or oil fired boilers, and a range of appliances including wood burners, solar and heat pumps. Please see the note on solid fuel below. Can anyone install an unvented hot water cylinder? Building Regulation G3 requires that installations of unvented hot water cylinders are notified to Building Control. If the installer is a member of a Competent Persons Scheme this will happen automatically following the installation. Can I install an unvented system on any mains water supply? 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. Can I run the safety discharge pipework differently from the way it is specified in the manual? The simple answer is no. Safety discharge pipework (sizes, materials and orientation etc) are 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.

Can I use an unvented cylinder with a solid fuel boiler? There are circumstances where this is possible provided that once the cylinder reaches temperature the solid fuel heat source can be safely shut down or its heat diverted safely elsewhere. Do I have to use a sealed system boiler with an unvented cylinder? 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. Do unvented cylinders come with all the necessary controls? 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. Must I fit the two-port valve supplied with the indirect cylinder? It is generally necessary 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. Using an unvented cylinder with a solar thermal system Current G3 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

What are the benefits to specifiers and installers to be gained from using a UVHWSS? The unit can be installed virtually anywhere within the building which reduces costs and allows greater flexibility of design. The higher pressure performance should ensure 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. In basements there may be issues arranging discharge, and a tank and sump pump may need to be installed. In loft spaces access may be an issue. What are the householder benefits? 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. What components do I need to supply in order to operate the cylinder? 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/tun dish. Other components, which you may need to supply, are: - *Programmer, secondary circuit pump and control components. What do functional controls do and what components do they consist of? They are required to protect the mains water supply from contamination and the unvented unit from over pressure, and consist of the following:- A line strainer which filters debris from the water supply to reduce the risk of damage to downstream controls. A pressure reducing valve reduces the mains water pressure to the specified cold working pressure of the unit. A single check valve prevents contamination of the mains water supply from backflow and cross flow between hot and cold distribution pipes. An expansion valve 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.

What is the difference between the modern high pressure unvented hot water storage system (UVHWSS) and the traditional low pressure vented system? 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 roof space. 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. What is the purpose of the safety controls? 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. What materials are used for unvented cylinders? They are manufactured from either copper, glass-lined steel or stainless steel, are covered with thermal insulation material and finished with a case in most products. What minimum pressure is required to operate an unvented cylinder? 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. Why do I need an expansion vessel? 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 a pre set 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. Does my unvented System need to be serviced? Yes. Most HWA members are registered with the Benchmark Scheme which requires an annual service. This service will include testing the function of all safety components of the system and the operation of the system.