Unvented Hot Water Storage

Unvented Hot Water Storage Systems are subject to the legal requirements of Building Regulation G3 and can only be installed by a “competent person” in strict accordance with the relevant manufacturer’s Instruction Manual, Building Regulation G3 and the current Water Byelaws.

Overview

An early pioneer of unvented hot water systems, Reliance continues to offer leading technology for storage, control and distribution within a sealed system.

Reputedly devised by an Englishman as far back as 1861, unvented systems are commonplace throughout Europe, South Africa, North America and Australia, although the UK remains one of the last bastions of the traditional low pressure vented system. However, times are changing. The demands from discerning householders for increased performance, sophisticated taps and showers, and aesthetic styling have produced a growing trend towards the installation of unvented systems in the UK, which will continue to increase in popularity.

Unvented hot water storage systems are subject to the legal requirements of Building regulation G3 and can only be installed by an “approved installer” in strict accordance with the relevant manufacturer’s Instruction Manual, Building regulation G3 and the current Water Byelaws.

house art 2

How the unvented system works

house artThe basic difference between the high pressure unvented systems and a traditional low pressure vented system is that we no longer require the cold water storage cistern, open vent pipe or cold feed pipe.

Instead, the unvented unit is fed direct from the mains cold water supply via an inlet control group preset to the relevant manufacturers specified pressures. Hot water drawn from the unit is then replenished from the mains supply.

As water expands when heated and (as the name implies) there is no vent pipe, the expanded water within the system is accommodated by either an external expansion vessel or, in the case of bubble-top units, an internal air bubble within the unit which is generated when the system is commissioned.

To maximise the benefits of the unvented system, a sealed heating system, if applicable, can also be installed providing the boiler used is compatible.

The significant benefits to be gained from the installation of an unvented hot water storage system can be divided into two categories: benefits to the specifier/installer and benefits to the householder.

Specifier / Installer BenefIts

  • Choice of location – unit can be installed virtually anywhere within the building structure, allowing greater flexibility of house and system design and reducing costs.
  • Superior performance – ensures constant high flow rates at all outlets and allows rapid filling of baths.
  • No cold water storage cistern – reduces pipework. No requirement for tank stand or pipework/tank insulation – reduces costs. Roof space can be utilised for additional living accommodation.
  • Balanced pressures – far wider choice of taps and showers can be used.
  • Quicker to install – reduces pipework and installation costs.
  • Reduces noise in system – no filling of cold water storage cistern.
  • Smaller diameter pipework can be used – reduces costs.
  • Eliminates risk of freezing and burst pipes in roof space.
  • No shower pump required – reduces costs.
  • Eliminates risk of contamination – no cold water storage cistern in roof space.
  • Superior performance and flow rates, ideal for multi-bathroom dwellings.
  • Energy efficient – reduces fuel costs.
  • Ease of maintenance – no access to roof space required.
  • Frees roof space – can be utilised for loft conversions.
  • Aesthetic looks.

Householder BenefIts

  • Superior performance - ensures constant high flow rates at all outlets and allows rapid filling of baths.
  • Balanced pressures - far wider choice of taps and showers can be used.
  • Reduces hydraulic noise in system - no filling of cold water storage cistern.
  • Eliminates risk of freezing and burst pipes in roof space.
  • Eliminates risk of contamination - no cold water storage cistern in roof space.
  • Frees roof space - can be utilised for loft conversions.
  • Energy efficient - reduces fuel costs.
  • Aesthetic looks.

Choosing an unvented system

From the early unvented systems of the seventies, the continued growth of installations in the UK has now produced the sophisticated, high performance units demanded by today’s householder. The availability of unvented systems from manufacturers in the UK, Europe, South Africa and North America now offers the specifier/installer an unparalleled choice to suit virtually any application.

Manufactured from either copper, glass-lined steel or stainless steel, almost all of the unvented systems available in the UK are supplied with an easy to clean finish. Using inlet pressures ranging from 1.5 Bar to 3.5 Bar, depending on the relevant manufacturer’s specification, and available in both direct and indirect form, today’s unvented systems can be divided into three main categories: bubble-top unit, external expansion units and direct gas- or oil-fired units.

Bubble-top units

bubbltopBubble-top units, manufactured from either copper or stainless steel, accommodate the expansion of the heated water by using an internal air-bubble, which is generated and trapped at the top of the unit during commissioning.

External expansion unit

Cylinder 2As the name implies, external expansion units utilise an expansion vessel to contain the expanded heated water. The expansion vessel can be installed directly onto the unit, or, if required, remotely sited with the inlet control group. These units are manufactured from copper, glass-lined steel or stainless steel.

The direct-fired unit, normally manufactured from glass-lined steel or stainless steel, again uses an expansion vessel to accommodate the increased volume of the heated water. Predominantly installed in larger domestic or commercial premises, these units can be either gas or oil fired.

A recent development now available in the UK is the combi storage unit. This type of unit incorporates the benefits, features and controls associated with unvented systems but has a limited storage capacity of hot water.

Ideally, the mains supply pressure to the premises should be a minimum of 2 Bar to ensure the superior performance associated with the unvented system. All unvented systems will operate at supply pressures as low as 1 Bar, but the benefits of will be reduced.

Cylinder 1Unvented Systems can also be used in conjunction with tank fed supplies and booster pump sets, although these installations are usually confined to commercial premises.

Unvented Systems are suitable for use in conjunction with electric, gas, LPG or oil heating appliances but MUST NOT be used with solid fuel appliances.

The majority of unvented units available today are certified to comply with the requirements of Building regulation G3, either by the BBA (British Board of Agrément) or the WRc (Water Research Evaluation and Testing Centre) and are supplied with a complete package of controls – either factory fitted or supplied as an unvented kit – which must be installed in strict accordance with the relevant manufacturer’s instructions

Installation, commissioning and maintenance of unvented systems

Although the basic principles are similar, the installation, commissioning and maintenance of an unvented system must be in strict accordance with the relevant manufacturer’s instructions and all installations must be carried out by a competent person approved to install unvented systems.

Controls for unvented systems

As the dominant supplier and pioneer in the development of controls for unvented systems, Reliance’s range of controls incorporates unique features to ensure superior performance: ease of installation, long and trouble-free service, and ease of maintenance and replacement.

Controls used on the majority of unvented systems in the UK fall into two specific groups: Functional Controls and Safety Controls.

Functional controls

Required to protect the mains water supply from contamination and the unvented unit from over pressure, these controls consist of the following components:

  • Line Strainer - filters debris from the water supply to reduce the risk of damage to downstream controls.
  • Pressure Reducing Valve – reduces the mains water pressure to the specified cold working pressure of the unit.
  • Single Check Valve – prevents contamination of the mains water supply from backflow and crossflow between hot and cold distribution pipes.
  • Expansion Valve – protects unit from over pressure caused by failure of pressure reducing valve, failure of expansion vessel or loss of internal air bubble.

Originally supplied as separate components, Reliance has developed and engineered these controls to form a one piece or two piece inlet control group for units requiring DN20 Nominal size controls, and a three piece inlet control group for units requiring DN25 size controls.

Inlet control group - one piece (Multibloc)

Originally developed for use on bubble-top units – there is no provision for an integral expansion vessel connection – this control is now found on all types of unvented system and offers the following features:

  • Integral coaxial stainless steel strainer.
  • Drop tight balanced seat pressure reducing valve – very stable under fluctuating inlet pressure conditions.
  • Unique one piece pressure reducing cartridge with noise reduction comb - facilitates ease of maintenance, reduces system noise further and prevents cavitation.
  • Balanced cold water connection – equal pressures to mixer taps/showers.
  • Unique stainless steel seat expansion valve – dramatically reduces premature failure from seat erosion, particularly in hard water areas.
  • Superior flow rates – up to 55 l/m.
  • Multi-orientational, providing installation flexibility.

Inlet control group - two piece (Core Unit)

Forerunner of the Multibloc one piece control, this inlet control group is also used on all types of unvented system and comprises of a separate pressure reducing valve/strainer and combined check/expansion valve core unit offering the following features:

  • Separate pressure reducing valve with integral coaxial stainless steel strainer – enables remote siting of the pressure reducing valve if required.
  • Droptight balanced seat pressure reducing valve – very stable under fluctuating inlet pressure conditions.
  • Unique one piece pressure reducing cartridge with noise reduction comb – facilitates ease of maintenance, reduces system noise further and prevents cavitation.
  • Balanced cold water connection – equal pressures to mixer taps/showers.
  • Separate check/expansion valve core unit with integral expansion vessel connection.
  • Unique replacement expansion valve cartridge and seat – enables refurbishment of existing valve without removal.
  • Superior flow rates – up to 55 l/m.
  • Multi-orientational, providing installation flexibility.

In addition to the standard specification DZR brass expansion valve seat of the core unit, Reliance has developed a stainless steel seat version for the exclusive use of one unvented systems manufacturer, proving Reliance’s commitment to continual development to offer the best available control package.

Inlet control group - 1" Highflow

Used on all types of unvented system, this inlet control group is for larger domestic and commercial installations where a high flow rate is required to satisfy peak demand.

  • Separate pressure reducing valve with integral coaxial stainless steel strainer, droptight balanced seat design and one piece cartridge with unique noise reduction comb.
  • Check valve manifold with integral connections for expansion valve, expansion vessel and balanced cold water supply.
  • High discharge capacity expansion valve with replaceable cartridge and seat feature.
  • High flow rates – up to 77 l/m.
  • Multi-orientational, providing installation flexibility.

Although the various inlet control groups are the most common functional controls supplied with today’s units, some unvented systems still use individual, separate controls. All controls supplied by Reliance in this format offer similar technical features to the inlet control groups but do not allow provision for a balanced cold water connection or integral expansion vessel connection – these must be supplied by the approved installer, using additional fittings in the pipework.

All Reliance inlet control groups are supplied with comprehensive installation and maintenance instructions to supplement the relevant unvented system manufacturer’s instructions.

The final functional control – a means to accommodate the expanded water to prevent waste of water – is also supplied with the unvented system. This is achieved in two ways – either by using an external expansion vessel or an internal air bubble generated within the unit.

External expansion vessel

Supplied in various sizes, dependent on the water capacity of the unvented system, the expansion vessel has an inner butyl rubber membrane which is surrounded by a cushion of air, the pressure of this air cushion corresponding to the set pressure of the relevant manufacturer’s pressure reducing valve. When the water is heated, the additional volume flows into the bag-type membrane, compressing the air cushion and accommodating the expanded water. On cooling down, the volume of water decreases, thereby deflating the inner membrane.

The expansion vessel must be sited either directly on the unvented unit or, as in most unvented systems utilising this method to accommodate the expanded water, the inlet control group.

The Reliance range of expansion vessels used on domestic unvented systems are supplied in a white finish to complement the appearance of the unvented unit.

All potable expansion vessels with a nominal capacity of 16 litres or greater incorporate a replaceable membrane to facilitate replacement in the unlikely event of failure.

Internal air bubble

In this type of unvented system, the expanded water is accommodated by compressing the internal air bubble generated when the unit is commissioned. Again, on cooling down, the volume of water contained within the unit decreases and the internal air bubble returns to its nominal capacity. This type of unvented system is commonly known as a bubble-top unit.

Safety controls

Required to protect the householder and prevent the temperature of the water in the unvented system exceeding 99°c, these controls provide a three tier level of protection.

  • Control thermostat – usually set between 60-65°C.
  • Energy cut out device with manual reset – usually set between 85-89°C.
  • Pressure & temperature relief valve – set to 90°C + 5°C.

Both the control thermostat and energy cut out device are supplied by the manufacturer, the control thermostat offering the first level of protection against over temperature of the water. The energy cut out device will offer the second level of protection in the event of control thermostat failure. The energy cut out device must incorporate a manual reset feature and must not be self-resetting.

The pressure & temperature relief valve is the third and final level of protection. Although this valve does incorporate a pressure relief function, the sole purpose of this control is to prevent the system water temperature exceeding 99°C; the pressure relief function is not used.

Reliance pressure & temperature relief valves are typically twist-top and are available in DN15, DN20 and DN25 nominal sizes. These can be supplied with an extended temperature sensor probe, depending on the relevant manufacturer’s design and specification.

All pressure & temperature relief valves should be factory fitted to the unvented unit by the relevant manufacturer.

Twist-top pressure & temperature relief valve

Developed following the demands of manufacturers for a more aesthetic, modern design to complement the finish of today’s unvented units, the twist-top valve now supersedes the lever type valve, and has the added benefit of direct compression connections for the discharge pipework. The twist-top pressure & temperature relief valve has also been designed to conform to the requirements of the latest European Standard.

Under fault conditions, the pressure & temperature relief valve will discharge water to prevent over temperature of the system. The drain pipework from the valve must discharge through a tundish, positioned within 500mm of the pressure & temperature relief valve, to a safe and visible position to alert the householder of a fault condition with the unvented system and to prevent damage to the property.

The drain discharge pipework must comply with the relevant manufacturer’s instructions and the requirement of Building regulation G3.

The expansion valve will also discharge water under fault conditions and it is acceptable practice to allow the drain pipework from the expansion valve to discharge into the same tundish used for the pressure & temperature relief valve provided the requirements of both the relevant manufacturer’s instructions and Building Regulation G3 are maintained.

Spare components

The inlet control groups supplied by Reliance for use with unvented systems have proven to be extremely reliable in field service but, in the unlikely event of failure, the following spare components are available from the relevant manufacturer of the unvented system in question.

Pressure reducing valve cartridge – supplied at the manufacturer’s required setting. One piece design facilities replacement. Supplied complete with integral strainer.

Replacement expansion valve cartridge and seat – unique design enables damaged valves to be refurbished in situ without need for removal.

Problems and remedial action

All manufacturers of unvented systems available in the UK today supply a comprehensive instruction manual with each unit, giving full details of fault diagnosis and remedial action required.

©Reliance Water Controls Ltd 2008 All images ©Reliance Water Controls Ltd 2008

Unvented Controls

Unvented Controls

The Reliance Water Controls range of unvented controls incorporate features to ensure superior performance - ease of installation, long life, trouble-free servicing and ease of maintenance and replacement.