What Are Vented & Unvented Hot Water Storage Systems?

How Do Vented Hot Water Storage Systems Work?

Within a vented system the cold water is heated either directly from an immersion heater within the hot water cylinder or indirectly via the central heating boiler or solid fuel appliance. A cold water tank is also used to remove trapped air and act as an expansion tank to accommodate the expansion of the hot water during the heating process and the occasional chance of an overheat situation.

As they are relatively simple in their construction and operate at lower pressure, vented hot water cylinders tend to be cheaper than unvented. The biggest challenge is they may deliver a lower pressure of water. The system also takes up more space in the home with the combination of the cold water tank and the hot water cylinder.

How Do Unvented Hot Water Storage Systems Work?

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.

The 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 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 – units can be installed virtually anywhere within the building structure, allowing greater flexibility of house and system design and reducing costs.
  • 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.
  • Smaller diameter pipework can be used – reduces costs.
  • No shower pump required – reduces costs.
  • Ease of maintenance – no access to roof space required.

Householder Benefits

  • Superior performance – ensures constant high flow rates at all outlets and allows rapid filling of baths.
  • 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.

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.

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.

What Can Reliance UK Offer?

Reliance UK is a division of Reliance Worldwide Corporation, a global leader in water control systems and our products span a wide range of applications including thermostatic controls, valves to regulate water flow and pressure, heating systems and plumbing solutions. All are equally at home in domestic, commercial and industrial applications. Take a look at our Heating System Components, Temperature Control and Flow Control product ranges to see what solutions we can offer you.