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What is an Unvented Cylinder?

As the name suggests, unvented cylinders are sealed to the atmosphere, whereas traditional vented cylinders are open to the atmosphere; this is significant and is further explained below (Pressurised hot water).
The water contained within the cylinder is stored under relatively high pressure (typically 3 bar which is equivalent to 30 metres head), and this pressure is created by the inlet water supply (usually the incoming water main). Unvented cylinders do not generate or increase water pressure, they simply use the inlet pressure to supply the outlets. Like traditional vented systems, the heat source can be electric immersion heaters, a boiler, some other heat source or any combination of the aforementioned.
An important difference is that unvented cylinders must be installed by a competent and qualified G3 engineer; this is a legal requirement in the UK and all installations must be certified and registered with the local council.

Unvented systems can supply high pressure hot and cold water to all outlets, and the increased pressures make for better quality showers, without the need for pumps or booster sets. As both hot and cold services are fed directly from the mains supply, there is no requirement for a cold water storage tank, thereby keeping loft spaces clear, and pipework to a minimum.

Pressurised Hot Water

In traditional open vented systems, as the water is heated, it simply expands into the water tank in the loft. If there's a fault with the thermostat, the water will continue heating and may eventually reach 100°C, and boil. The boiling water will turn to steam and evaporate into the atmosphere.

With an unvented system, the water is typically held under pressure at 3 bar. If the thermostat fails, and the water continues heating, it could reach 100°C. In a vented system the water would boil and evaporate, but remain at 100°C.
With an unvented system, at say 3 bar pressure, the boiling point of water would be 134°C, so the pressure and temperature will continue to increase. At above 100°C, the water would instantly flash into steam when released into normal atmospheric pressure, for example if opening a tap!. This can be extremely dangerous to both people and anything in the surrounding area. It is critical that unvented systems are installed with the correct safety controls to ensure the water never reaches boiling point, and if the temperature or pressure exceeds safe predetermined limits, the safety valves will release to rapidly reduce the temperature and/or pressure.

Mains Water Supply

Unvented cylinders are dependant on the 'Pressure and Flow' of the water supply feeding them, and in order to function properly requires a combination of good mains pressure and flow rate. Most manufacturers recommend a minimum supply pressure of 1.5 bar, and 20 l/m flow rate. This may be sufficient for a single bathroom property, but for larger multi-bathroom properties, this is highly unlikely to meet user expectations. In this scenario, other components would be required to address the performance issues (See Accumulators, Pump Sets and Water Boosters). The correct design is key to achieving a high performance water system.
A Pressure Reducing Valve is required on all unvented cylinders to limit the pressure of the incoming mains water to a safe level at which the cylinder is approved to operate. The difference between pressure and flow is important to understand, and whenever installing a new unvented system these should both be checked and verified.

Expansion

Unvented hot water systems must have a means of accommodating the water expansion as it is heated up. Manufacturers typically use one of two methods:

Expansion Vessel1. External Expansion Vessel:
An expansion vessel is basically a trapped volume of pressurised air, held by a rubber diaphragm within a steel vessel. The vessel is connected by pipework to the unvented cylinder, typically onto the mains cold inlet pipework, and as water in the cylinder expands it is pushed into the expansion vessel, compressing the air within the diaphragm. As the air is compressed, its pressure, as well as that of the water in the cylinder, will increase - the larger the expansion vessel, the smaller the increase in pressure. Minimum recommended sizes must be adhered to, as must the appropriate charge pressures. As a general rule, the vessel charge pressure should be 0.2 bar below the setting of the pressure reducing valve.

2. Internal Air Bubble: Another way of accommodating expansion is by trapping an air bubble in the top of the cylinder. This bubble compresses as the water expands, much in the same way as air in an expansion vessel. This method makes for a simpler design, without the need for having to fit or ever replace an expansion vessel. However annual servicing of the cylinder is usually required to recharge the air bubble and prevent safety discharges from occurring.

Safety Requirements

There are two basic dangers that unvented systems must address. First is over-pressurisation, caused by a failed pressure reducing valve, or by back pressure, from a faulty mixer valve for example. The other is overheating. If an unvented cylinder should ever overheat and reach 100°C, then instead of boiling and evaporating, as it would with a vented system, the water will continue to rise in temperature and pressure until the cylinder can no longer contain the pressure and splits. At this time, the sudden reduction in pressure resulting from the split will cause water to 'flash' rapidly to steam. The higher the pressure at failure the more steam, and cylinders capable of taking higher pressures will fail more dramatically or even explode.

This is enforced using a mandatory three tier level of protection:

To prevent this from ever happening, there are safety requirements that have to be fulfilled. As well as the mains Pressure Reducing Valve to limit the incoming water pressure, additional protection must be taken.

Pressure Relief ValveAn Expansion Relief Valve is required to allow water to be discharged during heat up if the means of accommodating expansion has failed to operate correctly. This valve is often combined with the Pressure Reducing Valve to form a single inlet control device.

A Temperature and Pressure Relief Valve, fitted near the top of the cylinder, is required to allow water to be discharged when store temperatures or pressures start approaching unsafe levels.  Any water discharged in this way will typically be replaced by incoming cold mains water that will prevent store temperatures from rising further.

Both relief valves make use of a Discharge Pipe and Tundish to allow water to be released from the store under fault conditions. Discharge pipes are typically 15mm up to the Tundish, and then on in 22mm to a safe discharge point outside. Certain systems layouts and longer pipe runs will require larger pipe sizes as laid down in the G3 Building Regulations.

To complete the array of safety controls, all indirect unvented cylinders require that the flow from the boiler to the store is fitted with a Motorised Isolating Valve which will close when an Overheat Thermostat (manually reset) fitted to the store detects overheat. Likewise, immersion heaters must have a built in overheat thermostat.

Megaflo CylinderMegaflo Design

Megaflo is a brand of Unvented Cylinder, manufacturered by Heatrae Sadia. They are constructed from duplex stainless steel and offers superior resistance to corrosion, especially in aggressive water areas.
Every unit is pressure tested in the factory to 15 bar – five times the normal operating pressure – making Megaflo eco suitable for all domestic and commercial applications.

Megaflo offers very fast heat recovery times and deliver superb flow rates of up to 72 litres per minute, making it ideally suited to a wide variety of applications.

The cylinders are insulated with a 60mm thick void-free layer of high-performance polyurethane foam. This fills the space between the inner vessel and outer casing for maximum insulation efficiency and compliancy with future European-wide legislation.

Megaflo Features

Megaflo Dimensions

As a guide, the diameter of a standard Megaflo Eco cylinder is 579mm, and the height varies according to the storage volume.
For example, a 145 litre cylinder is 1229mm, and a 210 litre (popular size for a medium sized house) is 1486mm.

Other Unvented Cylinders

Most major heating manufacturers offer unvented hot water cylinders; Vaillant UniStor, Worcester Greenstore, OSO, Telford, ACV to name a few. These all offer comparable performance to the Megaflo, and some have additional benefits/advantages over the Megaflo. These typically come with an industry standard warranty - 25 years on the stainless steel cylinder and 1-2 years on the components.

As an independant company we are not tied to any particular brand. We don't have preferential pricing or special incentives from any supplier, we simply recommend the best product for the job!

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