Megaflo Description

What is a Megaflo Unvented Cylinder?

Unvented hot water cylinders store water supplied from a pressurised water supply. This is typically the mains water supply. The heat source may be a gas boiler, electrical heating elements, renewable energy (solar, heat pumps, etc) or any combination of these.

The cylinder stores water under relatively high pressures – typically in the region of 2 to 3 bar (20 to 30 metres head). As such, unvented systems can service properties with multiple outlets, including those at high levels. For example, in a loft space.

Higher pressures provide more invigorating showers, without the need for pumps. Since both hot and cold services are  supplied direct from the mains supply, no cold water storage tank is required keeping loft spaces clear, and pipework to a minimum.

Mains Water Supply

Performance of unvented cylinders depend on the ‘power’ of the mains water supply that feeds them. Good water pressure and flow rates are essential to achieve high performance. A minimum of 1.5 bar pressure and 20 l/m flow rate is recommended by most manufacturers. Usually more is required depending on the property type and usage.  A Pressure Reducing Valve is used to limit the incoming mains water pressure to a safe level, at which the cylinder is approved to operate. The difference between pressure and flow is important to understand, and this should be checked whenever installing a new unvented system.


Unvented hot water systems must have a means to accommodate the expansion of the water as it heats up. Manufacturers use one of two methods:

1. External Expansion Vessel: An expansion vessel is basically a volume of trapped air, contained by a rubber diaphragm within a steel vessel. With this connected to the cold inlet of the unvented cylinder, it regulates the system pressure. Water heated within the cylinder expands to compresses the air in the expansion vessel. It follows that a larger expansion vessel affords greater expansion capacity. However, minimum recommended sizes and charge pressures must be adhered to. 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: Expansion can also be handles by an air bubble in the top of the cylinder. The air bubble is compressed as the heated water expands, similar to that of an expansion vessel. This method is 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 maintain the air bubble and prevent safety discharges from occurring.

Safety Requirements

Unvented Systems must address two basic dangers. Firstly, over-pressurisation. This is caused by a failed pressure reducing valve, or by back pressure from a faulty mixer valve, for example. Secondly, overheating. If an unvented cylinder continues heating beyond 100°C, the increasing temperature and pressure may cause the cylinder to split. Or even explode! If this case, the sudden reduction in pressure will cause flash steam with tremendous force. Clearly, this is extremely dangerous, hence the importance of safety controls.
This is enforced using a mandatory 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.
  • Temperature and pressure relief valve – set to 90°C, 10 bar.

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

An Expansion Relief Valve is required to allow water to discharge if the means of accommodating expansion has failed. This valve is often combined with the Pressure Reducing Valve to form a single inlet control device.
Temperature and Pressure Relief Valve is fitted near the top of the cylinder. It enables water to discharge if the temperature or pressure approaches unsafe levels.  Any discharged water will be replenished by the incoming cold supply, and helps prevent store temperatures from rising further.

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

To complete the array of safety controls, all indirect unvented cylinders require a means of cutting off the heat source in a fault condition. Typically, a Motorised Valve closes when an Overheat Thermostat (manual reset) detects an overheat condition. Likewise, immersion heaters also have a built in overheat thermostat.

Megaflo Design

Megaflo cylinders 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.

Megaflos have fast reheat 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

  • Higher flow rates for efficient hot water delivery
  • High performance showering throughout the home and fast-filling baths
  • Constructed in Duplex stainless steel for maximum strength – pressure tested to 15 bar.
  • Patented internal floating baffle system to accommodate water expansion.
  • No anode – eliminates costly anode maintenance.
  • Insulated casing for the temperature and pressure relief valve to minimise heat loss.
  • Unique ‘L-shaped’ long-life Superloy immersion heater reaches deeper into the tank giving more hot water.
  • Titanium immersion heaters as standard on all direct models
  • Patented cold water inlet diffuser minimises mixing of cold and hot water.
  • High performance ‘coil-in-coil’ heat exchanger.
  • Exceeds CHeSS* Best Practice for recovery times and insulation.
  • Improved insulation for maximum heat retention.
  • New water inlet controls increase flow rates by up to 20%.
  • Improved performance at low pressures down to 1 bar.
  • Installer friendly design.
  • Complies with European User Products Directive.
  • Lifetime transferable cylinder guarantee.
  • Made in the UK.

Megaflo Dimensions

As a guide, the diameter of a standard Megaflo Eco cylinder is 579mm. 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.
For a full list of specifications and dimension, please refer to the Megaflo_Range_Brochure.

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