Megaflo Description

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What is a Megaflo?​

Megaflo is an unvented hot water cylinder that provides pressurised hot water. It is recognised for high performance.

A Megaflo is an unvented hot water cylinder, and as the name suggests, it is a closed vessel and sealed to the atmosphere. Conversely, traditional vented cylinders are open to the atmosphere; this is significant, and further explained below (see Pressurised hot water).

What is a Megaflo?

Megaflo cylinders store water at relatively high pressure; typically 3 bar which is equivalent to 30 metres head. 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.

Cylinders of this type give ultra-fast recovery times for hot water and improved efficiency and performance. They are designed for fully pumped hot water systems, incorporating a motorised zone valve and thermostat temperature control.

Unvented cylinders must be installed by competent and G3 qualified engineers. In the UK, this is a legal requirement 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. It follows that the increased flow gives more powerful showers, without the need for pumps or booster sets. As both hot and cold services are fed directly from the mains, there is no requirement for a cold water storage tank. Thereby keeping loft spaces clear, and pipework to a minimum.

Pressurised Hot Water

When water is subject to heating, it expands!

With traditional open vented systems, the water simply expands into the expansion tank in the loft. However, if the thermostat develops a fault, the water may reach boiling point (100°C). In this scenario, although not ideal, it simply evaporates into the atmosphere as steam, but crucially, remains at 100°C.

However, with an unvented system, at say 3 bar pressure, the boiling point of water is 134°C. Therefore, under fault conditions, temperature and pressure may increase to dangerous levels. For temperatures above 100°C, the water would instantly flash into steam when released into normal atmospheric pressure, for example opening a tap! 

This is extremely dangerous to people and anything in the surrounding area. It is crucial that unvented systems are installed with the correct safety controls to ensure temperature and pressure never reaches unsafe levels. Should the temperature or pressure exceed predetermined limits, the safety valves will release to rapidly reduce the temperature and/or pressure to safe levels.

Mains Water Supply​

Unvented cylinders are dependant on the ‘Pressure and Flow’ of the water supply feeding them, and to function properly requires a combination of good mains pressure and flow rate.

Manufacturers recommend a minimum supply pressure of 1.5 bar, and 20 l/m flow rate. This may be sufficient for properties with a single bathroom, but for larger multiple bathrooms, this is highly unlikely to meet user expectations. In this scenario, additional components would be required to address the performance issues – see Accumulators, Pump Sets and Boosters. The correct design is key to achieving a high-performance water system.


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:

1. External Expansion Vessel
Megaflo Expansion Vessel

An expansion vessel is a steel container with an internal rubber diaphragm. One side contains pressurised air, and the other side contains water. As the water in the cylinder is heated, it expands and pushes back into the expansion vessel, thereby compressing the air within the diaphragm.

As the air is compressed, its pressure, as well as the water in the cylinder, increases. 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 potential dangers that unvented systems must address. First is over-pressurisation. This may be caused by a failed pressure reducing valve, or by backpressure from a faulty mixer valve, for example.

The other is overheating. Under fault conditions, the heated water will reach temperatures of 100°C. Being unvented and sealed to the atmosphere, the water cannot evaporate as steam but will continue heating to dangerous levels. The temperature and pressure will continue rising until catastrophic failure. 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 must be satisfied. As well as the mains Pressure Reducing Valve to limit the incoming water pressure, additional protection must be taken.

Expansion Relief Valve

The 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.

TPR Valve

The Temperature and Pressure Relief (TPR) valve is fitted near the top of the cylinder. It enables water to discharge if store temperatures or pressures ever approach unsafe levels. Any water discharged in this way is replaced by the incoming cold supply, which helps 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 22mm to a safe discharge point outside. Certain system layouts and longer pipe runs may require larger pipe sizes as laid down in the G3 Building Regulations.

To complete the array of safety controls, a Motorised Isolating Valve is fitted between the boiler and cylinder. This shuts when an Overheat Thermostat detects an overheat condition. Likewise, immersion heaters must have a built-in overheat thermostat.

Megaflo Design

Megaflo is a brand of Unvented Cylinder, manufactured by Heatrae Sadia. Constructed from duplex stainless steel, they offer 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 independent 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!

Why Choose Us

At Megaflo Unvented, we do what we say we’ll do. Our integrity and competence values demand this. We are accountable for our words and actions, and that we deliver a service that meets or exceeds all expectations of quality and timeliness.

Customer service is our highest priority and to reinforce this, we adhere to the following guidelines:

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