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“regardless of which unvented cylinder you choose, be sure the overall system is suitably designed…”

The design and installation of any Megaflo or Unvented hot water system is of paramount importance. Correctly sized pipes ensure optimum performance and efficiency in delivering high pressure hot and cold water to all outlets – get this wrong, and the whole system falls apart!

All unvented installations are subject to the legal requirements of Building regulation G3 and should only be installed by an “approved installer”. This requirement serves to enforce the basic and mandatory safety requirements of any Megaflo or unvented cylinder. Unfortunately, this does not regulate the quality, design or installation of the associated pipework, fittings and components! This is primarily down to the experience and competence of the installer.

Megaflo Unvented specify and design systems to maximise the performance of an unvented hot water system. A poorly installed Megaflo system will underperform a traditional vented system and will fail to offer any real benefits – don’t get this wrong!

With an unvented hot water system, the cold water supply must have adequate pressure and flow rate to meet the maximum flow demand for both hot and cold water outlets.

Pressure: the force of water delivered.
Flow rate: the volume of water carried in the pipework.

It is generally recommended that, as a minimum, the incoming mains is at least 25mm with pressure of 1.5 bar and 20 l/m flow rate.

Is Your Mains Supply Suitable?

Firstly, it is important to understand that with any mains fed system (i.e. without stored water), the performance is directly related to the incoming flow and pressure.

In existing properties, the water supply can be gauged by opening a tap connected directly to the mains supply. Typically, these are the outside tap or kitchen tap. If you open more than one tap, the ability of each one to maintain the flow rate is an indication of the water available. Your local water supplier should be able to assist in testing pressures and flow rates properly. We always test the flow and pressure when we carry out a survey.

Flow Rate

Simply the maximum amount of water that can flow into the property, measured in litres per minute, this is key to any mains fed water system. The easiest way to measure the flow rate is to time how long it takes to fill a bucket to a known volume. For example, if it takes 30 seconds to fill a 10-litre bucket, the flow rate is 20 litres per minute. The flow rate will indicate how many water outlets can run at the same time.

It is important to have readings for total throughput, not just one outlet. Try have all taps running at the same time and total up the figures. Better still to isolate the supply into the property externally (using road valve) and take the flow reading from the actual mains supply pipe within the property – although this does involve breaking into the pipework as close as possible to where the mains enters the property.


This is recorded using a pressure gauge, typically connected to an outdoor tap. The pressure indicates the potential gain in vertical height. For example, 1 bar pressure will reach a height of approximately 10 metres, 2 bar up to 20 metres, 3 bar up to 30 metres, and so on. The pressure is the force behind the water.

However, it is possible to have a very high-pressure supply, with only a small amount of water coming into the property. This happens when the pipes carrying the water are undersized, or very long, or both!
Some pressure is ‘lost’ in forcing water through the pipes and fittings.

Improving Flow

The first thing to check is that all valves are fully opened, including the main stop tap. Even with a good supply into the property, if internal pipework is undersized, damaged or restricted in some way, this will produce a detrimental effect on the flow rates. Additionally, some water meters and water softeners can add further resistance and pressure drop.

Unfortunately, the water undertaker only needs to supply a minimum of 1 bar pressure and 9 litres/minute flow. This is very low for any mains fed system.
The flow rate into the property is dependant on the pipe size and length.

Often upgrading to a larger bore supply pipe will improve flow rates. Often upgrading to a larger bore supply pipe will improve flow rates. This can be a very costly and disruptive procedure depending on the location of the pipework. Where there is good local pressure and pipes are known to be small, this is a viable solution, cost permitting!

If the mains supply pipe work is unable to provide the peak water requirements then there are various options available:
See Water Boosting (accumulators and pump sets).

That said, most properties have more than adequate water supplies, and mains fed hot water systems are an attractive and viable option.


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:

As the Megaflo or Unvented system is pressurised, it can be sited virtually anywhere; it does not rely on a header tank to provide a head of pressure.

Common locations are within an airing cupboard, a downstairs cupboard, utility room, loft, garage or basement. However, consideration should be given to the requirement for a safety discharge pipe to run from the cylinder to either outside or to a soil and vent pipe.

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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