Megaflo Explained: What It Is in Plain English

What is a Megaflo, in one paragraph

A Megaflo is a sealed, insulated cylinder full of the hot water you actually wash in, kept at the pressure of the cold water main in your street. It is a brand of unvented cylinder, and it is not a boiler. It does not make heat of its own in normal use. Something else heats the water, usually a boiler pumping hot water through a coil inside the cylinder, or an electric immersion heater, and the cylinder stores the result until you open a tap.

Take the word apart and it tells you the design. Vented means open to the air, which is what an old copper tank is: open topped, fed from the loft, at whatever pressure gravity gives it. Unvented means sealed shut. Water expands as it heats, so a sealed cylinder needs somewhere for that expansion to go and safety valves to protect it, which is why a Megaflo has more attached to it than a copper tank ever did.

The practical version most people care about: a Megaflo gives you strong hot water at every tap and shower in the house, at the same pressure as the cold, without a pump and without a tank in the roof.

Megaflo is the registered trade mark of its manufacturer. We are an independent London plumbing and hot water firm. We repair and replace these cylinders, and we are not an approved, authorised or official agent for any manufacturer.

How a Megaflo differs from the old vented tank

The difference is where the pressure comes from. A traditional vented system has a cold water storage tank in the loft, and everything downstairs is fed by the height of the water above it. The pressure at your shower is decided by how far the shower head sits below that tank, which in a typical house is not very far, which is why upstairs showers on that system are so often disappointing. A Megaflo throws the loft tank away and connects the cylinder to the cold main directly, so your hot water arrives with the same force as the cold water at your kitchen tap.

Everything else follows from that. The copper tank was open to the air, could not build pressure and needed no safety valves. A sealed cylinder can, so it carries controls that stop it. The open tank also held a reservoir of cold water, so a mains failure in the street still left you supplied for a while. A Megaflo stores no cold, so it goes off when the main does.

• Pressure: a vented tank gives gravity pressure set by the height of the loft tank. A Megaflo gives mains pressure.
• Loft: a vented system needs a cold tank and usually a small feed and expansion tank. A Megaflo needs neither.
• Balance: on a vented system the hot is weak and the cold is strong, so mixer showers fight each other. On a Megaflo both sides match.
• Safety kit: an open tank needs none. A sealed cylinder carries an expansion arrangement, a pressure reducing valve, relief valves and a discharge pipe.
• Maintenance: a copper tank was fitted and forgotten. A Megaflo is designed to be checked once a year.
• Material: most old vented tanks are copper. A Megaflo is stainless steel, which is why they are lighter than they look.

How can I tell if I have a Megaflo?

Look at the cylinder and look at the loft. A Megaflo is a tall white or cream cylinder with a hard foam jacket moulded onto it, so there is no loose red quilted jacket tied on with tape and no bare copper showing. The brand name is normally on a label on the front, with a model name and the capacity in litres, and that label is the thing worth photographing before anybody quotes you for anything.

The second tell is the pipework. An unvented cylinder has a group of valves on the cold feed going in, a small pipe running from a relief valve to a tundish, which is a little open funnel usually mounted just above the cylinder, and a pipe from that tundish that leaves the building. If you can see a white open funnel in the pipework, you are looking at an unvented cylinder of some kind.

The third tell is upstairs. If there is no cold water tank in the loft, or there is an old one sitting dry and disconnected, the system below is almost certainly mains fed. A large plastic tank still plumbed in and full of water means you have a vented system, whatever anybody told you.

Worth saying plainly: not every mains pressure cylinder is a Megaflo. Several manufacturers make unvented cylinders and they all do the same job, so the cupboard may hold a different brand entirely. The word has simply become the shorthand. The comparison of unvented and vented cylinders sets out the two families in more detail than this page needs to.

What are all those pipes and valves attached to it?

They exist because the cylinder is sealed. Water expands as it heats, and in a sealed vessel that expansion has to be allowed for and then, as a last resort, released. Nothing on the outside of a Megaflo is decorative and none of it is optional.

On the cold feed there is a valve set. It includes a pressure reducing valve, which takes whatever pressure your street happens to deliver and hands the cylinder a lower, steady figure, an expansion relief valve, and a non return valve that stops heated water pushing back into the cold main. Some unvented cylinders carry a separate expansion vessel mounted alongside, while the Megaflo design keeps the expansion space as an air bubble trapped inside the cylinder itself, which is one reason the unit looks tidier in a small cupboard.

Higher up there is a temperature and pressure relief valve screwed into the cylinder, and both relief valves discharge into the tundish. The tundish is the open funnel, and it is there so that a discharge is visible. Water running through it is the system telling you something. From the tundish, a pipe carries any discharge safely away, normally to the outside of the building.

You should not adjust any of it. What is useful is knowing what each part is for, so that when an engineer says the relief valve is passing, the sentence means something.

Water running from a tundish, or a pipe outside the house dripping when the hot water heats, is not normal and is not something to live with. It is the safety kit doing its job, and the reason behind it needs finding.

Does a Megaflo heat the water itself?

Usually not on its own. In most London homes the heat comes from a system boiler, which pumps hot water through a coil sealed inside the cylinder. That coil warms the stored water without ever mixing with it. The cylinder is the storage and the boiler is the heat.

There is almost always an immersion heater as well, and on some models two. An immersion is an electric element screwed into the cylinder that heats the stored water directly. On a boiler fed installation it is a backup, which is why a house with a dead boiler can still get a bath by switching it on. In flats with no gas, an immersion or a pair of them may be the only heat source, running overnight on a cheap electricity rate.

The word to be careful with is combi. A combination boiler heats water as it flows and stores none, so a house with a combi has no cylinder at all. If you have a Megaflo you do not have a combi. Which arrangement suits a particular house is a real decision, covered in our piece on choosing between a combi and a system boiler with a cylinder.

Will a Megaflo give me a better shower?

Usually yes, and sometimes no, and the reason for the no is worth understanding before anybody spends money. A Megaflo delivers hot water at the pressure and flow rate of your incoming cold main. If the main into your property is strong, the shower is transformed. If the main is weak, the hot water will be exactly as weak as the cold, because the cylinder cannot create pressure it was not given.

This is the commonest disappointment with mains pressure hot water, and it is not a fault in the cylinder. Victorian terraces and conversions across London are fed by old, narrow supply pipes, sometimes shared between flats in the same building, and a cylinder cannot widen a pipe under the pavement. Any honest assessment starts with measuring the flow rate at the kitchen tap rather than with picking a cylinder from a brochure.

The other thing a Megaflo changes is balance. With both sides at mains pressure, mixers and thermostatic showers behave as designed. You also lose the shower pump that was fitted to paper over the old system, which is one less noisy thing in the cupboard to fail.

• Strong incoming main: a mains pressure cylinder delivers a genuinely better shower at every outlet.
• Weak or shared incoming main: the hot will be as weak as the cold, and the supply is the thing to fix first.
• Two bathrooms at once: a cylinder holds a fixed volume, so simultaneous demand is about size as much as pressure.

What does living with one actually involve?

Once a year, someone qualified should service it. That is the main practical difference from the copper tank your parents had, which nobody ever touched. A service checks the expansion arrangement, the safety valves, the controls and the immersion, and it exists because the safety components on a sealed cylinder are mechanical parts that can stick or wear. What that visit covers is set out in our page on the annual unvented cylinder service.

Day to day there is nothing to do. There is no tank to look at and nothing to top up. What you should do is notice things. Water at the tundish, a drip outside when the hot water heats, hot water that takes much longer than it used to, or water at the base of the cylinder are all worth a call rather than a wait.

Hard water is the London factor. Almost the whole supply area is hard, and scale forms on the hottest surfaces in the system, meaning the boiler coil and the immersion element. It rarely announces itself. It shows up as a cylinder that reheats more slowly every year, or an immersion that fails sooner than it should.

Where are they usually installed in a London home

In a Victorian terrace or a conversion flat, the cylinder is normally in a first floor cupboard, often the old airing cupboard on a landing, and often reached through a doorway narrower than the cylinder. That matters more than people expect on a replacement day, because a tall cylinder has to come out and a new one has to go in, and the route is part of the job rather than an afterthought.

In purpose built blocks the picture varies. Some flats have an unvented cylinder in a hall cupboard, others have a thermal store or a communal arrangement that looks similar from the outside but works differently, so the label on the front is worth reading before assuming.

In newer builds the cylinder tends to sit in a dedicated plant cupboard, sometimes beside the manifold for underfloor heating, with pipework buried in liquid screed under the floor. That is not a problem in itself, but nobody should be cutting into a floor on a guess.

When does a Megaflo need replacing rather than repairing?

Most faults on these cylinders are repairs, not replacements. Immersion heaters, thermostats, relief valves and expansion arrangements are all serviceable parts, and a cylinder that is otherwise sound is normally worth putting right. The point where the conversation changes is when the vessel itself has failed, meaning the cylinder is leaking from the body rather than from a fitting, because the stainless vessel is the one component that cannot be renewed in place.

Parts are the other consideration. Designs change, and older models can use components that are no longer straightforward to obtain, including proprietary immersion heaters rather than the plain screw in element a merchant keeps on the shelf. That is a serviceability point rather than a criticism, but it is real, and it is why a repair on a very old unit sometimes waits on a part while a newer one does not.

What drives the cost of a replacement is not really the brand. It is the size of cylinder your household needs, whether the incoming main can feed it, how much surrounding pipework has to be altered, how the old unit gets out of the building, and how much electrical work the controls need. We set out our rate on our pricing page rather than in an article, because a real figure needs a real look at the cupboard.

If the cylinder is leaking from the body of the vessel rather than from a joint or a valve, turn the water off at the mains stopcock and get it looked at. A sealed cylinder holds a large volume of very hot water, and a first floor cupboard sits directly above a ceiling.

Megaflo, Megaflow and Megaflo Eco: which is which

Megaflo is the correct spelling and it is the manufacturer brand. Megaflow, with a w, is how most people type it, and it is the same thing. Neither spelling changes what is in the cupboard.

Megaflo Eco is the later family, and within it there are variants for different setups: indirect models heated by a boiler, direct models heated by immersion heaters alone, and versions built to work with a heat pump, which run at lower flow temperatures and use a larger coil surface as a result. The differences matter when parts are ordered or a like for like replacement is specified, and very little to how you use the hot water.

The label on the front of your cylinder settles all of it. Photograph it. The model name and the litre capacity answer more questions in ten seconds than an hour of description over the phone, and they are what lets anyone tell you whether a fault is a part, a service or a replacement before attending.