Megaflo Cylinders Explained

What is a Megaflo cylinder

A Megaflo is a mains pressure, unvented hot water cylinder. The stainless steel vessel is fed from the incoming cold main through a set of inlet controls, so hot water arrives at the outlets at close to mains pressure rather than at the trickle a loft tank provides. There is no cold water storage cistern and no vent pipe, which is why so many London loft conversions are built around swapping a vented tank for one of these.

It is a store, not a heater. The water inside is heated either by a coil connected to a system boiler or heat pump, which is the indirect version, or by immersion elements alone, which is the direct version. The name on the front is a brand, not a type: most of what follows about controls, safety valves and immersions applies to any unvented cylinder, and the parts that genuinely differ are called out below.

Because the vessel is sealed and at mains pressure it carries layered safety devices rather than an open vent: a thermostat, a thermal cut out behind it, a temperature and pressure relief valve on the cylinder and an expansion relief valve on the inlet group. Whatever those valves discharge runs to a tundish and out to a visible discharge point. This is work for an engineer holding the relevant unvented qualification, not general plumbing.

Megaflo is a trade mark of its manufacturer. We are an independent plumbing and hot water firm with 20 engineers and we work on these cylinders every week. We are not an approved, authorised or official agent of any manufacturer.

Which Megaflo models you will meet in London

In practice you will meet two generations. There are older Megaflo cylinders from before the Eco range, still working in plenty of houses and flats, and there is the Megaflo Eco range that has been the current family for well over a decade. The Eco cylinders carry a model code that tells you the litres and how the cylinder is heated, so a badge reading 170i is a 170 litre indirect and a badge reading 145dd is a 145 litre direct with immersion elements and no boiler coil.

Within the Eco family there are several variants for different installations. The standard indirect and direct cylinders cover most of the domestic range, commonly between about 70 and 300 litres. Slimline versions use a narrower diameter and more height for cupboards that will not take a standard barrel, which is genuinely useful in a converted flat. Solar versions add a second coil for a second heat source. There are also versions supplied with the pack of controls and valves an installer would otherwise assemble on site, and versions built with larger coils for heat pumps, where a standard boiler coil would never move enough energy at a low flow temperature.

Treat any range list, this one included, as a starting point. Manufacturers revise ranges and change part numbers, so a cylinder fitted eight years ago may sit in a family that no longer exists under that name. The badge on your own cylinder is the only reliable source.

• Pre Eco Megaflo: older generation, still common, parts support thinner as models age.
• Megaflo Eco indirect: coil from a boiler or heat pump, with an immersion as backup.
• Slimline: narrower diameter for tight cupboards, more height for the same litres.
• Heat pump versions: much larger coil area, specified at purchase, never retrofitted.

How to read the badge on your Megaflo

The information you need is on the rating plate, a printed label on the front of the white casing, usually near the top and often half hidden by pipework. Photograph it before you ring anybody. It carries the model name and code, the nominal capacity in litres, a serial number, the maximum operating pressure and temperature, and the manufacture date. With that photograph an engineer can identify the inlet group, the correct immersion element and the service kit before leaving the yard, which is the difference between one visit and two.

Read the model code first. The number is the litres and the letters say how it is heated. An i is indirect and has a coil inside. A dd is direct and has no coil at all, so if somebody proposes connecting your new boiler to it, the specification is wrong. The serial number matters because it dates the cylinder, and age is what decides whether the parts you need are current stock or a hunt.

If the label has faded to nothing, which happens in a hot airing cupboard over twenty years, the fittings still tell an experienced engineer plenty: the immersion boss positions, the style of the inlet control group, and whether there is an external expansion vessel on the cold feed. That last one is the fastest way to tell a Megaflo apart from most of the market.

Photograph the rating plate with the flash on, then take a second shot showing the pipework around the top of the cylinder. Those two pictures answer most of what anyone will ask you on the phone.

Why a Megaflo has no expansion vessel on the wall

Water expands as it heats and an unvented cylinder is sealed, so the extra volume has to go somewhere. Most manufacturers give it somewhere by fitting a separate vessel with a rubber diaphragm on the cold feed. Megaflo instead holds a pocket of air inside the top of the vessel, behind an internal baffle, and that trapped air compresses as the water expands. Nothing hangs off the pipework, nothing external can perish, and the installation looks far neater in a cramped cupboard.

The trade off is that air dissolves slowly into water. Over months the pocket shrinks, and once it has gone the expanding water has nowhere left to go, so it lifts the expansion relief valve and you see water running through the tundish every time the cylinder reheats. That is not a broken cylinder. It is a cylinder telling you the air gap needs restoring, which is a manufacturer specific procedure carried out at the annual service: isolate, drain to a set point so air re enters the vessel, then refill.

This is the biggest practical difference between a Megaflo and a Telford, Gledhill, OSO or Albion cylinder, and it cuts both ways. There is no external diaphragm to fail, which removes a common wearing part. But the design depends on a step that only happens if somebody services the cylinder each year and knows to do it. Where a Megaflo has been left alone for five years and now drips through the tundish on every reheat, this is the first thing we check and usually the answer.

• Lost air gap: discharge through the tundish during or shortly after a reheat.
• It is a service procedure, not a part, so there is nothing to order in advance.
• Never cap or plug a discharging valve. It is discharging for a reason worth finding.

The parts that actually fail on a Megaflo

The vessel is rarely the problem. The parts that fail are the wearing components bolted to it, and the list is short and predictable: the immersion element, the controls attached to it, the valves in the inlet control group, the temperature and pressure relief valve, and on indirect cylinders the motorised valve and control gear that decide when the coil gets hot. Every one is changeable with the cylinder left where it stands.

Immersion elements are the most frequent single failure in London, and the reason is water rather than manufacture. An element scales, the scale insulates it, the insulated element runs hotter than it was designed to and eventually gives up. The Megaflo element is a manufacturer part rather than a plain merchant element, so an engineer needs the model before arriving. That is a dependency worth understanding rather than a fault, but it does mean poor preparation turns a morning job into a return visit.

The inlet control group is next. It carries a strainer, a pressure reducing element, a check valve and an expansion relief valve, and any of them can silt up or begin passing after years on hard water. A passing expansion relief valve produces the same tundish drip as a lost air gap, which is exactly why the two need telling apart rather than guessing. The temperature and pressure relief valve deserves separate respect: it is the last defence against a store that has lost control of its temperature, so if it lifts, the question is what made it lift.

• Immersion element with its thermostat and thermal cut out.
• Inlet control group: strainer, pressure reducing valve, check valve, expansion relief valve.
• Temperature and pressure relief valve on the cylinder body.
• Lost internal air gap, restored at service rather than replaced.

What hard London water does to a Megaflo

Almost the whole London supply area is hard water, and hard water ages a hot water cylinder here faster than it ages the same cylinder elsewhere. Scale comes out of solution on the hottest surfaces, which means the immersion element and the outside of the boiler coil. A scaled coil transfers less heat, so the household notices slow recovery long before anything fails. A scaled element runs hot under its own crust and burns out. The vessel is unaffected, which is why a twenty year old Megaflo can be perfectly sound while its third element sits in the back of a van.

Three habits reduce it. Specify an Incoloy or titanium sheathed element rather than the cheapest copper one, because the better sheath tolerates scaling far longer. Do not run the store hotter than the household needs, because every extra degree accelerates deposition. And have the cylinder serviced annually, because that is when the strainer gets cleaned, the relief valves get checked and the air gap gets restored.

Scale also explains a fault people misread as a dead cylinder. Rumbling or ticking during reheat is usually scale on a heating surface rather than anything structural. It is a maintenance finding, not a replacement verdict.

If your Megaflo is heated by immersion elements alone, the element is the entire heat source. The cheapest one is a false economy in water this hard.

Should you repair a Megaflo or replace it

Repair, unless the stainless vessel itself is weeping. That rule holds for nearly every call we attend. A dead immersion, a failed thermostat, a passing relief valve, a silted strainer, a lost air gap or a stuck motorised valve are all ordinary component failures on a cylinder with plenty of life left, and replacing the whole appliance because of one of them is the most expensive mistake available in an airing cupboard.

The vessel is the exception because it cannot be patched. Once it is weeping the cylinder is finished regardless of how healthy every other part is. The tell is water appearing at the cylinder itself rather than at a joint, a valve or the tundish, and it needs confirming before anybody quotes, because a weeping compression fitting and a weeping vessel look identical as a damp patch on a cupboard floor.

Two other things push a working cylinder towards replacement. The first is age combined with parts support: on a pre Eco cylinder a component that is no longer produced turns a simple fix into a search, and at some point the sensible answer changes. The second is that the cylinder never suited the property. A store that runs out halfway through the second shower is not a fault to repair. It is a specification that was wrong, and no repair fixes that.

• Repair: immersion, thermostat, thermal cut out, relief valves, inlet group parts, air gap.
• Replace: a weeping stainless vessel, confirmed rather than assumed.
• Consider replacing: obsolete parts on an old model, or a cylinder that never fitted demand.

Megaflo cylinders in real London properties

Most of the Megaflo cylinders we work on sit in one of three situations. In a Victorian house or conversion the cylinder is usually in a first floor cupboard, replacing a vented tank, and the recurring problem is access rather than the cylinder. An immersion element needs real room to withdraw, and we regularly find a cylinder boxed in so tightly that a twenty minute element change turns into dismantling joinery. Tundish discharge pipework is the other conversion headache, because it has to reach a safe visible point outside and the route through a period house is rarely convenient.

In a new build the cylinder often sits in a utility cupboard alongside underfloor heating in liquid screed, and the complaint is almost never storage volume. It is that the hot water and heating sides want different flow temperatures and the design has not separated them properly. That is a system design conversation, not a cylinder fault.

In a purpose built block, stop before you assume. Plenty of London flats have a thermal store such as a Pulsacoil rather than an unvented cylinder, and it is a completely different appliance with different components, failures and service. Both are white, live in a cupboard and make hot water, so the two get confused constantly. Establish which one you have before anybody quotes to replace it.

Before any new cylinder is ordered, the incoming mains flow rate and static pressure should be measured at the property. An unvented cylinder only delivers what the supply feeds it.

What drives the cost of replacing a Megaflo

The cylinder itself is only part of it, and the variables are predictable enough that you can sanity check a quote without knowing the trade price of anything. Capacity and variant drive the appliance cost, so a slimline or a large heat pump rated cylinder costs more than a standard barrel of the same litres. Access drives the labour, because carrying a heavy cylinder up a narrow Victorian staircase into a cupboard with a fixed door frame is a different day from wheeling one into a ground floor utility room.

After that it is how much of the surrounding installation has to change. Reusing sound pipework, an existing discharge route and a suitable position is the cheap version. Moving the cylinder, running a new discharge pipe to an outside wall, upgrading the cold feed, altering the heating side controls or making good joinery all add work that has nothing to do with the brand on the front. Our own rates live on our pricing page rather than in an article, and any quote should itemise appliance, labour, parts and making good separately so you can see what is driving it.

The one cost worth avoiding entirely is replacing a cylinder that did not need replacing. Get the fault diagnosed first. In most cases an unvented cylinder repair puts the system right without anybody touching the vessel, because the failed item is an element, a thermostat, a valve or a lost air gap. Where the steel has genuinely gone, a hot water cylinder replacement is the correct answer and the specification decisions above become the ones that matter, because you are choosing the appliance you will live with for the next two decades. Diagnosis first, brand second.