What Is a Megaflo? Is It Just an Unvented Cylinder?

A Megaflo is a brand of unvented hot water cylinder, made by Heatrae Sadia. It is a sealed, stainless steel cylinder fed straight from the cold mains, so it gives mains-pressure hot water to every tap and shower. The name has become shorthand for any unvented cylinder, much as people say Hoover for a vacuum cleaner, which is why an installer may call your new cylinder a Megaflo even when it is made by someone else.

Is a Megaflo the same as an unvented cylinder?

Every Megaflo is an unvented cylinder, but not every unvented cylinder is a Megaflo. Megaflo is one manufacturer's product line; unvented is the type of system.

Heatrae Sadia sits within Baxi Heating, and Megaflo was one of the first unvented cylinders to become widely known in UK homes. Other manufacturers make unvented cylinders that work on the same principle, including Gledhill, Kingspan, Telford, Joule and OSO, and several boiler makers sell their own, such as Vaillant's uniSTOR and Worcester Bosch's Greenstore. They all store hot water under mains pressure, and they all carry the same set of safety devices. When a quote says "Megaflo-type cylinder", ask for the actual make and model so you know what you are paying for. If you are still working out whether you want a sealed cylinder at all, our comparison of unvented and vented replacement cylinders covers that choice first.

How does a Megaflo work?

Cold water enters from the mains through a pressure reducing valve, is heated inside the cylinder by a coil from the boiler or by an immersion heater, and leaves at close to mains pressure when you open a tap. There is no tank in the loft and no pump.

When water heats up it expands, and in a sealed cylinder that extra volume has to go somewhere. Most unvented cylinders use a separate expansion vessel, the small red or white tank mounted beside them. Megaflo has long been known for handling expansion with a space inside the cylinder instead, often described as an internal air gap or air bubble, although some models and sizes are supplied with an external vessel. It is worth knowing which one you have, because the two are maintained differently at the annual service. Either way, the cylinder also has an expansion relief valve, a temperature and pressure relief valve, thermostats and a thermal cut-out, all discharging through a tundish to a safe point outside.

Which Megaflo do you need?

The right Megaflo is decided by three things: how the water is heated, how much hot water the household uses, and the space available. The brand comes after those.

• Indirect models have a coil heated by a gas boiler, with an immersion heater as backup. This is the usual choice alongside a system boiler or a regular boiler converted to a sealed system.
• Direct models are heated by electric immersion heaters only, and suit homes without a boiler, typically all-electric flats on an off-peak tariff.
• Heat pump models have a larger coil, because a heat pump heats water at a lower temperature than a boiler. If a heat pump is on the cards in the next few years, choosing this version now avoids a second cylinder change later.
• Solar models have a second coil so a solar thermal panel can pre-heat the water.

Capacity is where most mistakes happen. Too small and the second shower runs cold; too large and you pay to heat water nobody uses. Our guide to what size hot water cylinder you need works through bathrooms, occupants and recovery time.

Is a Megaflo better than other unvented cylinders?

Not in any way that should decide your purchase on its own. A well-sized cylinder from any mainstream maker, fitted and commissioned properly, will outperform a badly sized or badly fitted Megaflo.

Megaflo's advantages are familiarity and spares: most heating engineers in London have fitted and serviced them, and parts for the valves and controls are widely stocked. Other brands compete on price, on slimmer or horizontal shapes for awkward cupboards, and on bundled warranties when the cylinder is bought with the same maker's boiler. Warranty terms vary by brand and usually depend on the cylinder being registered and serviced every year, so compare the written terms on each quote rather than the headline. What matters more than the badge:

• the cylinder is sized to the household, not to whatever the van carries;
• the mains supply has been measured and can feed it;
• there is a compliant route for the discharge pipe from the tundish;
• the installer holds the G3 unvented qualification and notifies the work under Building Regulations.

What does replacing a cylinder with a Megaflo involve?

It depends on what you have now. Swapping an old unvented cylinder for a new one is usually a contained job; converting from a vented cylinder with loft tanks is a bigger one.

Like for like, the installer isolates and drains the old cylinder, fits the new one with its valve set, reconnects the boiler coil, immersion and controls, and checks that the existing discharge pipe still meets the regulations. On a conversion from a vented system, the loft tanks and vent pipe come out, the cold feed is re-run from the mains, the discharge pipe is routed to a safe termination point, and the boiler side may need converting to a sealed system if it is still open-vented. The job also has to be notified to Building Control, and it can only be done by someone holding the G3 qualification. Our note on checking your cylinder installer is G3 qualified explains how to confirm that before work starts. Any work on the gas boiler itself must be done by a Gas Safe registered engineer.

When is a Megaflo the wrong answer?

When the mains supply cannot feed it, when a combi would do the same job more simply, or when there is nowhere safe to discharge to. In those cases, a new unvented cylinder of any brand is money spent on the wrong fix.

An unvented cylinder can only deliver what the incoming main supplies. In many older London terraces the supply pipe is narrow lead or old iron, and flow drops badly when a neighbour runs a tap; measuring flow and pressure at the survey tells you whether the pipe needs upgrading first. In a one-bathroom flat, a combi boiler often gives the same mains-pressure hot water with no cylinder at all, and frees the cupboard. And in some flats there is simply no compliant route for the discharge pipe, which rules unvented storage out regardless of brand. If a vented cylinder with a shower pump is working well and the pressure suits you, keeping it is also a legitimate choice.

If you want a survey to measure your mains, size the cylinder and say plainly whether a Megaflo, another unvented cylinder or a combi fits your home, see our hot water cylinder replacement service.