What Is a Megaflo System Unvented Hot Water Cylinder?

If you’ve ever stayed somewhere with powerful showers and fast-filling baths, there’s a good chance the property was using a Megaflo-style hot water system.

A Megaflo system is an unvented hot water cylinder connected to the mains water supply, designed to deliver strong, consistent hot water pressure across multiple taps and bathrooms at the same time.

For many UK homeowners upgrading their heating system, it offers a modern alternative to traditional tanks and gravity-fed cylinders — especially in larger homes.

At Boilers2go, we usually explain it like this: a Megaflo system stores hot water like a traditional cylinder, but delivers it at mains pressure like a combi boiler.

What a Megaflo System Actually Is

When people say “Megaflo system,” they’re usually referring to an unvented mains-pressure hot water cylinder, often paired with a system boiler or regular boiler.

Instead of storing cold water in a loft tank, the cylinder takes cold water directly from the mains supply, heats it, and stores it ready for use.

That means hot water pressure matches your mains pressure, giving stronger showers and more consistent flow around the home.

Inside the cylinder, water is typically heated by the boiler through a coil, although immersion heaters can also provide backup or standalone heating.

Because the system is sealed and pressurised, expansion and safety are managed by internal air gaps, expansion vessels, and temperature-pressure relief valves, rather than a loft tank.

How a Megaflo System Works in a Home

A Megaflo-type setup is actually quite simple once you understand it.

Cold water enters the cylinder directly from the mains supply through a control valve assembly that regulates pressure. The boiler then heats the stored water inside the insulated cylinder.

When a tap or shower is opened, mains pressure pushes hot water out of the cylinder while fresh cold water enters from below, keeping the supply steady.

This is why multiple bathrooms can use hot water at the same time without losing pressure, which is something many traditional systems struggle with.

Why Homeowners Choose Megaflo Systems

One of the biggest reasons people install Megaflo-type systems is hot water performance.

Because the system runs at mains pressure, showers feel more powerful and baths fill much faster than with gravity-fed cylinders. Hot and cold supplies are also more balanced, which helps keep shower temperatures stable when other taps are used.

Another major benefit is removing loft water tanks. Without a cold water cistern in the attic, homeowners gain storage space and reduce the risk of tank leaks or freezing pipework.

Megaflo systems are particularly popular in family homes with two or more bathrooms, where demand for hot water is higher and a combi boiler might struggle to keep up.

Modern cylinders are also well insulated and efficient, meaning stored water stays hot longer and reheats quickly. They can also integrate with solar thermal systems, heat-pump cylinders, and smart heating controls, making them suitable for future upgrades.

Things to Consider Before Installing One

Megaflo systems aren’t the right solution for every home, and the biggest factor is mains water pressure.

Because the system relies on incoming mains supply, poor water pressure or flow will limit performance. In some properties, the incoming main may need upgrading before installation.

Cost is another consideration. Installing a Megaflo-type system is usually more expensive than replacing a combi boiler, because it involves a cylinder, safety controls, and specialist installation work.

Unvented cylinders are also covered by G3 building regulations, which means installation and servicing must be carried out by a qualified engineer with unvented certification.

Space matters too. You’ll need a suitable cupboard or plant area to house the cylinder and allow safe access for maintenance.

Direct vs Indirect Megaflo Systems

There are two main types of Megaflo-style system used in UK homes.

Direct Megaflo System

A direct Megaflo system uses an immersion heater to heat the stored water using electricity. The cylinder fills from the mains, heats the water internally, and stores it until needed.

Because the heating element is built into the cylinder, this type of system is self-contained and doesn’t require a boiler, which can make it useful in certain properties.

Indirect Megaflo System

An indirect Megaflo system is connected to a boiler, usually a system boiler. The boiler heats the water inside the cylinder via a coil, and the hot water is stored until required.

Most indirect cylinders also include a backup immersion heater, which can be used if the boiler fails or needs maintenance.

One important tip we always give homeowners is not to run the immersion heater and boiler at the same time, as this wastes energy and can cause unnecessary overheating.

Indirect Megaflo systems are by far the most common setup in UK homes.

Megaflo System Size Guide

Choosing the right cylinder size is essential for good performance. The size you need depends on property size, number of bathrooms, and how much hot water the household uses.

As a general guide:

Hot water demand also depends on the number of people in the home. On average, one person uses around 35–45 litres of hot water per day, so larger households usually need bigger cylinders.

At Boilers2go, we always recommend a proper assessment before installation. A qualified heating engineer will look at bathrooms, usage patterns, boiler output, and mains pressure to make sure the cylinder is correctly sized.

Getting this right is what ensures showers stay powerful and hot water doesn’t run out too quickly.

How Much Does a Megaflo System Cost?

The cost of a Megaflo cylinder on its own starts at around £1,000 just for the unit. If you aren’t a professional and need someone to fit it then the total cost of installing a Megaflo-type system depends mainly on three things:

• Cylinder size
• Model
• Installation complexity

Because unvented cylinders require specialist installation and safety components, they typically cost more than a simple combi boiler replacement. However, for larger homes, the performance improvement can be significant.

Is a Megaflo System Better Than a Combi Boiler?

It depends on the home and how much hot water you use.

A combi boiler heats water instantly and saves space because there’s no cylinder. That works well in smaller homes with one bathroom.

A Megaflo system, on the other hand, stores hot water and delivers it at mains pressure, which makes it ideal for larger homes with multiple bathrooms or higher hot water demand.

In real-world terms, combi boilers suit smaller households, while Megaflo systems suit busy family homes.

Who a Megaflo System Is Best For

Megaflo-type systems usually make the most sense in homes where:

• Multiple showers may be used at once
• Strong shower pressure is important
• You’re removing old loft tanks during renovation
• You want flexibility to add renewable heating later

They’re generally less suitable for small flats or homes with limited space, where a combi boiler is often simpler and more cost-effective.

Boilers2go Advice

If you’re considering installing a Megaflo system, the most important first step is checking your incoming mains pressure and hot water demand.

When the property is suitable, a Megaflo-type system can transform hot water performance and make everyday use far more comfortable — especially in busy homes.

At Boilers2go, we often see homeowners upgrading from older gravity systems and immediately noticing the difference in shower pressure, bath-fill speed, and overall reliability.

It’s not always the cheapest option, but for the right property, it’s one of the most effective hot water upgrades you can make.