Megaflow Boilers and Unvented Hot Water Systems: How Much Do They Cost?

Megaflow boilers (also known as unvented hot water cylinders or systems) give you good water pressure throughout your home, regardless of how many taps or showers are running at the same time. This makes them a great choice for large families or shared houses.

Unvented hot water cylinders cost around £400 to £2,000, depending on how much water they're able to store. As a rule of thumb, the more bathrooms you have, the bigger the cylinder you'll need.

Brands like Heatrae Sadia and Worcester Bosch make a range of megaflow systems for domestic properties. Installation costs from £300 up to about £1,000, and must be carried out by an engineer with a G3 qualification.

A megaflow boiler sounds exciting and dramatic; an unvented hot water cylinder distinctly less so. Well, they’re the same thing. Designed to provide a good, strong flow of water to every bath, shower and tap in your house, even during the morning rush, megaflow boilers save space by doing away with the large cold water tank in the loft. Instead, they heat water - using immersion heaters inside the cylinder - directly from the mains supply.

Just before we go on: there is a particular brand of cylinders and boilers called Megaflo, which is so well known that its name has become synonymous with megaflow cylinders. However, this article is about megaflow boilers in general.

What's in This Guide to Megaflow Boilers?

Megaflow Boiler System Explained

Until recently, it was the norm for hot water for homes to come from a big cold water tank in the loft, supplied from the mains. Under this system, the water sits around in the tank until someone puts the boiler on, when it is fed into a hot water cylinder (usually in the airing cupboard), heated up, and then sent to the taps nice and hot.

With a megaflow system, there is no cold water tank. The water comes into the cylinder straight from the mains, where it is heated by either a boiler, energy from solar panels or an electric water heater called an immersion heater (so-called because it's inside the water). The cylinder, which will be made from either copper or stainless steel, will store the water until you need it.

What's the Big Advantage of a Megaflow System?

Hot water pressure megaflow boiler

The big advantage is that it eliminates one of the problems you can get with a traditional system: poor water pressure. A system with a tank in the loft uses gravity to draw the water down from the tank, through a pipe called the vent pipe and into the cylinder. But relying on gravity means that the higher you are in the house, the lower the water pressure. Some people install electric pumps to combat this problem.

A megaflow system doesn’t use gravity in this way. Instead, when you turn a tap on, cold water flows into the cylinder at mains pressure, pushing the hot water out of the top at mains pressure too. You can have a powerful shower or a nice deep bath that doesn’t take half an hour to fill.

But hang on, doesn’t water expand as it’s heated up?

Yes, it does. And if you have a tank in the loft, any expanding water that the cylinder can’t hold flows back into the tank. Megaflow cylinders, in contrast, come with their own expansion facility to stop the pressure getting too high. This can take the form of an air bubble trapped at the top of the unvented cylinder, which allows the water to expand, or an external container to hold the excess water (usually called an expansion vessel).

The expansion vessel should be a minimum of 10% of the cylinder’s capacity. So, for example, a 250-litre cylinder should have a 25-litre expansion vessel.

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To answer that, you’ll need to look at the advantages and disadvantages of an unvented hot water system, and how it compares with both a traditional system and a combi boiler.

Benefits of Megaflow Boilers

You can run several hot water taps at the same time ideal for large families, shared houses or anywhere where several people might want to enjoy a shower, bath or sink wash at once.

You’ll save space compared with a traditional system you don’t need to have a loft for your tank, and if you do have a loft, you can use it for other things. Megaflow systems also tend to be less noisy than traditional systems.

It’s suitable if your home doesn’t have a gas supply because you can heat the water with an electric immersion heater.

The water can’t get contaminated it’s not sitting around in a tank for days or weeks on end. An unvented cylinder system is completely sealed.

There’s no risk of frozen loft pipes or of the subsequent flooding when they then burst.

You can put the unvented hot water cylinder almost anywhere you like – because you’re not relying on gravity to move the water around your home.

Disadvantages of Megaflow Boilers

You still need space for the unvented hot water cylinder a combi boiler is the most space-saving option as it has no tank or cylinder.

You don’t have unlimited hot water on demand you have as much as the cylinder holds, and if it runs out or cools over time, you’ll be washing in cold water until it heats up again.

Any unused hot water wastes energy – if you don’t use all the hot water in the cylinder, it will eventually go cold, meaning you’ve wasted money and energy heating it. Good insulation of the unvented hot water cylinder should reduce this issue.

It needs to be installed and serviced annually by a specialist engineer not all plumbers are qualified to install unvented hot water cylinders. Your cylinder must be installed by an engineer who has what is known as a G3 qualification (G3 refers to the section of the national Building Regulations with which unvented cylinders must comply.) Your engineer should have obtained or renewed the qualification within the last 5 years, and should be able to provide a card issued by a body such as the Institute of Plumbing or the Construction Industry Training Board.

Megaflow cylinders aren’t compatible with solid fuel burners or any other boiler without a thermostat, unless you install extra safety measures to combat the risk of the water overheating. They’re also not compatible with some power showers and mixer taps, or ascending spray bidets.

Other Things to Think About

What is your mains pressure? Your mains water pressure should be at least 1 to 1.5 bars, and your flow rate at least 20 litres per minute, for a megaflow system to be worth installing.

Your installer will be able to check this for you, or you can check it yourself with a pressure gauge, a measuring container called a Weir gauge or indeed a measuring jug and a stopwatch.

There are solutions to poor pressure and flow, including accumulator tanks, pumps and even laying a new pipe to supply your home, though this is costly and disruptive.

How old are your radiators? If your heating system is quite old, an unvented hot water cylinder could produce water at a higher pressure than your pipes and radiators can handle – meaning you’d need to have work done to fix this.

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The Difference Between Direct and Indirect Unvented Cylinders

There are 2 kinds of unvented hot water cylinders: direct and indirect. These terms refer to the way the water is heated.

Direct unvented hot water cylinders use an immersion heater, or possibly 2, inside the cylinder.

Indirect unvented hot water cylinders use external sources of heat, i.e. your boiler (using a heat exchanger) or solar panels. They may also have an immersion heater which you can either switch on at the same time to heat the water more quickly, or use as a backup if your boiler breaks down.

Indirect cylinders tend to heat the water faster than direct cylinders, because a boiler will be more powerful than the immersion heater.

They will also probably be cheaper to run, as electricity is almost always more expensive than gas, though a direct cylinder may be cheaper to install as it doesn’t need to be connected to the boiler.

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Megaflow Boiler System Cost

As with any type of boiler or electric water heater system, costs can vary wildly. The cost for a megaflow hot water cylinder alone can be anywhere from around £400 for a small 120-litre cylinder to more than £2,000 for large high-end models.

The table below shows you the price range of different sized unvented hot water cylinders made by some of the best megaflow system brands:

Cylinder Capacity (Litres)
Price Range
Heatrae Sadia
70 to 500
£550 to £2,100
Worcester Bosch
90 to 300
£700 to £1,150
90 to 1,000
£950 to £3,400
90 to 400
£300 to £800
90 to 400
£170 to £1,650

Unvented Hot Water Cylinder Installation Cost

For the installation, you can probably expect to pay between £400 and £1,200 to install an unvented cylinder from scratch. For a replacement, you’re looking at anything between £275 and £450.

As with all boiler and heating installations, this is a job that cannot be done on the cheap; it's not worth risking some potentially serious consequences. Most unvented cylinders come with a 25-year guarantee.

What Size Cylinder Do You Need?

Your cylinder needs to be big enough to provide enough hot water for your home, but not too big, or you will be paying to heat and store water that you don’t need (plus a bigger cylinder will cost more in the first place). As we've already said, indirect cylinders heat up faster than direct cylinders and therefore can be smaller.

Here’s a rough guide to cylinder sizes for different homes:

Bathrooms and Showers
Indirect (litres)
Direct (litres)
75 to 120
120 to 150
1 or 2
210 to 250
250 to 300
Larger properties
2 or more

These figures are just a guide. The size of cylinder you need will be affected by:

• how many baths, showers and sinks you have

• how many people live in the house

• how many of them are likely to use the hot water at the same time

• and how much water each person uses per day

To help you work out roughly how much water your household uses on an average day, here are some estimates of how much water each of the following activities require:

Having a bath: 60 to 100 litres
Showering for 5 minutes: 50 to 90 litres
Using the washing machine: 50 to 100 litres
Using the dishwasher: 12 to 20 litres
Running a tap for 10 seconds: up to 1 litre

The Hot Water Association divides people into 3 groups:

• Low water consumption: 20 to 30 litres a day

• Average water consumption: 30 to 50 litres a day

• High water consumption: 50 to 70 litres a day

On this basis, it estimates that a typical 4-person household would use around 200 litres of hot water a day. But a household using 200 litres a day doesn’t need a 200-litre cylinder – you need the total amount of hot water that’s likely to be needed at any one time, plus a bit more to avoid running out.

To get an accurate quote from a qualified installer, hit the button below and complete our quick contact form.

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Best Megaflow System Brands

Heatrae Sadia

Founded in Norwich almost 100 years ago, Heatrae Sadia is now the UK's biggest manufacturer of electric water heaters and sells its products worldwide. It manufactures both Megaflo cylinders, having bought the brand in 1993, and the more budget-friendly PremierPlus range. Megaflo cylinders are tested to withstand 15 bars of pressure – 5 times their normal operating pressure.

Heatrae Sadia is part of the same company as Baxi, a traditional British boiler manufacturer, and Baxi makes the Megaflo boilers. Santon, another unvented hot water cylinder manufacturer, is also part of this group of companies.

Worcester Bosch

Widely respected as a reliable boiler manufacturer, and with a 5-star rating on the Trustpilot website, Worcester Bosch even has a royal warrant. Its Greenstore unvented cylinders are designed to work with its Greenstar boilers and Greenskies solar panels (though of course they can be used with other brands too) and range in size from 90 litres to 300 litres.


Established in 1917, Viessmann is one of Europe’s biggest heating manufacturers, employing 9,600 people around the world, and is generally considered a reliable brand. Its mighty Vitocell 100 comes in sizes up to a stonking 1,000 litres. Viessmann is increasingly focusing on renewable energy and actively encourages its customers to use solar power to heat their water.


Founded by brothers Norman and Joe Gledhill in Blackpool in 1926, Gledhill has manufacturing depots across the UK. Its Stainlesslite cylinders are among the lightest on the market. The Stainlesslite Plus Horizontal Indirect cylinder is, as the name suggests, fitted horizontally – meaning it can be installed in places where headroom is restricted, such as lofts.


A family-run business based in (you guessed it) Telford, the company has been making unvented cylinders since 2002 with product names including the Telford Tornado and the Telford Tempest. The latter comes with a lifetime guarantee, a capacity of up to 500 litres for domestic use and is made from stainless steel that’s up to 40% thicker than some of its rivals. And if you thought 500 litres was large, a commercial-sized Tempest can hold up to 4,000 litres.

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Are Megaflow Systems Safe?

The water inside a megaflow cylinder is stored at high pressure and will expand as it warms up. Fortunately, unvented hot water systems are fitted with a series of safety features designed to prevent the cylinder ever getting anywhere near the stage where it could be dangerous. These should include:

Thermostats – to stop the water from overheating. The thermostat should turn off the heat when the water temperature reaches 60 to 65ºC.

Safety cut-out – if the thermostat fails, and the water carries on heating up, there should be a back-up safety cut-out which switches off the heat at around 80 to 85°C.

Temperature and pressure release valve – if both of the above fail, the temperature and pressure release valve should open (usually at around 90 to 95°C, or when the pressure reaches between 6 and 7 bars) to let hot water and steam out into the safety discharge pipe.

Safety discharge pipe this must take away the hot water and steam and discharge it somewhere safe outside the building where it cannot scald anyone. The pipe should be made from a material like copper which can withstand extremely high temperatures. As the hot water flows away, fresh cold water will flow into the cylinder, helping to keep the temperature down.

Tundish – this is a vessel which collects the water from the release valves and feeds it into the safety discharge pipe. The point of the tundish is that it has to be somewhere where you can see it, so that you will see if water is running through it and realise something is wrong.

Pressure reducing valve – as the cold water comes in, there should be a filter to get rid of any grit, a pressure reducing valve to make sure the water comes in at a safe pressure, and a valve to stop water flowing backwards out of the cylinder.

Expansion release valve – this should open automatically to let out the expanding water if for some reason the expansion vessel or air bubble isn’t doing its job.

The probability of all these measures failing is extremely small. Megaflow systems are safe – they would not be allowed in homes otherwise. What this does mean, however, is that you must have the cylinder installed by a properly qualified engineer. You should also have it serviced annually to make sure that all these safety features are working properly. In this respect your unvented cylinder is no different from your gas boiler, your car or anything else which you have checked over each year for your own safety.

The servicing isn’t a legal requirement, but it should be done, and by a G3 qualified engineer. If nothing else, it keeps the warranty valid, nips any issues in the bud, will probably prolong the life of your cylinder and is quite possibly a condition of your home insurance.

Holding a G3 qualification means the engineer is on the Competent Persons Register, which is a list of experts who have been certified by the Government. The engineer has to tell your district or borough council that he or she has installed your unvented hot water system, and give you a certificate confirming that the work has been done.

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Combi Boiler or Megaflow Cylinder?

Combi boilers don’t have a tank or cylinder and heat the water as it comes off the mains, so may struggle to provide a decent supply of hot water to more than one tap or shower at the same time. This means they’re often not the best choice for homes where lots of people are likely to want continued hot water simultaneously.

However, if there is no issue with competition for hot water, a combi boiler is probably a better choice than a megaflow, as it is the most compact option and will supply enough heat and hot water for your needs – plus you’ll never have to wait for the water to warm up. A megaflow cylinder will take up space and cost you money heating more water than you need.

If you have solar panels, a megaflow system is probably a better option – a combi is not easily compatible with solar panels, as it heats the water only when it’s needed, whereas solar panels work continuously in daylight.

To find out what might best suit your home, hit the button below to complete our quick contact form and you'll hear from a qualified installer.

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