How to choose between MegaFlow or Combi

This article explores the difference between a Megaflow system and Combi boiler. If you’re choosing a new hot water system, then you may be confused with the vast choice. Do you go for Megaflow or a Combi boiler? What exactly is the difference?

This article explains the difference between Megaflow (or its misspelling Mega Flow) and Combi (short for Combination) boilers and what situations they are best for.

By the end of this article you’ll be better informed about whether Megaflow or Combi is the best option for you.

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Call us and with a few questions and with our comprehensive experience, we will be able to advise you with your’re best off with a MegaFlow or Combi. Contact us for a no-obligation phone consultation/quote. We cover Watford, North London, West London, Hertfordshire and Middlesex locations.

What is a Megaflow System?

A Megaflow is also known as a Megaflow system or a Megaflow unvented system. Sometimes it’s called a Megaflow Boiler. In fact, a Megaflow technically isn’t a boiler at all. It’s a water tank or cylinder made of stainless steel that heats water.

Water from the mains feeds into the cylinder and is kept at mains pressure. This is why it’s called ‘unvented’. The result is you get a high pressure supply of hot water.

The Megaflow heats the water within it. There’s two ways it can heat the water:

1. an immersion heater heats the water. This is also called ‘Direct’ Unvented Megaflo; and

2. the heat from your central heating system. This is called an ‘Indirect’ Unvented Megaflow.

A Megaflow system only heats water and doesn’t heat radiators.

With a Megaflow you have a constant store of hot water.

Megaflows can be connected to solar panels or the electric supply, which is perfect if there’s no gas supply to the property.

Megaflows are manufactured by British company Heatrae Sadia.

What is a Combi Boiler?

A Combi Boiler is also known as a combination boiler. Combi boilers deliver instant hot water on demand without the need for any hot water tank.

This means you save on space as there’s no need to have a separate hot water cylinder.

The big difference is that Combi boilers also heat your radiators.

Since there’s no hot water cylinder to keep hot, they are very energy efficient and therefore much cheaper to run.

Unlike Megaflows, a combi boiler doesn’t require a much water pressure to operate, which is handy in areas of low mains water pressure.

There’s a big choice of manufacturers who make Combi Boilers, including Worcester-Bosch, Ideal and Baxi. You can read our review of the choice of Combi boilers in our article ‘What’s the best Combi boiler on the market’ here.

Or you can contact us and we can help you choose the best one for your home’s requirements.

How to choose between Megaflow and Combi

Large homes that have 3 or more bathrooms or showers where they are likely to be used at the same time, then you’re best getting an unvented hot water storage vessel, such as a Mega Flow. That way there be plenty of hot water for multiple baths or showers.

If you’re home is smaller with just 1 bathroom, then a Combi boiler is the best solution.

That being said, if you have no gas supply to the property, then a Megaflow system is the option for you.

Megaflow pros and cons

Megaflow Pros


Lots of stored hot water


High pressure and volume of hot water up to 25 litres a minute


Can run up to 3 baths at the same time


No need for booster pumps for showers and baths


No need for cold water storage tanks as all off the mains water


Can use solar panels to heat water to reduce fuel costs


Can be installed when there is no gas supply to property


Minimum flow rate up to 25 litres of water per minute needed from water main with a minimum of 1 bar operating pressure on a 22mm diameter cold water main pipe

Megaflow Cons


Takes up storage space


Very heavy and bulky


More electrical and mechanical controls that could breakdown


Takes about 45 minutes to recover when all the stored hot water is used up


Less energy efficient to run than a combination boiler if connected to electric only


Large sized water supply


Requires servicing every year

Combi pros and cons

Combi Pros


Takes up very little space


Doesn't need much water pressure to operate only 0.7 bar. Or 7 litres a minute


Instant hot water when you need it only 20 seconds heat up time


You can have as little or as much as you wish of hot water as you like


Very energy efficient as there's no need to heat any storage vessel


Half the cost of an unvented cylinder and boiler installation


Cheaper to maintain and service


Therefore, perfect for smaller properties (3 bedroom and under)


No risk of legionella


Can run on oil, gas, electric and LPG


Fast installation


Less electrical and mechanical controls to breakdown


Up to 98% energy efficient and A class rated for efficiency which means they cheaper to run

Combi Cons


Can only run 1 bath or shower at a time


Will not cope with more than 2 bathrooms working at one time


May require bigger gas supply


Needs gas to property


Can only deliver a maximum of 16 litres a minute of hot water


Not suitable for large properties with more than 2 bathrooms

Mega Flow or Combi? Which is Right For You?

So which is right for you – Mega Flow or Combi? As you will have seen, it largely depends on the number of bathrooms you have and whether you have gas supply. If your property is 3 bed or under, then consider the Combi boiler. If you have more than 3 bedrooms and more than 1 bathroom plus you have the storage space, then a Mega Flow will be a good option.

What next

If you need more help choosing or want us to install your Megaflow system or Combi boiler, then contact us. We cover locations in and around Watford, London, Middlesex and Hertfordshire. 


Disclaimer: The information contained on this website if for general information purposes only. You should not rely upon the material or information on the website as a basis for making any decisions or actions and we don’t accept liability for anything you chose to do with the information.

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