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High Flow Rate Combi Boilers

Becky Mckay
By: Becky Mckay
Updated: 25th October 2023

A high flow rate combi boiler will make sure your home gets plenty of hot water.

Most combi boilers are suited to homes with one bathroom. But a model with a high flow rate and strong output rating can meet the demands in a home with two bathrooms. However, if your home has low mains water pressure, your combi boiler won't be able to deliver high flow rates.

So let’s find out if a high flow rate combi boiler is right for your home.

Are you interested in upgrading to a high flow rate combi boiler? We can connect you with up to 3 trusted heating engineers from our network to help you install a top-quality combi boiler.

After filling in a short form, we’ll send you up to 3 free, no-commitment quotes from qualified heating engineers in your area, which you can compare to choose the best offer. Simply click the button below to get started.

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What does flow rate mean?

A boiler's flow rate shows how much hot water it will be able to send to your taps in one minute.

However, this all depends on the flow rate of the mains water supply to your home. If the flow rate of your mains is lower than the flow rate of the boiler then it will never reach that level.

For example, a boiler may have a flow rate of 15 litres per minute (l/min). That's 15 litres of hot water for your home every minute. But if the flow rate of the mains water supply is 8 l/min, the boiler won't be able to reach 15 l/min.

Combi boilers are the only boiler type that have a flow rate. System and regular boilers both store hot water in a cylinder so don't have one.

The right flow rate for your home

The more bathrooms, taps and showers that you have around your home, the higher the flow rate will need to be. This is especially important if more than one hot water outlet will be used at a time. Blaze: High Flow Rate Combi Boiler

The mains water flow rate to most UK homes sits somewhere between 10 and 15 litres per minute. If the flow rate of the mains water supply to your home is higher than 15 l/min then your home has a good flow rate.

It's worth knowing that flow rate is different from pressure: flow rate is how much water you're getting while pressure is a measurement of the strength of that flow of water.

Watch out for water pressure that's too high as this can put the pipes under too much strain and lead to problems.

It's not all about flow rate

You need to make sure the combi boiler's central heating (CH) and domestic hot water (DHW) output ratings match the needs of your home too.

Combi boilers are available in a range of outputs but don't go for the most powerful you can afford. Do that and your heating bills could shoot up. You should also avoid going too low as the boiler will struggle to meet demand.

What type of shower can you have with a combi boiler?

The 3 most common shower types that are compatible with a combi boiler are: thermostatic mixer showers, electric showers, and digital showers. Thermostatic shower valves are traditionally considered the best shower valves to pair with your combi boiler. This type of shower means that it will maintain a pre-set desired temperature and will prevent large fluctuations in temperature.

It’s important to know that power shower valves are not compatible with combi boilers because they operate with an in-built pump that needs to be connected to a hot water cylinder - which combi boilers do not have.

High flow rate combi boilers

The Worcester Bosch Greenstar 8000 Lifestyle 50 is the combi boiler with the highest flow rate (20.3 litres per minute). No other boiler comes close.

Combi boiler Flow rate (litres per minute)
Worcester Greenstar 8000 Lifestyle 50 20.3
ATAG IC39+ 17
Navien NCB 40 16.9
Baxi 800 Platinum+ 40 16.4
Ideal Vogue Max C40 16.4

Even though there's daylight between the flow rate of Greenstar 8000 Lifestyle and the other combi boilers, they're still more than enough for most homes.

Worcester Greenstar 8000 Lifestyle

The Greenstar 8000 Lifestyle 50 is actually two boilers: the Greenstar 8000 Life and the Greenstar 8000 Style. They're essentially the same boiler but the Style units are available in black as well as white. Whichever you go for, they're both suited to homes with around 20 radiators and 2 bathrooms.

Greenstar 8000 Lifestyle
Central heating output 35 kW
Domestic hot water output 48.9 kW
Efficiency 94%
Max. Warranty 12 years
Typical price £2,100

ATAG IC39+

The ATAG IC39+ has a high flow rate of 17l/min which makes it a powerful option for homes with 2 bathrooms. While that flow rate isn't quite as high as the Greenstar 8000 Lifestyle, it does come with a longer warranty.

You should consider this boiler if your home has around 15 radiators and 2 bathrooms.

ATAG IC39+
Central heating output 31.2 kW
Domestic hot water output 42.5 kW
Efficiency 94%
Max. warranty 14 years
Typical price N/a

The price hasn't been made available by the manufacturer.

Navien NCB 40

Navien aren't as well known as the other boiler brands we've mentioned. But that doesn't mean their boilers aren't worth considering. The NCB 40 is a highly efficient combi that's covered by 7 year warranty. It's ideal for homes with up to 20 radiators, a bathroom and an ensuite.

Navien NCB 40
Central heating output 33.2 kW
Domestic hot water output 39.6 kW
Efficiency 93%
Max. Warranty 7 years
Typical price N/a

The price hasn't been made available by the manufacturer.

Baxi 800 Platinum+ 40

Baxi is a much loved brand amongst heating engineers. They're efficient, reliable and well protected when it comes to the warranty. This is their most powerful combi and makes a great choice for homes with up to 20 radiators, a bathroom and an ensuite.

Baxi 800 Platinum+ 40
Central heating output 34.4 kW
Domestic hot water output 40 kW
Efficiency 93%
Max. Warranty 10 years
Typical price £1,307

Ideal Vogue Max C40

A flow rate of 16.4 l/min may seem low compared to the Greenstar 8000 Lifestyle. But it's still very high and capable of meeting the hot water demands of almost any home. It's also more affordable than the Worcester while still being covered by a lengthy 10 year warranty.

If your home has 15-20 radiators, a bathroom and an ensuite then this could be the boiler for you.

Ideal Vogue Max C40
Central heating output 32 kW
Domestic hot water output 40 kW
Efficiency 93%
Max. Warranty 10 years
Typical price £1,323

Keep in mind that the prices mentioned in the tables above are estimations. The only way to get an accurate price for a new boiler is by contacting a heating engineer. We strongly recommend getting quotes from multiple heating engineers and not just accepting the first offer you receive. This way, you can ensure you’re getting the best deal.

Our team at Boiler Guide can provide you with up to 3 free, no-commitment quotes from qualified heating engineers in your area. Allowing you to find and easily compare local rates. Click the button below to get your free quotes and choose the best offer.

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High flow rate storage combi boilers

A storage combi boiler has a built-in cylinder which means it can meet higher demand for hot water. There aren't too many on the market but the models that are available have high flow rates.

Storage combi boiler Flow rate when cylinder is full Flow rate CH Output DHW Output Cylinder capacity
Vaillant ecoTEC Plus 938 20 l/min 15.9 l/min 28.6 kW 38.7 kW 15 litres
Viessmann Vitodens 111-W 21.2 l/min 18.1 l/min 29.3 kW 33.9 kW 46 litres

The only problem with storage combi boilers is their size. As there’s a cylinder within the unit, they can be quite bulky.

You can compare all combi boilers on the market with our Boiler Comparison Tool.

How to check your home's water flow rate

Not sure if your home can deal with a high flow rate combi? Then you need to check the mains water flow rate of your home. The good news is that you can easily do this yourself. All you need is a 1 litre jug and a stopwatch.

Start by turning off all the taps. Go round the house and make sure every single tap has been shut off. You'll need to turn the washing machine and dishwasher off too.

Now it's time to get your jug and stopwatch out. While holding the jug under the tap, turn it on full blast and time how long it takes to hit the 1 litre mark.

You're looking for the jug to fill up in less than 6 seconds. If it takes longer than 6 seconds, the flow rate to your home is less than 10 litres per minute:

60 seconds / 6 seconds = 10 litres per minute
So, if your home has a low flow rate then even the combi boilers with the highest flow rates won't be able to supply your home with enough hot water.

Is a combi right for my home?

Most combi boilers are suited to homes with one bathroom. But a high flow rate combi (with powerful hot water output) can be fitted into homes with two bathrooms. Just remember that for a boiler to deliver a high flow rate, your home needs a strong mains water flow rate.

The alternative to a combi is a system boiler. These are considered the best option for homes with two or more bathrooms as they store hot water in a cylinder. When the cylinder is full of hot water, there's plenty to go around. Even if two baths or showers are being run at the same time (something most combi boilers will struggle to do).

To make sure you get the right heating system, visit our guides to the best boiler for:

Whether you want a high flow rate combi or a system boiler, we can connect you with qualified heating engineers near you.

Up to 3 local heating engineers will soon be in touch to arrange a free boiler quote. Once you have your new boiler quotes, compare them and be confident you're getting the most competitive price. Simply click the button below to get started.

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Becky Mckay

About the author

Becky Mckay

Becky has been a writer at Boiler Guide since 2021. Her vast boiler knowledge means she’s ready to help with any home heating query, big or small!

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