Best High Flow Rate Combi Boilers In the UK 2024

Written by

Becky Mckay

Last updated

13 March 2024

Ever wondered why your hot water takes forever to kick in? This is connected to your combi boiler flow rate. If you don’t have the right rate for your home, you’ll be dealing with slow hot water flow to your outlets. 

So how can you fix this? Well, there are plenty of high-quality combi boilers available today that offer a range of flow rates. For those needing to increase their hot water supply, we’ve got you covered. 

In this article, we’ve found the top 5 best combi boilers for high flow rates – here’s a quick overview:

• Worcester Bosch Greenstar Highflow CDi
• Navien NCB700 ON
• ATAG IC39+
• Baxi 800 Platinum+ 40
• Ideal Vogue Max C40

We’ll also break down what you need to know about flow rates, boiler power and the important factors to consider when upgrading your combi boiler. So whether you have 1 or 4 bathrooms, we’ve found boilers to suit every need.

If you’re ready to replace your boiler, but don’t know where to start, we can help. Don’t spend hours searching the web for heating installers, comparing reviews and waiting for callbacks. Instead, you can use our quick form and be connected with up to 3 qualified, trusted installers from your local area.

These installers can supply you with free quotes for you to compare, so you can find the best deal without the stress. There’s no obligation to accept any quote, so there’s nothing to lose! To get started, simply click the button below.

What is the flow rate of a combi boiler?

The flow rate is a term given to describe how much hot water a combi boiler can send through your taps within a set timeframe. This is measured in litres per minute (l/min), the higher the number, the higher the volume of hot water. 

Flow rate only applies to combi boilers (as system and conventional boilers keep a store of hot water) and will typically be somewhere between 9 to 25 l/min. Higher rates are best for homes with multiple bathrooms, where there is more demand for hot water.

It’s important to note that the flow rate can be affected by the strength of your mains water supply. If the water pressure in your area is lower than the flow rate of your combi boiler, the hot water volume per minute will be limited and only ever as strong as your water pressure.

When looking at combi flow rates, it’s key that you also have an awareness of your mains supply flow rate so that you get the best performance from your boiler.

What is the right combi boiler flow rate for your home?

A good standard flow rate is anything between 10-15 litres per minute. This would be suitable for small to medium-sized homes. If you have more than 3 bathrooms, however, consider a combi boiler with at least 15 l/min or higher. This will ensure you can use multiple outlets simultaneously.

Below is a rough guide of suitable flow rates, based on the number of bathrooms in your home.

When choosing the right combi boiler, however, the flow rate is not the only factor to consider. As well as understanding the mains water pressure in your area, you should also look at the output level of the boiler. 

Output is measured in kilowatts (kW) and indicates how powerful a boiler is. For combi boilers, output levels apply to central heating (CH) and domestic hot water (DHW). The central heating level must be strong enough to serve the number of radiators in your home, whereas the hot water (DHW) must do the same for the number of outlets.

Larger homes with multiple bathrooms typically require higher hot water (DHW) output than smaller homes. The CH and DHW outputs must match the needs of your home, if you choose a boiler that is too powerful it will use more energy than you need, costing you far more in energy bills. However, if the power is too low, you will not get the heat levels required to serve your needs. 

If you are unsure of the best output or flow rate for your home, it’s recommended that you seek expert advice from a qualified installer. By speaking to a local engineer you can get fast, professional advice without the hassle of contacting a national company.

If finding an installer leaves you feeling out of your depth, or you have no clue where to start, we’re here to help. Using our simple form, with the click of a button, we’ll connect you with up to 3 trusted installers in your area. 

They can then provide you with the very best advice and offer quotes for you to compare. It’s completely free and there are no obligations attached. To get started simply click the button below.

Best high flow rate combi boilers

There are many combi boiler models on the market today which are highly efficient and capable of providing good hot water volumes. However, to help you find the best of those on offer, we’ve compiled a list of the top 5 high flow rate combi boilers. 

We’ve researched the major boiler brands, compared their combi boiler flow rates and other key features, and found the top 5 below. For each model, we’ve also listed their outputs, efficiency levels and an estimated combi boiler price range.

Worcester Bosch Greenstar Highflow 550CDi

The Greenstar Highflow 550CDi is an industry-leading combi boiler from Worcester Bosch. It’s well recognised for its high flow rate and hot water capabilities. 

This floor-standing boiler is great for larger homes with two bathrooms, as it can provide up to 25 litres per minute of hot water. This means multiple outlets can be used at once without any disruption.  

Worcester Bosch boiler prices are mid-high end in the market. However, this model is highly efficient and comes with a generous 10-year guarantee, meaning this boiler can serve you well for many years.

Navien NCB700 ON 54kW

This powerful combi boiler from Navien is the first of its kind, capable of providing hot water for three bathrooms simultaneously. 

The Navien NCB700 ON combi has a flow rate of 22 litres per minute and offers high-efficiency levels, for fast-flowing hot water and lower running costs.

Typically combi boilers are recommended for smaller homes, but this Navien model shows just how capable a combi can be, with its enhanced technology and unique hot water recirculation pump.

ATAG IC39+

The iC Economiser from ATAG is a specialised combi boiler range, which re-uses its waste flue energy to pre-heat water. This makes it highly efficient, with the 39kW model offering a flow rate of up to 17 litres per minute. 

This compact combi is best suited for large homes, with high hot water demand and multiple bathrooms. As well as great efficiency ratings, ATAG also offers a warranty of 12 years, which can be extended to 18 years – one of the longest warranties available.

Baxi Platinum+ 40kW

Baxi’s Platinum+ combi boiler range offers exceptional performance and reliability. Its 40 kW model can provide a flow rate of up to 16.4 litres per minute. 

The Platinum+ is well suited for medium-sized homes, with up to 2 bathrooms. It’s quick and easy to install, saving time, space and money. 

This boiler also comes with a magnetic system filter, for greater whole-system performance and longevity. Baxi boiler prices range from mid-high, but they also offer a 10-year warranty for further peace of mind that this boiler will last.

Ideal Vogue Max C40

The Vogue Max combi boiler range from Ideal Heating is another high-quality product with exceptional flow rate and performance. 

As part of Ideal’s premium range, the Vogue Max C40 offers a flow rate of up to 16.4 litres per minute. This is great for small to medium-sized homes with two bathrooms. 

Ideal boiler prices are amongst the lowest on the market. They also include a system filter to protect the boiler’s internal parts, plus a 10 or 12-year warranty for complete protection.

Each of the combi boilers featured in our list offers great flow rates, capable of providing your home with exceptional hot water levels. The exact price for each model will vary depending on the supplier and any extra features needed. You should also account for installation costs which can be between £650 – £2,500 in addition. The more complex the installation, the more the costs can rise. 

To find the very best price on a high flow rate combi boiler, however, we recommend comparing several quotes. By contacting local installers you can also reduce costs, as they typically charge less than national companies and have greater availability. 

Avoid the stress of searching for installers you can trust, by using our quick and easy form. With just a few details, we’ll find you up to 3 qualified installers in your area, who can provide you with competitive quotes – so you don’t have to! 

It’s free and easy and there are no obligations attached. Simply click the button below to get started and save money on your new boiler price.

High flow rate storage combi boilers

A high-flow rate storage combi boiler is an alternative to the traditional combi. Usually, a combi boiler takes its water from the mains to heat on demand, however, in a storage combi a storage tank is built into the unit. This keeps a larger supply of water heated, and ready for use. 

This is very similar to the way a system boiler operates, as it works with a large external hot water cylinder. The storage combi, therefore, offers a more compact, easy-to-install alternative. 

Storage combi boilers also offer exceptionally high flow rates. Below are some stand-out models which are worth considering if you have a much larger home and good local water pressure.

Viessmann Vitodens 111-W (B1LF) 32kW

Viessmann is one of the top-rated boiler brands, with award-winning ranges and high customer ratings. Their 111-W storage boiler has an integrated stainless steel cylinder which can store up to 46 litres of water. 

The 32kW model offers a high flow rate of between 18 and 21.2 litres per minute, making it suitable for large homes with multiple bathrooms and high hot water demand.

Vaillant ecoTEC Plus Combination Store 38kW (938)

Vaillant is another top brand, whose combi storage model offers a high flow rate of up to 20 litres per minute. This boiler is ideal for those with large homes and more than two bathrooms. 

This combi model is also highly efficient, with premium components and smart technology built in. It can reduce power consumption and save you money.

Vaillant boiler prices are low-mid range, with this award-winning manufacturer also offering a guarantee of up to 10 years. So you can enjoy peace of mind for the next decade.

How to check the water flow rate in your household

If you’re unsure what the current flow rate is in your home you can perform a simple test to find out. If you’re not comfortable doing these steps yourself, you can always ask a qualified installer to calculate the flow rate for you. 

Measuring flow rate

For this test, you will need an empty measuring container (such as a one-litre jug), and a stopwatch. 

• Turn off all taps in your home (as well as any appliances using water – dishwasher etc.)
• Place the jug under a tap you want to test
• Turn the tap on to its fullest point, start the stopwatch at the same time
• When the jug fills to 1 litre, stop the stopwatch

This will help you determine how quickly your system can provide you with one litre of water. To then calculate the litres per minute flow rate you can divide a minute (60 seconds) by the time you recorded.

For example: 

If the tap took 6 seconds to fill to 1 litre:

60 seconds / 6 seconds = 10 litres per minute

The ideal flow rate for any home is between 10 to 15 l/min, with 15 considered to be a high flow rate. For this test, you want the jug to fill up in less than 6 seconds, this will show your flow rate to be above 10 l/min. 
If your test takes more than 6 seconds, this indicates a low flow rate. This would mean any combi with a very high flow rate wouldn’t be suitable for your home. It’s important to match the boiler flow rate to your water supply otherwise you may end up with a boiler which can’t supply adequate amounts of hot water for your needs.

When to take action

If you have an inadequate flow rate in your home it can lead to problems. You may experience certain issues when using your water that can indicate a change is needed. These can include: 

• Inadequate hot water supply or frequently a lack of hot water, especially when multiple taps or showers are in use.
• Long wait times for hot water to reach your taps or showers.
• Fluctuating water temperature.
• The boiler frequently turns on and off while you use hot water (known as ‘boiler cycling’) this indicates it’s struggling to meet demand.
• Noises or vibrations from the boiler or pipes when using hot water.

If you’re experiencing any of these problems, it could be because your boiler cannot cope with the demands of your home. If these are new issues, it could be because your boiler is old and losing efficiency, or it could happen if you recently added a new bathroom, or water fixture, that the boiler flow rate cannot cope with.

Is a high flow rate combi boiler the right choice for your home?

Whether you’re experiencing issues or looking to upgrade your boiler, finding a suitable flow rate for your combi is essential. As we’ve seen in this article, the top 5 high flow rate boilers all offer an excellent range of options, with long warranties and high efficiency. 

A combi boiler flow rate of between 10 and 15 litres per minute is suitable for most homes, however, those with large properties (and over 3 bathrooms) might benefit more from a storage combi boiler. These can offer more hot water on demand at exceptional flow rates. 

If you’re looking for a new combi boiler with a high flow rate, any of our top 5 boilers would be a great choice. To get the best deal on these models, however, we recommend seeking several quotes from installers in your local area. 

Finding quotes from qualified installers couldn’t be easier, with our quick form. Within less than a minute you can share your details, so we can connect you with up to 3 trusted engineers local to you. 

It’s free, with no obligation, and by comparing quotes you can find the best deal for your budget. Our network of installers are qualified professionals, who are fully vetted and backed by positive customer feedback. 

So get your new boiler installation underway today, by clicking the button below.

Frequently asked questions