Best High Flow Rate Combi Boilers

The combi boiler has dominated the UK heating industry for quite some time now and is found in almost all properties with one bathroom. The combi’s success is due to its compact, space-saving nature and its high efficiency. However, there is one drawback to regular combi boilers; they have a limit on how much hot water they can deliver to a home’s hot water outlets per minute.

With such a limited hot water flow rate, these boilers will struggle to meet the hot water demands of big houses that might have several hot water outlets running at once. This results in homeowners having insufficient hot water coming out of their showers and waiting for ages for bathtubs to fill.

And to mitigate this, boiler manufacturers designed high flow rate combi boilers that discharge large volumes of hot water per minute, allowing homeowners to run up to two showers at once. But is a high flow rate combi boiler the right choice for your home? And what are some of the reputable high flow rate combi boilers currently in the market?

Best high flow rate combi boilers

1. Worcester Bosch Greenstar Highflow CDi Combi Boiler (25 litres per minute)

The Worcester Greenstar Highflow CDi is the combi boiler with the highest flow rate at 25 litres per minute. Unlike conventional combi boilers, the Greenstar Highflow CDi is a floor-standing boiler, meaning it won’t fit into your kitchen cupboard. This gas-powered combi boiler comes in three versions, the 29CDi, 440CDi, and 550CDi.

Greenstar Highflow 29CDi has the highest flow rate of 25l/min, followed by the 440 CDi at 20 l/min. The 550CDi isn’t considered a high flow rate combi boiler as it clocks a maximum flow rate of 14.3 l/min. All the boilers in this range come with a 5-year warranty and have an efficiency rating of 90% and above.

2. Viessmann Vitodens 111-W (21.2 litres per minute)

This is a bulky storage gas combi boiler with a built-in 46-litre hot water cylinder. The boiler has a maximum flow rate of 21.2 l/min when the cylinder is full and a standard flow rate of 18.1 l/min. It usually comes with a 33.9 kW DHW output rating and a 29.3 kW central heating output.

3. Worcester Bosch Greenstar 8000 Lifestyle 50kW Combi Boiler (20.3 litres per minute)

The Worcester Bosch Greenstar 8000 Lifestyle 50kW combi boiler refers to two similar Worcester Bosch combis; the Greenstar 8000 Life and the Greenstar 8000 Style. These two boiler models have a flow rate of 20.3 litres per minute and have a 12-year warranty. They also have a similar efficiency rating of 94% and cost around £2,100.

Greenstar 8000 Life and the Greenstar 8000 Style have a central heating output of 35kW  and a DHW output of 48.9kW, making them ideal for homes with two bathrooms and around 20 radiators.

4. Vaillant ecoTEC Plus 938 Combination Store (20.1 litres per minute)

The Vaillant ecoTEC Plus 938 is another storage combi boiler with a built-in 15-litre hot water cylinder. With a 38kW output rating and a 20.1 l/min output rating, this storage combi boiler is an excellent fit if you live in a large home with substantial hot water demand.

5. ATAG IC39+ (17 litres per minute)

The standard ATAG IC39+ combi boiler is a wall-mounted condensing unit measuring 700x440x355mm. It is also available in a 275mm depth version which can fit into most kitchen cupboards. The boiler comes in a range of outputs, starting from 27Kw for smaller homes and ending at 29kw, suitable for large homes with two bathrooms and around 15 radiators.

Some of the unique features of this 17 l/min boiler include an iCon heat exchanger, built-in flue gas recovery, and a standard outside weather compensator. It also comes with a standard 10-year warranty which can be extended to 14 years.

6. Navien NCB 40 (16.9 litres per minute)

Although not a popular boiler brand in the UK heating industry, Navien has established itself in the world of high flow rate combi boilers. The brand’s NCB 40 combi boiler can discharge up to 16.9 litres of water per minute, making it one of the top 10 high flow rate combi boilers.

Navien NCB 40 boilers come with a 7-year standard warranty and an LPG conversion kit, meaning they can run on both natural gas and LPG. Other unique features of this boiler include

7. BAXI 800 Platinum +40 (16.4 litres per minute)

The BAXI 800 Platinum +40 combi boiler is another popular high flow rate combi boiler. It can attain a maximum flow rate of 16.4 l/min, which, when coupled with its 40kW DHW output, makes it a perfect choice for properties with up to twenty radiators and two bathrooms.

8. Ideal Vogue Max C40 (16.4 litres per minute)

To wrap up the list is the Ideal Vogue Max C40 combi boiler which comes with a maximum flow rate of 16.4 l/min and a 10-year warranty. This boiler has a 32kW CH output and a 40 kW DHW output, making it an ideal fit for homes with up to 20 radiators, a bathroom, and an ensuite.

What is the boiler flow rate?

Boiler flow rate refers to the amount of hot water that the boiler can send to your hot taps in one minute. Flow rate is measured in litres per minute (l/min), and a higher flow rate means more water to your taps per minute. For example, a boiler with a flow rate of 15 litres per minute means that 15 litres of water will flow out of your hot water taps every minute.

Essentially speaking, the higher the number of hot water outlets in your home, the higher the boiler flow rate you will need. This is particularly the case if multiple hot water outlets will be in use at a time. The reason behind this is simple.

Boiler flow rate and mains flow rate

It is worth noting that the flow rate of your boiler can only be as high as the flow rate of your home’s mains water supply. Simply put, a boiler will only meet its designed flow rate if it draws water from a mains water supply with an equal or higher flow rate.  If the flow rate of the mains is lower than that of the boiler, the boiler will not achieve its specified flow rate.

Let’s consider a boiler with 15 litres per minute flow rate. If this boiler is connected to a mains with a flow rate of 15 litres per minute or more, it will be able to send 15 litres of hot water to your taps every minute. However, if the same boiler is connected to a mains with a flow rate of 10 litres per minute, it will only manage to send 10 litres of hot water to your pipes every minute.

What is considered a good mains flow rate for your home?

Most UK households have a mains water flow rate somewhere between 10 and 15 litres per minute. Anything below this range, say, a mains flow rate of 8 l/min, is considered low, while anything higher than 15 l/min is considered a good flow rate. Generally, high flow rate combi boiler would best realize its potential when paired with a mains flow rate of 15 l/min or higher.

How to measure your home’s main flow rate?

As earlier said, a high flow rate combi boiler won’t have an impact if your home’s mains flow rate is lower than the boiler’s. Therefore, you must know your home’s mains water flow rate before you can select a high flow rate combi boiler.

You can use a weir cup to accurately determine your mains water supply’s flow rate. If, however, you don’t have it, you can opt for a more DIY approach that requires you to have a 1-litre jug and a stopwatch. Here is a summary of the procedure:

1. Turn off all the taps in your home, and switch off the washing machine and the dishwasher
2. Select one tap, say the kitchen sink, and place the jug beneath it.
3. Turn the tap on full blast and start the stopwatch
4. Determine the time it takes for the water to reach the 1-litre mark
5. Divide sixty by the number of seconds it took to fill the 1-litre jug. For example, if it took five seconds, then the flow rate will be:

60 seconds / 4 seconds = 12 litres per second

What is considered a good flow rate for a combi boiler?

Combi boilers have flow rates ranging from 10 l/min to 25 l/min. A flow rate between 14 l/min to 16 l/min is considered good as it increases a home’s shower performance and reduces the wait time for filling a bathtub. Anything above 16 l/min is deemed high and is only found in high flow rate combi boilers.

When do you need a high flow rate combi boiler?

A high flow rate combi boiler has a higher output rating for DHW and CH than a typical combi boiler. And with a higher output rating comes a higher fuel consumption, which translates to enormous heating bills. In other words, you won’t have to go for a high flow rate combi boiler unless it is necessary.

Some of the situations that necessitate installing a high flow rate combi boiler include:

• The property has two or more bathrooms.
• The property has one bathroom and two showers.
• Several people are living in the house, meaning several hot water outlets are likely to be opened at once.