best combi boiler for a large house

What is the Best Combi Boiler for a Large House?

Last updated on January 29th, 2024

Finding the best combi boiler for a large house can be a large task as there are so many combi boilers on the market and so much information to crawl through.

I have been carrying out combi boiler installations for over a decade, from fitting the smallest combi boilers to some of the largest.

Here I will give you a rundown of what I think are the best options for your large house based on my experiences over the years.

There are some powerful combi boilers around and there are also storage combi boilers. A storage combi boiler has a small hot water cylinder inside, which means they are larger than a normal combi boiler.

High Flow Rate Combi Boiler

A high flow rate combi boiler is what is needed in large houses. The flow rate is how fast it can heat hot water.

The higher the number the better.

The flow rate is measured by how fast the boiler can heat the water to a certain temperature.

Different boiler manufacturers use different temperatures in their boiler manuals, so it can be hard to compare them accurately.

If you are looking for the highest flow rate combi boiler, you need to look at the temperature rise and the flow rate of the water combined.

For example, some boiler manufacturers say the maximum flow rate their combi boiler can heat your hot water is to a temperature rise of 30 degrees Celsius, and some will say 40. This is a huge difference.

A high flow rate is great, but combi boilers run off the mains water.

If the flow rate of your cold water is only 8 litres per minute, and you buy a high-flow rate combi boiler with the capability to heat 16 litres per minute, then you probably won’t need a high-flow rate combi boiler.

The flow rate of a combi boiler is telling you what it is capable of heating your water to, it does not increase the flow rate of your home’s water supply, it’s just a guide.

You can increase your mains water flow rate by increasing the water main supply pipe from the street to your house, or there are potential pump options also.

So, if you really want that high flow rate combi boiler with a low house flow rate, it is possible.

Best Combi Boiler for 3-Bed House

The best combi for a 3-bed house depends on how many bathrooms you have. In my experience, most 3 bedroom houses have one bathroom.

My recommendation for this setup would be a Baxi 830 combi boiler. Which is a 30 kW boiler and comes with a 10-year Baxi warranty and provides 12.2 litres per minute of hot water.

If you have one bathroom and one shower room in your 3-bedroom house, then maybe a Baxi 836 is the best option, which is 36 kW.

If you are likely to have 2 mixer showers running at the same time regularly, then this would suit your 3-bed house.

Best Combi Boiler for 4-Bedroom House

Not much difference to a 3-bedroom house, the best option for a 4-bedroom house will depend on how many bathrooms you have.

A 4 bed 2 bathroom house would benefit more from the most powerful Baxi 800 series of combi boiler which is the 36 kW 836 with 15 litres per minute of hot water.

But, if you only have one bathroom, then a Baxi 830 should be more than enough power for your heating and hot water needs.

Baxi also does the 600 combi boiler which is the same boiler as the 800 but has less of a warranty (7 years), and no central heating magnetic filter, but costs less.

Both compact combi boilers are the perfect size for fitting in kitchen cupboards.

Best 40 kW Combi Boiler

Baxi Platinum 40kW Combi Boiler
Baxi Platinum 40kW Combi Boiler

I tend to fit mainly Baxi and Ideal combi boilers as they are the best boiler brands in the UK and both are made in the UK. I will mostly fit these unless I need a different boiler for a specific house and set up, or if the customer specifies a certain brand or model.

My recommendation for the best 40 kW combi boiler is a Baxi Platinum+ 40 which provides 16.4 litres per minute of hot water and comes with a magnetic system filter, and a 10-year Baxi warranty.

The Baxi Duo-tec and Platinum combi boilers are big solid boilers and a solid favourite of many installers. They are not the best for fitting in kitchen cupboards, so if that’s what you need, you might struggle with one of these.

Most Powerful Combi Boiler

Not including storage combi boilers, the most powerful combi boiler on the market in the UK, that I’m aware of, is a Worcester Bosch Greenstar 8000 Life 50 kW combi boiler.

Delivering a whopping 18 litres per minute of hot water.

Being the most powerful combi boiler, with a central heating output of up to 35 kW and a hot water output of almost 50 kW.

There are two different versions of this model of combi boiler, the Life and Style. The Style version comes in a choice of high gloss black or white.

With a standard warranty of 5 years, and up to 12 years when installed by a Worcester accredited installer.

Combi Boiler Flow Rate Comparison

Here is a combi boiler rate comparison of some of the largest combi boilers on the market in the UK with the highest flow rates.

  • Worcester Bosch Greenstar Highflow 550 CDi Combi Boiler 25 l/min – 40 °C temperature rise
  • Worcester Bosch Greenstar 8000 Life 50 kW 18 l/min – 40 °C
  • Viessmann Vitodens 111-W (storage combi) 21.5 l/min – 30 °C (or 18.1 l/m at 35 °C)
  • Baxi Platinum+ 40 16.4 l/min – 35 °C
  • Baxi 836 16.5 l/min – 30 °C (or 15 l/m at 35 °C)

Combi boiler flow rate comparison can be tricky, as the numbers the combi boiler manufacturers supply can vary for the temperature rise. I don’t think it’s intentional, but it can be misleading.

For example, the top flow rate shown for the Viessmann Vitodens 111-W is 21.5 l/min, but that is only at a temperature rise of 30 °C. Whereas the Worcester 8000 50 kW is 18 l/min at a temperature rise of 40 °C.

That means if the flow rate of your water at home is coming out of the taps at 18 l/min, the Worcester 8000 can increase its temperature by 40 °C. The Viessmann can increase it by 35 °C (at 18.1 l/min), as stated in the boiler manual.

You can check the exact temperature rise by holding a thermometer under the hot tap whilst the combi boiler temperature settings are on maximum, and also do it under the cold tap.

The difference in temperatures is the temperature rise.

How to Measure Water Flow Rate at Home

To measure the water flow rate at home, you need a water flow cup (Weir gauge), or a jug or bucket that you know the exact capacity of. For example, a 1-litre jug or 14-litre bucket.

To use the water flow cup, you run a hot tap fully open, so it’s running at the full flow rate, then read the gauge.

To measure the water flow rate using a jug, run the hot water into your measuring jug for 6 seconds, then times the number of litres by 10.

For example, if the jug is filled to 0.7 litres, you calculate 0.9 by 10 = 9 litres per minute.

To measure the water flow rate using a bucket, you need to time how long it takes to fill, then calculate it by dividing the number of litres by the number of minutes taken to fill.

For example, if it takes 90 seconds (1.5 minutes), that will be 14 litres divided by 1.5 minutes = 9.3 litres per minute.

This is the flow rate of your hot water.


When looking for the best combi boiler for a large house, you will need to work out what size combi boiler you need based on your home’s heating and hot water demands.

Knowing what a combi boiler is capable of is crucial before making your decision as each one is different.

You might be thinking about changing to a combi boiler, but if your heat demands are more than what a combi boiler can offer, then you may want to look at getting a system boiler instead.

If you click a link on this page and buy any items, I may earn a commission.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


Is a combi boiler OK for a large house?

A combi boiler is OK for a large house, but it depends on how large the house is.

You can almost always find a combi boiler large enough for your central heating needs, but your hot water demand is what’s more important.

What is the best heating system for a large house?

The best central heating system for a large house depends on how large the house is.

A combi boiler is the best heating system for a lot of houses, including large ones, but if your hot water demands are very high, you might want to go for a system boiler with an unvented hot water cylinder.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
Share this post

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top