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Which is the best combi boiler for larger houses 2021?


Jo Alsop

Heating Hero

The Heating Hub

Tread very carefully when choosing a combi boiler for a larger home. They can be very inefficient for heating if not specified and set up correctly.


The truth is that even the smallest combi boilers will heat most larger homes and the best solution for you might not be a combi at all. The real driver for selecting the right boiler for a larger home is your hot water requirements. If you need lots of hot water, a hot water cylinder will be preferable.

Whilst large combis are available that provide lots of hot water, they can be very inefficient on the heating system side. Some combi boiler manufacturers have found the right balance of providing lots of hot water without compromising efficiency, but the maximum they can do is still only two bathrooms.

We guide you on careful combi selection for big homes with big hot water requirements, and big homes with small hot water requirements, plus offer some better setups for large households.

The unspoken truth about combi boilers

The average UK home needs just 6-8kW of heat on a very cold day, much less the rest of the year. Large homes might need 10-15kW, but again this is on a very cold day, and large, old and very draughty homes might go to 25kW assuming the whole house is heated.

Combi boilers commonly show outputs of 24kW-42kW, but this can be very misleading. These larger outputs on combi boilers relate to hot water performance. For example, a 24kW combi will give around 9 litres per minute (LPM) of hot water. A 38-42kW boiler will give 14-6 LPM. The heating outputs on combi boilers are 16kW and upwards, which will heat the majority of larger UK homes.

Hot water requirements are the real driver for combi boiler selection

The real driver for determining the best boiler for larger homes is the number of bathrooms you have. If you have 5 bedrooms and 1 bathroom, then a 24kW combi boiler will suffice. If you have 4 beds and 3 bathrooms, as is the case in many new build homes, then you will need a bigger combi, particularly if you have showers in use at the same time.

A 42kW boiler will give around 16 LPM, which is more suitable for multiple bathroom properties. However this will be halved to 8 LPM if you have two hot water outlets on at the same time, for example a hot tap and a shower.

Efficiency problems with larger combis

The main problem with combi boilers that give high rates of hot water is that many models can be very inefficient for your heating system. All boilers operate within a range for heating. For example 2-20kW or 8-30kW. The lower the bottom end of the range, the more efficient the boiler can operate all year round.

What we find on larger combi boilers is that the lower end of the range is much higher, commonly 8-12kW. Therefore if your 4-5 bedroom home needs 10kW on a very cold day (1-6kW for most of the year) then a big combi boiler that only goes down to 8-12kW as a minimum will be oversized for the heating system most of the time.

It is widely acknowledged that oversized boilers tend to cycle more and operate less efficiently. So how do we find a solution for homes with big hot water requirements?

Warning label

We want to answer the question of 'which is the best combi boiler for larger homes' honestly and provide some meaningful advice on this topic. But our recommendations come with some heavy caveats and warning labels, as follows:

1) A combi boiler is only as good as the installer that fits it.

2) A combi boiler is only as good as the heating control it is paired with.

So that even 'the best combi' boiler will massively underperform if setup incorrectly and paired with a basic heating control. 99% of installers have not been trained to setup condensing combi boilers to run at their A-rated efficiencies and basic heating controls have still not been banned under building regulations, so most boilers are B-E rated in the home. You can read more about our work to recruit the top 1% of installers and get independent advice to consumers on the correct and most efficient heating control with the help of our Expert Panel.


Multi-room heating controls

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Combi options for big hot water requirements

There are a few options available to households that have big hot water requirements without impacting the efficiency of the heating system:

1) Careful combi boiler selection

Opt for a combi boiler that gives good hot water performance but offers a really low kW output for heating. 

2) System boiler with hot water cylinder

Opt for a system boiler with unvented hot water cylinder. This is a standard alternative for larger homes. The hot water cylinder ideally should be located on the same floor as the bathrooms, or most of the bathrooms, and be big enough to meet your household hot water requirements

3) Priority domestic hot water

Consider a combi boiler with a small cylinder on a priority domestic hot water set up. This will provide a buffer of hot water that is always ready to go and when a hot tap or shower is turned on the boiler will automatically switch to reheating the cylinder.

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