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Combi Boiler Installation Guide

Have you decided the time has come to upgrade your boiler to a more energy-efficient model? Make sure you read this guide before getting a new combi boiler installed. 

Combi boilers are the most popular type of boilers in the UK because they offer many advantages over system boilers and heat-only boilers. By heating water on demand, rather than heating stored water, combi boilers can provide an unlimited supply of hot water with no waiting time. And not only are combi boilers compact and highly efficient units, but they’re also far less complex than traditional heating systems, which means installation is often cheaper.

If you want to replace your boiler but don’t know where to start, this guide includes everything you need to know about combi boiler installation in the UK. 

A combi boiler or combination boiler takes water from the mains supply and supplies hot water to your taps and radiators all by itself. Unlike a system boiler or heat-only boiler, a combi boiler doesn’t require a separate cylinder for storing hot water. 

Because a combi can provide heating and hot water without needing any additional components, it takes up much less space than traditional heating systems and makes the installation cheaper. And with no storage cylinder to heat up, a combi boiler can provide hot water whenever you need it. 

Can I replace a system or heat-only boiler with a combi?

Yes. If you own an existing system or heat-only boiler, you can make the switch to a combi boiler and get rid of your tanks and cylinders. However, the installation will be more expensive and take longer than replacing your current boiler with a boiler of the same type.

But remember, changing to a combi boiler will free up some space because you will no longer need any tanks or cylinders. 

Before making the switch, you need to consider your heating and hot water requirements. System and heat-only boilers are best suited to homes with higher hot water demands, so you may be better off going for a like-for-like swap if you often use multiple taps or showers at one time. 

Choosing a boiler type to match your requirements is something a heating engineer can help you with, and they can also help you size your new boiler. 

What size combi boiler will I need?

That’s a tricky question to answer because every household has unique needs, so the perfect size combi boiler for your home will depend on your heating and hot water requirements, but we can give you a rough idea. 

Before we carry on, we must point out that boiler size refers to the output of the boiler in kilowatts (kW) rather than the physical dimensions. The general rule is – the higher your heating and hot water requirements, the higher the kilowatt output will need to be. However, other factors also play a part in choosing the correct size combi boiler.   

You can use the table below as a rough guide to help you determine the correct combi size for your needs, but be sure to ask a Gas Safe registered engineer for a more precise recommendation.

BedroomsBathroomsRadiatorsBoiler Size/Output
1-31Up to 1024kW to 27kW
2-41-2Up to 1528kW to 34kW
3-52+Up to 2035kW to 42kW

Do all combi boilers have identical physical dimensions?

No. Different combi boilers have different physical dimensions, so you need to know how much available space you have before choosing a new combi for your home. But don’t worry if your home is a little on the small side because there are plenty of compact combi boilers on the market right now.  

Below are some examples of combi boilers that are small enough to fit inside most standard-sized kitchen cupboards. 

Combi BoilerPhysical Dimensions
Baxi 600 Combi700mm (H) x 390mm (W) x 285mm (D)
Ideal Logic Plus Combi700mm (H) x 395mm (W) x 278mm (D)
Glow-worm Compact Combi626mm (H) x 400mm (W) x 270mm (D)
Potterton Assure Combi700mm (H) x 395mm (W) x 285mm (D)
Vokera  Compact A Combi715mm (H) x 405mm (W) x 248mm (D)
Worcester Bosch Greenstar CDi Compact Combi680mm (H) x 390mm (W) x 280mm (D)

Some combi boilers are quite a bit larger than those listed above, so always check the physical dimensions before selecting a new boiler for your home. 

How much does a new combi boiler cost to install?

Combi boiler installation costs can vary significantly, depending on where you live, who you choose to do the job, and how much work is required. You can expect to pay between £500 and £750 for a straightforward combi boiler replacement in the same position on top of the cost of the combi boiler. 

If you want to install a new combi boiler in a new location, the installation cost will be higher. You will also pay more if you decide to switch from a system boiler or heat-only boiler to a combi boiler. 

We recommend you shop around and compare quotes on the cost of a new combi and the installation to save money. If you fill out our online form, you can get quotes for free from different trusted boiler companies and get the right boiler at a competitive price.   

Which combi boiler brand should I choose?

Reliability is a crucial factor that will likely influence your decision when choosing a new combi boiler. According to the most recent survey by Which?, the top three boiler brands are Worcester Bosch, Viessmann and Vaillant, and the top three cheap boiler brands are Baxi, Ideal and Glow-worm. 

You also need to pay attention to the warranty on offer. If a brand offers a competitive boiler warranty, that means they have confidence in the quality and reliability of their boilers. A lengthy warranty also gives you peace of mind should your boiler develop a fault or break down.  

Which brand offers the best boiler warranty?

Some brands offer better warranties or guarantees than others, and the warranty length differs from one combi boiler model to the next from each brand. The table below shows the different boiler brands and the best standard warranty or guarantee available from each of them.  

Boiler BrandLongest Available Warranty/GuaranteeCombi Boiler Model
Alpha10 YearsE-Tec Plus Combi
Ariston 12 YearsAtleas ONE Net Combi & Genus ONE Net Combi
ATAG10 Years iC Combination
Baxi 10 Years 800 Combi, Platinum Combi and Platinum+ 40kW Combi
Biasi7 YearsAdvance Plus Combi
Ferroli10 YearsBluehelix Tech RRT Combi
Glow-worm7 YearsEnergy Combi
Ideal10 YearsLogic Max Combi and Vogue Max Combi
Intergas10 YearsXclusive Combi and Eco RF Combi
Keston 5 YearsC30 Combi and C35 Combi
Main5 YearsMain Eco Compact Combi
Navien7 YearsNCB Combi
Potterton7 YearsTitanium Combi
Vaillant5 YearsecoTEC Pluc Combi and ecoTEC exclusive with Green iQ Combi
Viessmann5 YearsVitodens 100-W Combi
Vokera10 Yearsevolve C Combi and Unica Max C Combi
Worcester Bosch5 YearsAll Greenstar Gas Combi Boilers

Remember, these are standard warranties, and some manufacturers offer you the option to extend. For example, Worcester Bosch offers a 10-year or 12-year guarantee on many of its combi boilers when you choose a Worcester Bosch Accredited installer to fit your boiler along with a Greenstar filter system. That means you get different length guarantees depending on your boiler, accessories and who installs it. 

Other boiler brands offer similar options to extend, so it pays to do some research before you decide on a new combi boiler.