What is a Combi Boiler? Combi Boilers Explained

What is a Combi Boiler? Combi Boilers Explained

Combi boilers, or combination boilers, are the UK’s most popular type of boiler, so it’s no surprise that my customers are asking what they are, how they work and what makes them different to the other types of boilers available. There are many benefits but also some drawbacks involved with combi boilers and it’s important for you to know them before you make a decision about what boiler type you have installed. This is why I’ve decided to make these blog posts, explaining what each boiler type is and how they work.

There are many different factors that you must think about when you are deciding which type of boiler you want to have installed. Answering simple questions like “How many people are living in the household?” or “How many bathrooms do I have?” are very important when choosing your boiler system. Answer them as honestly and realistically as possible and you’ll be well on your way to finding the perfect boiler for you.

It’s always best to discuss these questions with a fully qualified gas safe registered plumber to get the right advice and recommendation. Knowing the basics about the different boiler systems will certainly help too.

This blog is going to explain how combi boilers, or combination boilers, work, but I’m also going to be writing blogs about system boilers and open vent (conventional boilers) boilers too.

Worcester Gas Combi Diagram

Combi Boiler Diagram

How Does a Combi Boiler Actually Work?

Combi Boilers are quite different compared to conventional and system boilers. Unlike conventional or system boilers, combi boilers are highly efficient water heaters as well as a central heating boiler – a combination of the two, which is why it is called a ‘combi’ boiler. They heat your hot water and your radiators so there is no need for any separate tanks.

Conventional and system boilers use extra storage space for separate central heating components, but a combi boiler is simply one compact, space and energy efficient unit. This is because combi boilers heat water directly from the mains when the hot water is needed, so it doesn’t need a separate storage unit to hold the water once it has been heated.

Therefore, it is cost effective and energy efficient because water is only heated if it is needed, you only heat what you use. System boilers, on the other hand, heat a certain amount of water at a time even if it is not always used. This wastes the energy used to heat it. Combi boilers produce hot water at mains pressure, so you will not need a separate pump to pump the water around your system. As long as you have decent mains pressure then you can enjoy a nice, hot and powerful shower.

Boiler Installation Dartford Kent

Combi Boiler Installation in Dartford Kent

Combination boilers make up for well over half of all the new domestic boilers installed in UK homes each year. But what are all of these pros and cons exactly?

To summarise I have made a list of the pros and cons of combi boilers:

Pros of combination boilers:

  • The compact size of the boiler makes it ideal for smaller properties, or homes with very little extra space, especially in the loft or roof area.
  • It is very space efficient because you don’t use any other tanks or storage cylinders.
  • Installing it is usually cheaper and faster than other boiler types because there is less pipework involved. (A benefit of the reduced pipework, particularly in the loft, is that there is zero risk of pipework freezing)
  • The water pressure is the same as the mains pressure so, providing that you have good mains pressure, you should have a strong flow of hot water through your kitchen and basin taps.
  • Unlimited hot water is available at your demand because water is heated straight from the mains.
  • Combi boilers condense excess energy and recycle it back into the central heating system to stop it from being wasted in the atmosphere.
  • It typically is over 90% efficient so it is one of the most energy efficient and cost effective boilers types available.
  • Any future maintenance costs are automatically reduced as there is only one unit to maintain.
  • The cold water from a combi boiler system is always safe to drink as it comes directly from the mains.
  • Combi boilers are usually cleaner and tend to accumulate less sludge because the water is not held in a tank where rust, dust or debris can collect.

Cons of combination boilers:

  • A combi boiler may take a few seconds to fully heat the water.
  • Depending on the strength of your mains water pressure, opening more than one tap at a time will reduce the pressure of the water as it is divided up.
  • Combi boilers make use of a condense pipe to drain the condensed vapour away from the boiler. You need to run a condense pipe into the closest drainage system or install a condense pump.
  • Because combi boilers combine central heating and heating water, if the boiler breaks down your household will be left without central heating as well as hot water.
7, 8, 9 or 10 Year Extended Guarantee on Worcester Bosch Boiler Installations

7, 8, 9 or 10 Year Extended Guarantee on Worcester Bosch Boiler Installations

So if you’ve read this blog post properly you now know what a combi boiler is, how it works as well as the advantages and disadvantages of installing one.

Now that you have the basic knowledge of combi boilers, it should be a little easier deciding whether or not it is the boiler for you. Remember that there are lots of things you need to consider before you make your choice, and there is a large variety of boilers out there that are better suited for different things. If you know what kind of household you have (i.e. the number of people, if you often need instant hot water etc) it will be much easier to understand what type of boiler you need. If you do think that a combi boiler is the perfect boiler type for your home then the next phase is choosing what combi boiler to actually install. Luckily for you, I wrote a separate blog post on this to help you – the Best Combi Boilers for 2018.

If you liked this blog post and found it useful I’d be grateful if you could help by sharing it with your friends and family on Twitter or Facebook.

If you’re still confused and need me to clarify anything feel to free to comment down below and I’ll answer the questions myself personally.

I would like to stress once again that it’s always important to get any gas related advice or installation work done from a fully qualified gas safe registered plumber.

If you live in South East London or Kent and need some advice in terms of what boiler system to choose for your home then feel free to get in touch. We’d love to hear from you.

Richard Costello

Richard Costello, from Dartford in Kent, is the owner of MultiPlumb and has been involved in the plumbing industry installing bathrooms and boiler systems for over 15 years. If you have any questions or suggestions for future blog posts feel to get in touch at richard@multiplumb.co.uk.


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1 reply
  1. Madison
    Madison says:

    Great blog, thanks for sharing this detailed breakdown of combi boilers! They are definitely rising in popularity and there’s no question why, combi boilers have many advantages from cost efficiency to environmental friendliness. Of course, location and size of a household play an important role in deciding whether a combi is right for you


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