How Do Combi Boilers Work? Everything You Need To Know

How do combi boilers work? How much are they? What are the advantages and disadvantages? As qualified Gas Engineers, these are questions we hear on a regular basis. This guide will help you understand how combi boilers work, and what the benefits are.

What is a combi boiler?

A condensing combi boiler is a small unit that heats central heating and hot water. Hence the name ‘combination boiler’ aka ‘combi boiler’. Since their introduction, around twenty years ago, combi boilers have become the most popular type of boiler in the UK. Thousands of homeowners convert traditional systems to combi boilers every day.

Combi boilers are suitable for most small to medium-sized homes with under two bathrooms. With their compact size, they don’t take up too much space and can be fitted practically anywhere. Plus, due to them not needing water tanks, they can free up valuable space when replacing more traditional style boilers. Combi boilers heat hot water instantly, as a result, there is no wait for a hot water tank to heat up.

How do combi boilers work

How do combi boilers work?

Hot water and heating

Combi b0ilers heat water instantly via a burner and a heat exchanger. Contrary to common belief, the hot water that comes from taps isn’t heated directly.

When a combi boiler detects a demand for hot water it fires up. Circulating central heating water round its main burner. The hot central heating water passes through a plate heat exchanger.

Coldwater coming into the boiler passes through the same heat exchanger and heats up indirectly. Combi boilers cannot provide heating and hot water at the same time.

Always giving priority to hot water, combi boilers switch from heating to hot water almost instantly due to the internal diverter motor.

Pressurised system

Unlike older open vent boilers (regular boilers), combi boilers work on a sealed system. Meaning, no (central heating) water can get in or out without someone putting it in, or removing it.

Water is added to the boiler via a filling loop underneath the boiler. To between 1 and 1.5 bar of pressure. Too much can reduce the life of the expansion vessel. If too much pressure is put in, it’s not dangerous like most people think.

Inside the boiler is a pressure relief valve that drains out water if the pressure gets too high. Pressure will rise as the water gets hot, which is completely normal.

Because of the added pressure, combi systems are less likely to have problems with airlocks. Also, radiators deteriorate slower as they aren’t exposed to aerated water as much.

Loss of pressure on a regular basis means there is a leak somewhere on the system. If your boiler is dropping pressure on a regular basis call a local gas engineer to come and have a look.

Also, if the pressure has dropped there is a chance air will have got in. Bleeding the radiators will remove any air and allow them to work properly. If there is a lot of air, pressure may have to be added more than once.

Condensing combi boiler

As with all modern boilers, combi boilers are energy efficient. Most combi boilers are 90% efficient, with some very new ones being up to 98% efficient. Meaning that at least 90% of the heat produced is used, and not wasted. To put that into perspective, older, regular boilers are around 60% efficient. 40% of the heat is wasted as a result.

Condensing boilers capture the heat from the flue gasses and recycle them. Drastically reducing the flue gas temperature from around 130•C to 50•C. Because of the drop in temperature, condensation is produced. The condensation collects at the bottom of the heat exchanger and exits the boiler into a drain via a waste pipe. Now commonly known as a condensate pipe.

When sized wrong, condensate pipes can freeze in cold weather. If your condensate pipe freezes every winter, it may be a good idea to contact a local gas engineer to upgrade the size of the pipe.

This video will show you how to safetly thaw a frozen condensate pipe.


Combi boiler advantages and disadvantages

There are a number of advantages and disadvantages to how combi boilers work. We’ve summarised all of them below but if you want a more in-depth read check out – Combi boiler pros and cons.


Saves on space – Combi boilers take up a lot less space due to not needing water tanks and cylinders.

Energy-efficient – At least 90% efficient, new condensing combi boilers are much more efficient than older counterparts.

Comprehensive warranties – All combi boilers parts are inside the boiler itself. All parts are cover by the warranty as a result.

Better water quality –  Fed directly into the boiler, water is less likely to get contaminated.

Selling point – A small factor, combi boilers make good impressions on potential buyers when it comes to selling a property.

Hot water on demand – Heating water instantly, combi boiler eliminate the wait for a water tank to heat up.


Not suitable for multiple bathrooms – Due to combination boilers heating water instantly, they struggle to heat more than one hot water outlet at once.

Repairs can be expensive – Parts can get expensive with some combi boilers, especially cheaper boilers.

No immersion heater – Because combi boilers don’t use hot water cylinders. The house is left with no heating or hot water if the boiler breaks.

How much is a combi boiler installation?

Since the introduction of combi boilers, most households in the UK have changed their older style boilers to combi boilers because of all the added benefits.

The cost of condensing combi boiler installations can vary, depending on the size of a property and the existing system.

The make of the boiler and the type of installation can have a big affect on the cost. If you require a system conversion, it can add a large sum to the price.

Usually, there is a boiler available to suit your budget. Where possible, we always recommend getting the best boiler you can afford. And always look for one with a good warranty. Following this advice will help save money in the long run.

Combi Replacement Prices

Here are a few typical boiler installation prices. These prices may vary by a few hundred pounds depending on the region you live in. All prices factor magnetic filters and digital roomstats. If a new gas pipe or vertical flue is required, expect to pay more.

Example prices based on a straight combi boiler replacement.

Worcester 30iBaxi 800Vaillant EcoTec Pro 28Vokera Compact 29Ideal Logic 30

Example prices based on a gravity fed system conversion to combi boiler.

Worcester 30iBaxi 800Vaillant EcoTec Pro 28Vokera Compact 29Ideal Logic 30


We hope this page has helped you understand how combi boilers work. Along with their advantages and disadvantages. We have a number of informative guides to help you decide what boiler is right for you and your home. Feel free to check them out.

Remember, if you’re looking to get a new boiler always use a Gas Safe Engineer for the installation!

Dont cut corrners with gas

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