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Written by Bernard Morgan (Engineer). Published: August 15, 2018. Updated: April 20, 2026

How Does a Combi Boiler Work?

A combi boiler can heat your central heating and supply hot water. This eliminates the need for a hot water cylinder or cold water tank. Combi boilers a very popular because they don’t need these tanks or cylinders.

So, how does a combi boiler work? Easy. It heats water to pump around your home, and it heats water for taps and showers. New combi boilers can be very efficient, with the Viessmann 200-W being 98% efficient with its fuel.

What does this guide cover?

How does a combi boiler work? ⇣
The heat exchangers ⇣
Why isn’t a hot water cylinder needed? ⇣
Combi boiler pipes explained ⇣
Advantages of a combi boiler ⇣
Disadvantages of a combi boiler ⇣
What are the costs of a combi boiler? ⇣
How long does installation take? ⇣
What’s the best combi boiler? ⇣
Do NOT try and install a combi boiler yourself ⇣

More Information About Combi Boilers

< Back to the main combi boiler installation and details page

Further Reading…

How does a combi boiler work?

A combi boiler uses a burner (or heat source) to heat your home and hot water supply.

Combi boilers can come in gas, LPG, oil or electric versions. From here, we’ll focus on gas and LPG, as they’re the most common fuels.

Combi is short for combination. A combi boiler combines home heating and on-demand hot water within the same compact unit.

The fuel enters a combustion chamber inside the combi boiler. This heats the primary heat exchanger, which heats the water for your radiators. This water is usually heated to between 50 and 85 degrees Celsius.

When you turn on a tap or shower, the secondary heat exchanger is used too. The hot water is diverted from the primary heat exchanger and heating system into the secondary exchanger. This hot water then heats fresh cold water instead. This means mains-pressure cold water is heated on demand.

In contrast, regular and system boilers heat water at set times and store it in hot water tanks. These are usually in the airing cupboard or attic. In these setups, you can run out of hot water. But, the hot water can be provided at higher flow rates than some combi boilers. This is why combi boilers may not be the best option for large homes.

This diagram shows the internal parts from the previous version of the Viessmann Vitodens 050-W.

The heat exchangers

There are two heat exchangers in a combi boiler. The primary and secondary heat exchangers.

Primary heat exchanger – heating the radiators

The primary heat exchanger heats the water that goes into the radiators. That’s how a combi boiler works to heat your house. This water travels through the radiators and then back into the boiler. The boiler keeps firing to reheat that water and keep it at around 50 to 85 degrees Celsius.

You wouldn’t want to drink or wash with this water. It can get quite dirty as it cycles through the system. This includes small parts of rust, metal or limescale. At Boilerhut we always include filters to help prolong the life of your boiler and central heating installation.

Secondary heat exchanger – clean hot water

The water that travels through the radiators is undrinkable. Combi boilers use water from the heating system to heat fresh, clean water. It does this by diverting water from the radiators through the secondary heat exchanger.

The secondary heat exchanger allows the closed-loop heating water to transfer heat to fresh cold water. It does this without mixing them. When this switching happens, you will hear some boilers switching on and off as you open the taps.

Learn more about heat exchangers in this guide >

Why isn’t a hot water cylinder needed?

Combi boilers don’t need cylinders because they produce hot water on demand. This is done by heating fresh, cold water from the mains supply as it enters the boiler. The mains pressure then continues to push the water to the open tap or shower.

This is why combi boilers can be so efficient; there is no 100-300 litre water tank to keep hot.

If you need more hot water than a normal combi boiler can provide a storage combi boiler can offer higher volumes of hot water because it has an internal hot water tank. For example, the Viessmann Vitodens 111-W has a 46-litre internal hot water tank.

Combi boiler pipes – explained

There are several pipes that a combi boiler requires They are:

Flow
The flow pipe is the hot water travelling away from the boiler towards the heating system and radiators.
Return
The return pipe is the water coming back from the radiators. It will be cooler than the flow due to heating up the radiators. The optimum temperature differential (drop) will be approximately 20 degrees Celcius. A 7-degree differential is optimal for underfloor heating.
Gas supply
This brings the gas to your boiler from your gas meter. For most boilers, this must be 22mm in diameter. This ensures the correct pressure and flow of gas to your boiler.
Cold water inlet
This is where the fresh, clean, cold water comes in for on-demand heating. This supplies taps and showers.
Condensate pipe
A condensate pipe is a white plastic pipe that allows acidic water to drain out of the boiler. All boilers made and installed after 2005 will be condensing boilers.
Flue
This is the large pipe that comes out of the top of the boiler. It has two functions: it pushes waste gases out of the property, and it can draw fresh air into the burner.

Advantages of a combi boiler

Extremely efficient
New combi boilers can be up to 98% efficient. Plus, not having to heat hot water two or three times a day can save a lot of money.
Hot water on demand
Never run out of hot water in the morning again.
Smaller size
Combi boilers are a single unit. This means it can give you your attic and airing cupboard back.
Easy installation and serving
As this is a single unit, installation is easier, and servicing means only gaining access to one unit.

Disadvantages of a combi boiler

Flow rate
Your hot water pressure will be about the same as your incoming cold, or less. The amount of water that can be heated by a combi boiler is proportional to its power in kilowatts.
Can’t run multiple taps at once
You may only be able to run two taps or showers at once. This is fine for most homes, but not everyone.
No backup
If your boiler breaks down there is no backup for heating.

How much do combi boilers cost?

Combi boilers costs vary due to a wide range of sizes and powers. In general, a cheap, or budget, model might cost £700-800. A premium brand, like the Viessmann Vitodens 050-W, currently costs £1,060 – not including installation. And Viessmann’s top model, the Vitodens 200-W costs approximately £1,970. Correct as of March 2024.

Installation cost will vary across the country and depends on a lot of factors. For example:

Are you replacing a combi boiler with a new combi?
Are you replacing a system boiler?
Where is the boiler going to be located?
Are your pipes and radiators up to building regulations?
Do you have the correct thickness gas pipe to supply the boiler?
Where will the flue go?
Where will the condensate go?
…and many more.

When you get an instant quote with Boilerhut, we ask the main questions online to give you a price. But, we will always try and call you to ensure nothing has been missed. Every home is unique, either the building or your requirements.

How long does it take to install a combi boiler?

In general, a swap from an old combi boiler to a new one will take 1 day. If you are converting from a system boiler to a combi boiler it could take 1.5 or 2 days.

The three main things that contribute to the timescales are:

If you are swapping or converting a heating system. Or if you are installing a full central heating system.
If the flue can go out of the wall or ceiling. Ceiling means going through a roof, which takes more time.
If you need a lot more pipework. This could be gas, water or condensate piping.

Which is the Best Combi Boiler?

It’s important to choose the right type and capacity (kW / kilowatt) boiler for your home. Please read our guide to get into the details of which combi boiler is best.

On top of the capacity of the boiler, it’s also crucial to choose the right manufacturer. So, which is the best combi boiler manufacturer in the UK?

According to our installers, and our in-house heating experts and engineers, the answer to this question would have been different 10-15 years ago. But in 2023, the answer is pretty simple. We think Viessmann are the best combi boiler manufacturer, not just in the UK, but in the world right now. In fact, the best boiler manufacturer, period.

Every Viessmann boiler has a patented Inox-Radial stainless steel heat exchanger. These are built in-house in the Viessmann factory in Germany – something no other manufacturer can boast about.

Along with other unique innovations like PRV (pressure release valve) and condensate combined into one unit, and in-built shock arresters, plus 10 to 12 year warranties (when installed by Boilerhut), you really can’t go wrong with Viessmann.

Viessmann Vitodens 050-W

Size: 25kW, 30kW, 35kW
Efficiency: 94%
Warranty: 10 Years
Stainless Steel Heat Exchanger
WiFi for Fault Monitoring

Read Our In-Depth Review

Viessmann Vitodens 100-W

Size: 11kW to 32kW
Efficiency: 94%
Warranty: 12 Years
Stainless Steel Heat Exchanger
WiFi for Fault Monitoring

Read Our In-Depth Review

Viessmann Vitodens 200-W

Size: 30kW & 35kW
Efficiency: 98%
Warranty: 10 Years
Stainless Steel Heat Exchanger
WiFi for Fault Monitoring

Read Our In-Depth Review

DO NOT try to Install a Combi Boiler Yourself

How combi boilers are installed can be very complicated and requires a Gas-safe engineer. Sometimes it’s assumed that because combi boilers are compact, and don’t require extra tanks, it should be easy enough to install it themselves.

It’s not easy, and you shouldn’t try it unless you are Gas-safe.

Not only is it extremely dangerous because of the fuel, but it’s also illegal to install a combi boiler if you’re not certified. Illegal gas workers are prosecuted on a daily basis, and for good reason.

Ignoring the law and our advice to try and save some money by breaking the law and installing the boiler yourself is dangerous. You may be in breach of the law and can face hefty fines and possibly prison.

There are “armchair experts” online who may try to convince you to save money and install a boiler yourself. Please be warned, you will be putting yourself and your family, tenants or property buyers in danger.

No matter how good your understanding of how a combi boiler works is, below are some reasons why you should NEVER install a boiler yourself:

A Gas Safe Registered Engineer spends 5 years studying and hit their true level of experience in 5-10 years.
Flue Gas Analysers are required by law, which cost between £600 & £1000.
Flues Gas Analysers need to be calibrated annually and hold a certificate.
The suitability of the boiler for the property needs to be checked.
Boiler siting needs to be safe and comply with Gas Safe & Building Regulations.
Manufacturer-specified appropriate materials and fittings have to be used.
If you’re not Gas Safe Registered and make a new gas connection to a boiler you will face heavy fines or prison.
Equipment like Manometers test the gas supply and make sure the pressures are safe.
Gas integrity needs to be tested prior to connecting the boiler, and again once connected.
Let-by test & tightness tests are required.
Flow rates need to be checked and adjusted.
Gas Safe Engineers need to gas rate the boiler and check gas pressures and flue integrity.

After reading all of the above, if you still decide to chance-it and install the boiler and make gas connections yourself, you will invalidate the boiler warranty.

If you would like a Gas Safe Registered Installer to install your combi boiler or a certified heating expert to survey your property or answer further questions on how a combi boiler works, start by getting an online quote – start below.

What’s next?

If you are looking to change your combi, or change to a combi boiler you can get a quick quote using our online tool. It’s free and only take 30 seconds. You’ll get a quote instantly, and you can then discuss it with us to ensure it’s correct.

More Information About Combi Boilers

< Back to the main combi boiler installation and details page

REVIEW: Viessmann Vitodens Storage Combis >
How do combi boilers work? >
What’s a storage combi boiler? >
Combi vs Conventional boilers >

How Does a Combi Boiler Work?

How does a combi boiler work?

The heat exchangers

Primary heat exchanger – heating the radiators

Secondary heat exchanger – clean hot water

Why isn’t a hot water cylinder needed?

Combi boiler pipes – explained

Advantages of a combi boiler

Disadvantages of a combi boiler

How much do combi boilers cost?

How long does it take to install a combi boiler?

Which is the Best Combi Boiler?

Viessmann Vitodens 050-W

Viessmann Vitodens 100-W

Viessmann Vitodens 200-W

DO NOT try to Install a Combi Boiler Yourself

What’s next?

More Information About Combi Boilers

Further Reading…

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