07th February, 2022

What Is a Combination (Combi) Boiler?

Is a combi boiler worth it? Would your home benefit from a combi boiler? We will find out..

What Is a Combination (Combi) Boiler?

You've probably heard of a combi boiler before, and there is quite a good chance that you actually have one in your home or business premises already, but not everybody is going to know what they actually are.

If you're going to be using one, it's a good idea to familiarise yourself with what they are, how they work, and how they differ from other boiler types.

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What's the difference between a combi boiler and a normal boiler?

What is a combi boiler, Combi boilers, (short for 'combination boiler') are a type of boiler that combines a water heating system and a central heating unit into a single body, acting as a unified way to handle heating and hot water without needing two separate boilers.

Combi boiler designs can vary, but they almost always take it directly from the mains and can provide hot water almost instantly as long as they're properly connected.

Because of this, there's also no need for a separate water tank system: boilers like this generally have one tank that they use to heat water, regardless of where it's going.

There are various different sub-types of combi boiler based on different features, designs and extra functions, such as a condensing combination boiler that is focused on recycling already-used water. The basic design is almost always the same, at least in terms of how hot water storage and the heating system are managed.

What are the benefits of a combi boiler?

Combi boilers have plenty of useful benefits, some of which are much more important than others. Since they all have the same kind of structure and layout in how they work, most of these benefits are constant across all types of combi boilers, but they might not be obvious straight away if you've just gotten a new boiler.

To read iHeat's best combi boilers of 2021, read our latest article!


One of the most notable benefits of a combi boiler is the amount of space they take up. Since you're combining two different boilers into one body, it cuts down on the amount of room they actually use in total and gives you more room for other things in your home.

Even if the boiler would be inside an attic or cupboard anyway, needing less space is always a good thing, especially in a smaller home where you might not have the necessary room to fit multiple appliances into a single area.

Since a combi boiler acts as a single appliance, it only needs one set of pipes, so installation is cheaper and faster. Not only that, but some new boiler designs have even been able to use the reduced size as an excuse to add larger water tanks - this isn't always that useful, but it can still be helpful in certain situations.


Another major benefit is the overall efficiency of the heating system. Some older water heater boilers are barely over 50% efficient - for every £1 you spend trying to heat water for your home, you're wasting almost £0.50 for no reason.

Not only are combi boilers generally the more efficient type of boiler, but since they're two units combined into a single new boiler, water is always ready to go into the heating system as soon as it's needed.

Some can go as high as 92% or even higher, dramatically cutting down the amount you're paying for your water heater compared to a standalone gas boiler.


A combi boiler almost always takes its water directly from the mains, meaning that it retains a lot of the mains pressure as normal water pressure.

Unlike a regular boiler, which will often use gravity to collect the water, this means that you get a much stronger flow that matches the original mains pressure more closely.

Aside from helping you get stronger jets of water, this also means that your boiler won't end up with as many areas where water can get stuck or pool in a curved pipe since there's more force behind it to keep it moving through the system.


Many people overlook it, but a good combi boiler can actually be a great way to make sure you get hot water nearly instantly and at any time. Unlike regular boilers, a combi boiler can basically produce it on demand since it's already a water heating system, which means that there can always be some ready to use.

You don't need to heat it up separately ahead of time or wait for it to warm up as long as it's already running properly, so your home can get a much more consistent supply of hot water as a result.

What is a combi boiler bad for?

Combi boilers still have flaws, but they're much less of a problem than you might think. Like all boiler types, there are some things they can't do quite as well as, say, a normal gas boiler, so you'll want to be aware of their limitations or shortfalls too.

Water Storage

A combi boiler system doesn't usually include storage tanks. Very few boiler designs even try to include a water storage tank, but a combi hot water system boiler will almost never include any kind of hot water tank.

Since the water cylinder is just a chamber to use for water heating, boiler designs like this rely on a constant flow from the mains, which means that you will need some other storage method if you want your boiler to keep running without a regular water supply.

There are ways around this, like installing your own tanks in your home, but this is a separate appliance that will take up more room.

Not all homes need a storage system like this: if you don't, or if your building already has one installed anyway, then this downside doesn't matter at all (apart from very specific situations that most people will never actually encounter).

Poor Pipe Systems

Since a combi boiler gets its pressure from the mains water pressure, this means that weaker pipe systems will lead to weaker pressure overall.

This isn't a problem with the combi boiler itself, but with the water heating systems and pipes: a bad flow rate and low pressure in the pipes will lead to the same issues in the boiler, which will cause taps, showers and other water-related systems to get weaker as well.

This makes a combi boiler much less useful for older pipe systems that might have been built around some other source of high pressure.

Alongside this, there is also the issue of old pipework. An older system can lead to lower energy efficiency and an increased chance of the combi boiler not being compatible with the connections.

Some boilers won't accept particular connections or pressure levels, and the only way to fix it is to either adapt the existing system or have part of the pipe structure replaced.

Is a combi boiler cheaper to run?

A combi boiler is one of the most efficient types of boilers you can install in your home, although each particular combi boiler model will have its own level of efficiency.

As mentioned earlier, a lot of older boilers barely manage to reach 60% efficiency, meaning that half of the money spent on heating is getting wasted each time you use it.

However, combi boilers have a higher efficiency level, meaning that it costs less to heat any given amount of water.

That's not the only improvement, though. Think about it: what exactly is a combi boiler? Since it combines two different types of boiler-related systems, you're often using half the power and resources to run it (at least in theory), meaning that most combi boilers are flat-out more efficient in terms of energy usage even if they have the same level of heating efficiency on paper.

This can save you plenty of money and make the installation cost worth it, even if the installation costs are higher than you expected to pay initially.

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Is a combi boiler better?

While combi boilers reduce your heating bills and take up less airing cupboard, kitchen cupboard or loft space, that doesn't necessarily mean that they'll always be the best in every single situation.

Always choose one that suits your home and your situation, since that's what matters most: every home is different and has different levels of heating usage, and even the smallest detail down to directions of certain pipes can affect the way that it performs.

No boiler design is perfect for all home types and living situations, so you still need to plan accordingly.

That being said, if you can get a combi boiler, it's often much more reliable and efficient for home use than a normal boiler as long as your house isn't too large.

The lower running costs and combined boiler controls can really help, especially if you use your central heating system a lot. If your boiler systems can support it properly and you have a direct connection to the mains, it's worth at least considering a new combination boiler, even if you'd only be saving a small amount of money and power every month.

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