10th May, 2022

Condensing vs Combi Boilers

Combi boilers and condensing boilers are two very different pieces of equipment, offering different features and levels of performance thanks to the way that they are constructed.

Condensing vs Combi Boilers

Deciding Between a Condensing & Combi Boiler

Whether you are upgrading from an older boiler, installing one from scratch, or just trying to figure out which one you may want to purchase in the future, it is important to know the difference between these two popular options. Here is a breakdown of condensing and combi boilers, and why one type may be more suitable for your circumstances.

If you are looking for the best condensing boilers then take a look at our upgraded 2022 guide.

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What is a Condensing Boiler?

A condensing boiler recycles the exhaust gases created during the boiler’s normal operation.

The water vapour and steam is re-used rather than just being lost, retaining some of the heat and keeping more water within the system.

Boilers are designed to provide heat and hot water to a space, generally by taking in cold water and heating it up. Considering that they use water, and water evaporates at high temperatures, it is always possible that some of the potential heat offered by water can end up being lost when it turns to water vapour.

This loss of water causes a loss in efficiency, which can often lead to the boiler being much less effective overall. In larger homes & buildings, it can sometimes cause certain radiators to not heat up at all, since the water has cooled by the time it gets there.

How do Condensing Boilers Work?

A condensing boiler makes use of a different flue system that stops the hot gases from being wasted. This condensing system captures the heat from the gases and uses it to heat up the water that returns from your central heating system, meaning that the actual burner in the boiler needs to do less work.

For example, when you first turn on the boiler, the burner will be the only heat source. As gas keeps being created, though, the heat from the gas supplements the boiler.

This essentially gives you two heat sources as long as the boiler keeps running and the water keeps creating hot gas.

All of this boosts efficiency, since you are not using as much energy or power to keep the burner running, and you are re-using a lot of the heat.

In simple terms, you are getting heat back from the boiler that you can use all over again, saving it from having to produce as much on its own.

Where are Condensing Boilers Used?

Condensing boilers are suited to quite a wide range of homes and buildings, with many of them heating up larger houses quite effectively. The size of the condensing boiler matters since a larger condensing boiler can heat up a greater area, but there are not really any specific places that condensing boilers need to be installed or used.

While they can't usually fit into very small spaces, a good condensing boiler can still be placed in an efficient way that maximise the amount of room you have available. They are also suitable for heating multiple spaces at once, so they might get used in a block of flats or in a small office building with multiple floors.

Condensing boilers tie into the heating system but do not really require any special connections, so they can be connected to the system just like any other boiler.

The Benefits of Condensing Boilers

Condensing boilers are efficient and provide a good level of water recycling, with some having more effective flue system designs than others. This can massively reduce how much energy you are using to heat cold water into hot water, especially if your new condensing boiler is a high-quality one.

A more efficient boiler means that you are paying less for every given amount of water you heat, and that is no different with condensing boilers.

Recycling the heat cuts down on how much you are paying without reducing the amount of heat you are creating, saving you money in the long-term.

While they are not the smallest boilers on the market, they also are not the biggest, so they can be a good upgrade from older designs that require an entire cupboard or attic space to properly contain. Many of them are easy to install simply because they can fit into spaces that larger models can’t.

The Negatives of Condensing Boilers

Since a condensing boiler draws out the acidic steam, that acid has to go somewhere. This often means that larger condensing boilers need their own acidic waste removal option, which usually happens to be a pipe. The steam also means that the boiler has to be placed in a safe area with appropriate ventilation.

Some of these boilers also include a fan to make sure that steam moves properly, which can add another moving part to the design – a moving part that could potentially break down. This can also add to the complexity of the design, making it a little harder to repair on your own.

What is a Combi Boiler?

We do have a more detailed guides on what is a combi boiler but in summary:

A combi boiler, or ‘combination boiler’, is a boiler design that combines hot water and heating options into a single body. This means that you can compact two appliances down into one, saving on space and making it much more convenient for the user.

These designs are quite common in modern buildings, especially smaller homes that might not have the space for something that requires a completely independent tank system. You will also see them in flats and other areas where space is at a premium since it removes a lot of the bulk that other boilers have.

Another major plus is the fact that they are very efficient, especially electric ones since there is not any separate tank that can slowly lose heat. The entire design is meant to be as self-contained as possible for maximum convenience.

The popularity of combi boiler designs has become obvious over the years, and they are quickly starting to become the default boiler type in many people’s eyes. However, they also have some flaws that can make them unsuited to larger areas.

If you're looking for a new boiler then check out our best combi boiler brands to see which is most suitable for your home.

How Do Combi Boilers Work?

Combi boilers take water straight from the mains and use a gas/electrical burner (along with a heat exchanger) to heat the water up. Unlike most other boiler types, this is all done in a single frame, and there is not a separate tank for heating or storing water. This creates hot water almost instantly.

There is sometimes always a little reservoir of hot water that is kept permanently hot, meaning that you will always have a small amount of hot water in reserve if you really need it. Either way, once this hot water is produced, it is sent into the heating system like normal.

Since these boilers are a combination of hot water systems and heating systems, they produce hot water no matter where it is needed and will start to create it as soon as you turn on a hot water tap. While it may have to take a moment to warm up, it does not waste energy by heating all of the water when you are not using it.

Benefits of Combi Boilers

A combi boiler is incredibly efficient and very easy to use, generally using modern digital interfaces that give you more direct control over the boiler’s various settings and functions. Even if they do not have those controls, they are still very simple and compact, making them easy to understand.

Since they only produce hot water when it is needed, you do not have the problem of your boiler constantly using energy to heat water during the night or while you are away. It will still heat some, but only as a reserve, and only if that boiler model includes a reservoir at all.

Their size is perhaps their biggest benefit, though. They can be stored in cupboards and easily fit into smaller spaces, which makes them incredibly convenient. Since a combi boiler acts as two different appliances, you are taking two units and compressing them into a single, even smaller body.

On top of that, they generally reduce your carbon footprint through the increased efficiency, especially electric ones. This can be a great thing for people who care about the environment, but even if you do not, it is likely to save you quite a lot of money in the future.

Negatives of Combi Boilers

There is not a water tank alongside a combi boiler, and that means that it can only produce or store so much hot water at once. If everybody in the house needed hot water from a different tap at once, it would struggle to provide it all effectively, and the water may start off cold for longer.

There is also the issue of size working against you. These boilers are much more compact, but their size means that they often have to sacrifice special features unless they are specifically built into the design. It is rare to find a combi boiler with condensing features, for example, although they still exist here and there.

Since there are no water tanks in combi boilers, they rely on the pressure of the mains, as well as a constant supply of water. With no reservoir, a mains issue means that you have no water at all, either hot or cold.

Finally, these boilers can be quite weak compared to some other offerings. While they are more efficient, some larger buildings (such as offices) may actually need two or more to manage every single radiator, especially if that business relies on having hot water readily available.

Condensing vs Combi Boilers

Condensing and combi boilers are two very different things when it comes down to mechanics and components, but they also serve a similar purpose. This means that you might end up having to choose between them, especially in a house that is getting a new heating system installed.

However, even if one option seems like the obviously correct choice, that does not mean that it actually is. There are lots of cases where you can end up needing a specific feature or level of efficiency to make sure that the boiler functions properly, and that means comparing them.

Of course, all boilers are different, and there are not any "standard" boilers to choose from. Different brands offer different things, including features that are exclusive to a specific product line.

Boiler Efficiency

Efficiency is actually one of the hardest things to compare since each boiler is designed to a different level of efficiency. The type of power or fuel used, also matters, since electricity is by far the most efficient compared to gas, oil and coal.

For a standard gas-based condensing boiler, it is quite common to see efficiency ratings around 94% or higher, especially if the design comes from a higher-tier brand or is specifically made for maximum efficiency above everything else.

On the other hand, a standard gas-based combi boiler can vary quite heavily, with most sitting above 90% as a low point. This can mean that some come as close at 96%, or even almost exactly 100% if the boiler is completely electric and extremely well-designed.

Remember that efficiency is not necessarily everything and that you can sometimes beat the cost of a less efficient design if it still does not cost that much to run. There is no clear winner in this regard, so it is best to compare specific models or designs that you are interested in, especially non-electric ones.

Carbon Footprint Comparison

Technically, by re-using heat, condensing boilers are more eco-friendly overall. This is not counting differences between overall power and efficiency, of course: if two completely equal boilers were compared, the one that re-uses heat from the water would be better for the environment.

An electric condensing boiler would technically be the best option for the environment overall, but you would probably have to sacrifice actual power and heat if you decided to use it.

Remember that low-power boilers can still be bad for the environment if they are inefficient since you have to use more energy heating up any given amount of cold water. This could cancel out some of the other environmental benefits, so there are times where it is better to use a stronger boiler that heats the water up much faster.

Space Required

Combi boilers are the smallest and most effective choice when it comes to saving space since they are two appliances in one. Even then, they are often smaller than either of those individual appliances together. The smaller the design, the less room they will take up.

However, smaller models might also take longer to heat water, and they won’t have as big of a water reserve (if they even have one at all). This could mean that you will actually want to find a good middle ground between size and features.

Condensing boilers are larger, but re-use their own heat, which can sometimes be worth the extra space it takes up. These boilers can still sometimes be fairly small, but this comes with the same reduction in heating power, so it is a choice you will have to make on your own.

Potential Savings

Boilers can be expensive to run, especially inefficient ones that use pricey fuels. Because of this, even a small amount of daily savings can quickly add up into a massive amount of extra money, and it is not hard to end up finding a boiler that offers more opportunities to keep your costs down.

Condensing boilers re-use the heat you are already creating, which can be a great way to ensure that you are not using too much fuel. It also serves as a way to make the boiler more efficient once it has started running since some heat is getting used twice rather than just being dumped into the area around the boiler.

Combi boilers are much more efficient overall, but their efficiency does not scale up: it runs at the speed it runs at, and generally, there are not any features in place to make your energy go further.

Remember that this does not even include the cost of installation, which is often higher with condensing boilers, as well as the fuel you are using. Gas will continue to get more expensive over time, but electricity will generally be around the same price, so long-term consistently favours electrical combi boilers.

Of course, everybody is different, and there may be times where you simply need to stick to a gas design for the raw heating power it can offer. This means that you will need to choose whether you want higher overall efficiency or a way to re-use heat you already have (a more direct type of efficiency boost).

Can You Have Both?

Condensing combi boilers are a very real thing. Because a condensing boiler relies on the gas and heat of the boiler itself, there is nothing stopping certain designs from incorporating condensing boiler flue elements into a combi boiler body. However, this also strips away one of the biggest benefits of a combi boiler, which is the small size and simplicity.

A condensing combi boiler is closer to a 'pure' condensing boiler, and that means that they are generally treated in the same way. It has the same benefit and drawback of no water reserve tank, too, which limits places that it might be effective. A condensing combi boiler is not ideal for every home.

Condensing combi boilers are an option, but they are best used if you already know that your home can support both properly. If you are not sure, then you may end up paying more for something that does not make much of a difference or even gets in the way at times. Each condensing combi boiler is still just as good as its design, so quality matters too.

Which do I Choose?

There are not any correct choices when it comes to boilers, since every single person will have their own particular needs and preferences for how a boiler should work. In one instance, efficiency might be everything, while in another there might be a strong need for high heating power.

Because of it, it is all up to what you personally need, as well as the kind of environment you are heating. Larger buildings call for larger boilers, whereas smaller ones may only need a basic compact model. If you do not have a pipe system already set up, then you will need to get that installed as well.

Smaller Homes

A smaller home usually needs a combi boiler, and this can be condensing or non-condensing. If you do not have room for a water tank, then a combi boiler is really your only option since combi designs include the tank as part of the main body.

Mid-Sized Homes

If your home has room for a slightly larger boiler, you can upgrade from a combi-boiler into a more conventional one, like a smaller condensing boiler. These can better supply a bigger house with heat and hot water, especially if that hot water needs to be sent up a floor to a shower.

Large Homes

If you are lucky enough to have plenty of space for something bigger then a larger condensing boiler might be the way to go. Recycling water vapour for some free 'bonus' heat can be very effective if you have a larger home since that heat can save a huge amount of money in the long run.

Whatever you end up choosing, it should suit your home or workplace properly. There is no point spending money on a boiler that does not work properly in that space, especially if you go for a more expensive model that still won’t provide any useful benefits.

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