Choosing a new boiler should be simple. Unfortunately, the heating world decided to call everything by at least three different names, add a few cylinders, throw in some loft tanks, and then expect everyone to keep up.
So, let’s make it easy.
The three main types of boilers you’ll find in UK homes are:
Regular boilers, also called heat-only or conventional boilers
The right one for your home depends on your hot water demand, property size, available space, water pressure and current heating setup.
Pick well and you’ll have reliable heating, strong hot water performance and a boiler that actually suits your home. Pick badly and you could end up with weak showers, wasted space, or a heating system that behaves like it’s been personally wronged.
Let’s find out…
Boiler types at a glance
Boiler type | Best for | Hot water cylinder? | Loft tank? | Good for multiple bathrooms? |
Combi boiler | Flats, smaller homes and homes with one bathroom | No | No | Usually no |
System boiler | Medium to large homes with higher hot water demand | Yes | No | Yes |
Regular boiler | Larger homes, older systems and low water pressure areas | Yes | Yes | Yes |
Electric boiler | Smaller homes without gas | Sometimes | No | Usually no |
Oil or LPG boiler | Off-grid homes | Depends on setup | Depends on setup | Depends on setup |
Which type of boiler do I need?
As a quick rule:
Choose a combi boiler if you have a smaller home, one bathroom, good mains water pressure and limited storage space.
Choose a system boiler if you have a larger home, more than one bathroom, good mains water pressure and space for a hot water cylinder.
Choose a regular boiler if you have an older heating system, low mains water pressure, high hot water demand, or an existing setup with a hot water cylinder and cold water tanks in the loft.
That’s the short answer. But boilers, much like your group chat, get more complicated once you look under the surface.
What is a condensing boiler?
Before we get into combi, system and regular boilers, there’s one important thing to clear up.
A condensing boiler is not a separate boiler type. It’s a boiler technology.
Modern condensing boilers are designed to capture and reuse more heat from the gases that would otherwise escape through the flue.
This makes them more efficient than older non-condensing boilers. Modern condensing boilers are more efficient because they reuse more heat from gases that would normally escape through the boiler flue.
Your boiler is likely to be condensing if:
The flue is made of plastic.
There’s a plastic condensate pipe coming from the bottom of the boiler to a drain.
It’s a gas or LPG boiler installed after 2005.
It’s an oil boiler installed after 2007.
In other words, most modern boilers are condensing boilers, whether they’re combi, system or regular.
A bit like how most modern phones have cameras. It’s not the type of phone, it’s just something the phone does. Except your boiler probably takes worse selfies.
Different Types of Boilers
Combination Boilers (Combi)
A combi boiler, short for combination boiler, provides both heating and hot water from one compact unit.
It doesn’t need a separate hot water cylinder. It doesn’t need a cold water tank in the loft. It simply heats water directly from the mains when you turn on a hot tap or shower.
This makes combi boilers one of the most popular choices for UK homes, especially smaller properties where storage space is more precious than a parking space in central London.
They're the holy grail of heating and according to Statistica are the UK's most popular type of boiler replacement with a whopping 54% of households owning one by 2019. So what took King Arthur his lifetime, is readily available for you.
How does a combi boiler work?
Combi boilers utilise a metallic heat exchanger to instantly heat hot water on demand, they achieve this via combustion e.g. by burning fuels such as gas or oil.
This is the biggest distinction between combi boilers, as unlike traditional boilers they do not require a separate hot water storage tank.
Instead, the hot water is heated immediately from a single unit, so you save space for some additional Christmas decorations or an unnecessary hobby.
Learn all about combi boilers in our what is a combi boiler guide and discover which are the best combi boilers in the UK.
Curious to know which boiler brand performed worst? Find out in our report on the worst boilers.
🎥 Prefer video? Check out our rundown of the different types of boilers in our video below:
Related combi boiler reading:
Pros of combi boilers
If you are looking for a boiler that's highly energy efficient, a combi (combination) boiler might be the solution. They can be used to heat the home as well as its water, saving you a wedge on heating bills.
Their compact size makes them ideal for even the smallest of properties, and since they use the mains to heat water, you will not have to make space for a cumbersome water tank or cylinder.
Another plus is that because they can instantly heat the water you need; you will not have to wait around for that hot, sultry shower. It’s less time browsing your phone in the bathroom, but quicker hygiene, so that’s a fair tradeoff.
Cons of combi boilers
There are a few drawbacks to consider.
Water flow rates are reduced when two or more outlets are being used at the same time.
So, if you have a larger property with a few bathrooms (which may be used at the same time), a combi might not be the right choice for you. Gordon Ramsay, who’s got five kids and regularly uses his kitchen sink, might want to opt for a different type.
Ultimately, a combi boiler can only heat the water which gets fed to it from the mains. So, if your mains water pressure is low, your combi boiler won't work very well.
Combi boilers are best for:
Flats and apartments
Smaller houses
Homes with one bathroom
Properties with limited storage space
Homes with good mains water pressure
Households that don’t often use several hot water outlets at once
A combi boiler may not be best for:
Larger homes
Homes with multiple bathrooms
Properties with poor mains water pressure
Households where several people shower at similar times
Homes that may switch to a heat pump later and need a hot water cylinder
Combi boilers can be less suitable for homes with multiple bathrooms or higher hot water demands, especially where hot water is needed in several places at once.
Looking to pay for your new boiler monthly? Refer to our boiler on finance guide for more info.
Learn all about having a new combi installed in our guide to combi boiler installation and find out what type of boiler you have here.
Heat Only (Regular Boilers)
A heat-only or regular boiler has three main components – the boiler itself, a separate hot water storage cylinder and a cold water storage tank.
The boiler supplies hot water to both the central heating and storage cylinder, so can provide both hot water and heating water.
How do heat-only boilers work?
Regular boilers work by feeding hot water directly to the central heating system i.e. the radiators and storing hot water within a hot water storage cylinder for on-demand use.
Coldwater is supplied to the boiler via a cold-water tank (typically located in the attic/loft), which fills the boiler with the help of gravity. Whether or not Isaac Newton helped invent this part of boiler systems, we don’t know.
Pros of heat-only boilers
Heat-only boilers can provide a greater flow rate and as a result, are more suitable for larger properties with greater hot water demands e.g. those with multiple bathrooms or very dirty children
As they do not require water to be supplied at high pressure via the mains, they are a good option for older heating systems that may not tolerate the high pressure of a closed system. While the law of gravity isn’t always kind to human seniors, it’s almost soothing to an old heating system.
Alternatively, they are also a good choice for areas that have low water pressure mains supplies, which typically struggle to supply adequate water pressure required in combi and system boilers.
Check our new boiler costs guide and use our boiler size calculator if you want to get a better understanding of how our fixed-price boiler quote tool works.
Cons of heat-only boilers
A heat-only boiler may not be compatible with some newer heating systems, and you might have to wait a while for the water to reach the right temperature.
Also, bear in mind that you will need somewhere roomy to store the boiler's cylinder and water tank. If you’ve got your London studio optimized according to the best tiny-space practices of a famous Swedish furniture store, a regular boiler probably isn’t going to work.
Since the cold-water cistern needs to be installed directly above the boiler, this may also limit where you can install the boiler too.
Having issues with your combi boiler? Read our boiler problems and solutions for quick troubleshooting guidance.
Regular boilers are best for:
Larger homes
Homes with multiple bathrooms
Older heating systems
Properties with low mains water pressure
Homes that already have a cylinder and loft tanks
Situations where a like-for-like replacement makes sense
A regular boiler may not be best for:
Flats
Smaller homes
Homes with limited storage space
Homeowners who want to remove tanks and simplify their system
Properties without suitable loft space
Is your boiler leaking or losing pressure? Use our quick and easy tool to get your fixed price boiler replacement.
System Boilers
System boilers are essentially an upgraded version of the regular or heat-only boiler. They can provide both hot water on demand and heating water for your radiators.
How do system boilers work?
Just like a heat-only boiler, they have a separate hot water tank, except that it is unvented.
All parts such as the pump and expansion vessel are internal too, which also means it does not require a cold-water cistern tank and water is supplied to the boiler via the mains.
Check out our full guide on choosing a boiler for a large home.
Related heating guides:
Pros of system boilers
Just like heat-only boilers, their large hot water storage cylinder means they are ideal for properties with high hot water demands.
Since they have all their components internalised and do not require a cold-water cistern, it means that they do not require as much space as a heat-only boiler. It also means there is more flexibility with the installation location. Now you can have a system boiler as a (hideous) coffee table.
System boilers do not require any components in the attic, it also means that you do not have to worry about any potential leaks or freezing during the winter months.
Considering underfloor heating? Read our complete guide on underfloor heating with a combi boiler.
Cons of system boilers
The hot water flow rate within the home largely depends on the pressure of the main supply, so if it’s low, the flow rate within the home will be limited. For this reason, they are not ideal for places that suffer from low water pressure.
The high-pressure nature of this system means that it may not be suitable for properties with older systems or systems with sensory issues.
System boilers need a hot water storage cylinder so if you are swapping it for a combi boiler, you will need to find adequate storage space.
System boilers are best for:
Medium to large homes
Homes with two or more bathrooms
Larger families
Properties with good mains water pressure
Homes with space for a hot water cylinder
Households that often use hot water in more than one place at once
A system boiler may not be best for:
Small homes with limited storage
Flats with no cylinder space
Properties with poor mains water pressure
Very old heating systems that may not suit higher pressure
Want to have your boiler moved to a different location? Check out our "cost of moving boiler" guide.
Gas, oil, LPG and electric boilers
So far, we’ve covered boiler types by system setup: combi, system and regular.
But boilers can also be grouped by the fuel they use.
The main options are:
Gas boilers
Oil boilers
LPG boilers
Electric boilers
Gas boilers
Gas boilers are the most common option for homes connected to the mains gas grid.
They use natural gas to heat your home and hot water. If your property already has a mains gas connection, a modern condensing gas boiler is usually one of the most cost-effective boiler options to run.
Most boilers in England, Scotland and Wales are gas boilers, and that a modern condensing gas boiler is usually the cheapest to run compared with oil, electricity, LPG and house coal if you have a mains gas connection.
Gas boilers can be combi, system or regular boilers.
Oil boilers
Oil boilers are commonly used in homes that are not connected to the gas grid.
Instead of using mains gas, they use heating oil stored in a tank, usually outside the property.
The main thing to remember with oil boilers is that you need space for the tank and you’ll need to arrange oil deliveries.
Oil boilers are often found in rural homes, because apparently living somewhere peaceful and scenic means being personally responsible for your own fuel logistics.
LPG boilers
LPG boilers use liquefied petroleum gas, which is stored in a tank or bottles.
They are another option for off-grid homes without access to mains gas.
LPG boilers work in a similar way to gas boilers, but LPG is usually more expensive than mains gas.
LPG is the most expensive of gas, oil and LPG, although it has lower CO2 emissions than oil.
Electric boilers
Electric boilers use electricity to heat your home and hot water.
They don’t burn fuel inside the property and they don’t need a flue, which can make installation simpler in some homes.
However, electricity is usually more expensive per unit than gas, so electric boilers are generally better suited to smaller properties with lower heating and hot water demand.
They can be useful for flats, small homes or properties where gas, oil or LPG are not practical.
What size boiler do you need?
When people talk about boiler size, they usually mean the boiler’s power output, not how physically large it is.
Boiler output is measured in kilowatts, or kW.
The more heating and hot water your home needs, the higher the boiler output may need to be.
As a rough guide:
Property type | Likely boiler type | Typical requirement |
Flat or small home with one bathroom | Combi boiler | Lower heating and hot water demand |
2-3 bedroom house with one bathroom | Combi or system boiler | Depends on water pressure and usage |
3-5 bedroom house with two bathrooms | System boiler | Higher hot water demand |
Larger older property | System or regular boiler | Depends on existing setup and pressure |
Your installer should calculate the correct boiler size based on your radiators, bathrooms, insulation, water pressure and hot water habits.
Getting this right matters.
An undersized boiler may struggle to keep up. An oversized boiler may cost more than necessary and run less efficiently.
So, no, simply choosing “the biggest one” is not the power move it sounds like.
Want to save money and energy on your heating bills? If so, you should consider installing one of the best smart thermostats.
What is Boiler Plus?
Boiler Plus is a set of efficiency standards for domestic boiler installations in England.
Under Boiler Plus, new and replacement gas boilers in English homes need to meet a minimum ErP efficiency of 92%.
Time and temperature controls are also required, and combi boiler installations need an additional energy-saving measure, such as weather compensation, load compensation, flue gas heat recovery or smart controls.
In normal human language, that means modern boiler installations need to be efficient and controllable, not just technically capable of making radiators hot.
Can you change from one boiler type to another?
Yes, you can often change boiler type, but it depends on your home.
Changing from a regular boiler to a combi boiler
This is common, especially in homes where the owner wants to remove the hot water cylinder and loft tanks.
It can free up space and simplify the heating system.
However, it may not be right if:
Your mains water pressure is poor.
You have multiple bathrooms.
You use lots of hot water at once.
Your pipework needs major upgrades.
Changing from a combi boiler to a system boiler
This can be a good option if your household has grown, you’ve added another bathroom, or your combi boiler struggles with demand.
You’ll need space for a hot water cylinder, but you should get better hot water performance for multiple outlets.
Changing from a regular boiler to a system boiler
This can reduce the need for loft tanks while keeping a hot water cylinder.
It may be suitable if your mains pressure is good and you want a more modern setup without going fully combi.
Final verdict: which boiler type should you choose?
The best boiler type depends on your home, not just the boiler.
Choose a combi boiler if you want a compact, efficient option for a smaller home with one bathroom and good mains water pressure.
Choose a system boiler if you have a larger home, multiple bathrooms and space for a hot water cylinder.
Choose a regular boiler if you have an older heating system, low mains water pressure or an existing setup with tanks and a cylinder.
Still not sure? That’s fine. Nobody is born knowing whether they need a 30kW combi or a system boiler with an unvented cylinder. That would be weird.
The easiest way to find out is to answer a few quick questions about your home and get a fixed price quote.
Next Steps For Your New Boiler Journey:
When planning to install a new boiler for your home, there are several important factors to consider. Make sure to refer to the following guides to help you make informed decisions:
To dive deeper into these topics, head over to our advice section, or check out our YouTube channel to learn more.
Get a Quote for One of the Best Boilers Today
If you’re aiming to get the best deal on a new boiler, consider getting a quote from us, here’s why:
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To learn more about boilers, visit our advice section, check out our YouTube channel, or read customer testimonials here.
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