In this article

1. What type of boiler do you need?

2. Choosing the right fuel type - Gas, oil or LPG boiler

3. Condensing vs non-condensing boilers

4. Boiler efficiency

5. Getting the right boiler size

6. Buying a reliable boiler

7. Get ahead of the boiler ban

Replacing your existing boiler will typically be one of the more expensive home improvements purchases you will make. Regardless of whether you’re replacing a boiler because it has packed up or upgrading an old model to a more energy-efficient boiler, there are a few important things to consider.

Fortunately, we’re here to help you in your search for a new boiler and help you navigate through what sometimes seems like a jungle of available heating options. Our expert guide will help you pick the perfect boiler for your home and fuel type you’re currently using. We’ll walk you through the different boiler types - combi, conventional and system boilers - explain how they work, which one would benefit you most, and a number of other things to look out for.

1) What type of boiler do you need?

There are several things to consider when purchasing a new boiler such as the size of your property, the number of radiators, your mains water pressure, the number of bathrooms and your property type, and we’ll talk about these as we look at which boiler types are the most practical for certain properties. First, let’s start with the types of boilers that are available.

There are three common types of boilers:

  • Combi boilers,

  • Conventional boilers, and

  • System boilers.

You may have also heard of another type of boiler, called a back boiler. These are conventional heating systems which sit behind a gas fire and provide hot water and heating. They are outdated nowadays but were a popular choice until 2005, when regulations came into force that made high efficiency condensing boilers mandatory.

The problem back boilers have is that they’re very inefficient – typically less than 70% efficient – which means that for every £1 of gas, you’ll only get 70p worth of heat while the remaining 30p are simply wasted. Back boilers are no longer produced but can be replaced with safer, more energy efficient and more compact heating units, usually combi boilers.

What is a combi boiler?

A combination or ‘combi’ boiler is the most popular boiler in the UK and combines both your water and central heating into one highly efficient and compact unit, which is why a combi boiler can be a great way to save space in your home and help you do your bit for the environment. Combi boilers heat your water directly from the mains whenever you use a tap, giving you unlimited hot water on demand, with no need for an additional water tank.

Combi boilers are typically very efficient (receiving an A efficiency rating), helping you to reduce your heating bills, and with ultra-compact models available, their small size can pack a serious punch.


  • Highly efficient

  • On-demand hot water and central heating

  • Ultra-compact models available

  • Water delivered from the mains – not from storage

  • Easier to maintain and install

  • Clean and safe


  • Run multiple showers or taps at the same time will lead to low water pressure

  • Can be expensive to fix

  • Requires a good strength of mains water pressure

  • If your boiler breaks down, you lose both your hot water and central heating

What is a conventional boiler?

A conventional boiler, also known as heat-only, traditional or open vent boiler, is a heating system that is often found in older and / or larger properties. A conventional boiler setup includes a large tank of cold water in the loft and a hot water cylinder – typically in an airing cupboard – in addition to the boiler unit.

So how does a conventional boiler work? A conventional boiler is supplied with cold water from the mains, which fills up the tank of water in the loft. This cold water is then fed by gravity into the boiler and heated. Once heated, the water is pumped from the boiler to the hot water cylinder in the airing cupboard, and stored for use with heating, and for your taps or showers. This setup means that conventional boilers are great for supplying multiple rooms at once with hot water, but they do take up extra room in your home. You also need to make sure that the hot water cylinder is large enough to cater for the hot water needed in your home on a daily basis.


  • Can supply hot water to multiple bathrooms at once

  • Works well even in homes with low water pressure

  • Easy to maintain

  • Can be combined with an immersion heating unit


  • Requires space for two water tanks

  • Some heat loss will occur if the cylinder is not insulated

  • It can take a while for hot water to reach the taps More expensive to install due to the setup

  • The size of the hot water cylinder determines how much hot water you have at your disposal

What is a system boiler?

A system boiler is another form of traditional heating system found in homes, which operates in a very similar way to a conventional boiler but has the advantage of requiring less space in your home. A system boiler requires an additional hot water storage cylinder alongside the boiler unit – but it doesn’t require a cold-water storage tank in your loft, which you would need with a conventional boiler.

With a system boiler, nearly all the components needed to supply your heating and hot water are built into the boiler itself – except for the hot water storage cylinder. Cold water is drawn in directly from the main supply to the boiler. Once heated, this water is then transferred to your hot water cylinder to be stored and used in your home. As with conventional boilers, a system boiler is a great option for homes with multiple bathrooms, as it allows hot water supply to multiple rooms at the same time.


  • Easier to install than conventional boilers

  • Takes up less space than a conventional boiler

  • Can supply hot water to multiple rooms at once

  • Water supplied from the mains meaning stronger water pressure

  • No need for an additional cold-water tank in the loft


  • Requires space for a hot water cylinder

  • It can take a while for hot water to reach the tap

  • Some heat loss will occur if the cylinder is not insulated

  • The size of the hot water cylinder determines how much hot water you have at your disposal

2) Choosing the right fuel type - gas, oil or LPG boiler?

As we’ve mentioned briefly, most UK homes will have a gas heating system but there are around 4 to 5 million homes in the UK who cannot install a gas boiler as they’re not connected to the gas network. These off-grid homes require an alternative fuel type such as oil or LPG (Liquefied petroleum gas). The good news is that the three different boiler types mentioned above are available for each oil and LPG users as well. However, more generally speaking, heating with oil or LPG are two of the most expensive ways of heating a home and one the worst choices when it comes to carbon emissions. Oil and LPG users should look into whether an air source heat pump could make a positive impact on their heating bills and carbon footprint.

And while gas is currently the cheapest fossil fuel source with which to heat your home, there may still be savings to be made when converting to an air source heat pump as running costs can be lower, not to mention the reduced carbon emissions.1

3) Condensing or non-condensing boilers

We’re often asked what the difference between a condensing and a non-condensing boiler is. At its core, all boilers burn fuel to create heat which is used to heat up water. But in the process of doing so, a boiler also produces waste gases (carbon dioxide and water vapour).

While non-condensing boilers simply release these waste gases through the exit flue, a condensing boiler features either a larger heat exchanger or a secondary heat exchanger, which uses the excess heat in the flue to preheat the incoming mains cold water, and as a result less gas needs to be used.

The effect of transferring the additional energy causes condensation to form within the flue and drain out through the bottom of the boiler, hence the term "condensing boiler". This results in a higher energy-efficiency, enabling condensing boilers to be up to 90% efficient.

At the start of 2005, the government made condensing boilers mandatory for all new boiler installations, so if you replaced your boiler after 2005, it’s very likely that it’s a condensing boiler.

So, what are the benefits of a condensing boiler? As mentioned earlier, by recycling waste gases, condensing boilers can be up to 90% energy efficient compared to the approximately 70% energy efficiency a non-condensing boiler achieves. This can translate into lower costs and reduced heating bills. The exact savings vary depending the age and efficiency of the old boiler but the Energy Saving Trust estimates that you could save up to £350* per year if you replace a G-rated boiler with an A-rated condensing boiler. And there’s other benefits too:

  • Save space in your home
    Condensing combi boilers don’t require additional water tanks.

  • Simple and smart controls
    On-demand heating and hot water eliminates the need for complicated timers.

  • Reduce your carbon footprint
    The increased efficiency of condensing boilers helps you to lower you carbon emissions. You won’t be able to achieve the low levels of carbon emissions an air source heat pump can achieve but it is a step forward.

4) Boiler efficiency

Boiler efficiency is one of the biggest things to consider when it comes to choosing a new boiler. The Energy Saving Trust estimates that heating your home will typically account for around 55% of your annual energy bill, so this makes choosing an efficient boiler key to reducing your energy bills and your carbon footprint.2

As with many appliances, each boiler will come with an energy efficiency rating, with the most efficient boilers being A rated. This efficiency rating is a good indicator of how much your boiler will cost to run and you can use this to compare different models to decide which will save you the most money and waste the least amount of energy. One thing to note is that because all modern boilers are required to be a condensing boiler, they tend to have an efficiency rating of around 90%. We would recommend looking at your existing boilers efficiency rating to compare the difference upgrading could make and our heating experts are happy to help you with this.

By replacing your traditional non-condensing boiler for a modern (A-rated) condensing boiler, you can seriously improve the efficiency of your home setup and could make some big savings on your annual energy bills. For example, the Energy Saving Trust estimates that, if you are living in a detached house and make the switch from an old G rated boiler, you will experience savings of over £300 per year.3

5) Getting the right boiler size

A boiler will be one of the most important investments you make in your home – and you should therefore make sure that the size of your boiler is the most appropriate for the size of your home. Choosing the right size boiler will ensure you have sufficient heating and hot water when you need it and depends on factors such as the number of bathrooms in your home. Here’s what you need to consider when choosing the size of your boiler:

  • Boiler size (kW)
    Boiler size is measured in kilowatts (kW), and this refers to the energy that is output from the boiler in the form of heat, rather than the physical size of the unit. The more heating or hot water you require – the higher number of kilowatts you will typically need.

  • Number of rooms / bathrooms
    The number of rooms and bathrooms in your home will influence your requirements for heating and hot water, and, in turn, the size of boiler that is needed. Larger homes will have more taps, showers and radiators, meaning a more powerful boiler is needed. If you live in a 1-bedroom flat, a combi boiler could be perfect, whereas if you live in a larger home with multiple bathrooms, you will require a larger combi, conventional or system boiler to run multiple showers or taps at the same time.

  • Water pressure
    For newer homes this won’t be much of a concern, with a mains water supply typically supplying adequate water pressure. However, for older homes this will need to be considered, and identifying flow rates is an integral part of the survey process.

  • Heat loss
    As well as the size of your home, the heat demand of your home plays an important part in choosing your boiler size. Newer or better insulated homes will have a lower heat demand, whereas older or less well insulated homes may require a larger boiler to compensate for the higher heat demand.

  • Boiler dimensions
    Lastly, you should measure the space in your home and compare it with the dimensions of the boiler to ensure that there is enough space for the install. If you live in a small flat, you may want to consider an ultra-compact combi boiler.

To help you with your search, here are a few examples of the best boiler sizes for different types of home.

  • Best boiler size for a one-bedroom flat
    If you live in a small one-bedroom flat with one bathroom, a combi boiler will be perfect for you. The high efficiency combined with the small dimensions make this type of boiler perfect. We would recommend looking at a boiler with a size of 24kW or less.

  • Best boiler size for a two-to-three-bedroom home
    For slightly larger homes between two to three bedrooms, we would recommend a medium-sized combi boiler or a small conventional or system boiler. A medium-sized combi will typically be powerful enough for your home but if you have multiple bathrooms that are often used at the same time, the conventional or system boiler route is the way to go.

  • Best boiler for size for four or more bedrooms
    For larger homes of four or more bedrooms, we would recommend a large combi or a system boiler. Your property will typically have room to store the additional water tanks – and a system boiler will allow multiple bathrooms and taps to be used at the same time. Dependant on how well-insulated your home is, we would recommend a mid-size to large boiler size, around the 28kW or 30kW mark.

These boiler sizes are intended purely as rough guidance, and we would always recommend speaking to one of our heating experts to clarify your requirements and get the best boiler for your home.

6) Buying a reliable boiler

As with all electric appliances, there is a wide range of available brands and boiler models on the market with varied reputations for reliability and quality. You want to ensure that your investment is a worthwhile one and you’re going to purchase a boiler that won’t develop any fault anytime soon. Here are our tips for buying a reliable boiler:

  • Research the best brands
    There are many boiler brands available to purchase which can at times seem overwhelming. However, there are a number brands that have a long-standing reputation for being industry leading in reliability, including Vaillant, Viessmann and Worcester Bosch. Impartial review sites such as Which? can help you with your decision.

  • Speak to an expert
    When in doubt speak to an expert. Heating engineers will have years of experience and can recommend you the best set up for your home. Receiving at least two quotes from different installers can help you to make an informed decision.

  • Warranty and aftercare
    Your boiler will on average last you 10 years, so you want to know you’re adequately covered in the event of a break down or any repairs that may be needed. Check the manufacturer warranty length of your chosen boiler and think about any additional breakdown cover. Nothing is worse than a broken boiler during the winter months.

7) Get ahead of the boiler ban

A large part of the government plans to cut the UK’s carbon emissions to zero by 2050 is aimed at abandoning fossil fuel heating in favour of low-carbon heating solutions. 95% of all UK households are centrally heated4, the majority of which by fossil fuels such as gas and oil. Burning these releases carbon dioxide (CO2) into the atmosphere, which is a major contributor to climate change.

While the government has made financial grants available that incentivise the uptake of renewable low-carbon heating - see below for the Green Homes Grant and the Renewable Heat Incentive - it has gone one step further and introduced a gas boiler ban, although the ban currently applies to new builds only. From 2025 onwards, it will be no longer permitted for newly built properties to have a gas central heating system installed.

An alternative to fossil fuel heating, and not only for new builds, can be found in air source heat pumps, which are a renewable and low-carbon way of heating a home, and can also be a great way to cut heating bills.5

Air source heat pumps have several other benefits:

  • Up to 4 times more efficient than an equivalent boiler

  • Lower running costs

  • Reduced carbon footprint

  • Efficient even at low temperatures

  • Low maintenance and a 15 to 20-year lifespan

  • Easy installation process6

Depending on the age of the heating system, LPG and oil boiler owners can save up to £1,300 or £550, respectively, on their annual heating bills when switching to an air source heat pump. If you are switching from a gas boiler, you can save up to £100 on your yearly heating bill.7 These savings assume you are replacing a G-rated boiler - the older the boiler, the higher the savings.

But there’s more. The government is currently offering funding to install an air source heat pump, most notably the Green Homes Grant (GHG) and the Renewable Heat Incentive (RHI). The former is intended to help with financing the upfront cost of an air source heat pump while the latter allows you to get paid for every unit of renewable heat your new air source heat pump generates.

For a property to be suitable for the installation of an air source heat pump, the requirements are low: you will need to be a homeowner (or have the landlord's permission), live in a house (rather than a flat), and have an up-to-date EPC certificate.8

If you’d like to learn more about air source heat pumps, check out our guide to air source heat pumps or speak directly to one of our heating experts who will be able to answer any questions.

1 Depending on system design and usage.




5 Depending on system design and usage.

6 Depending on system design and usage.

7 Your actual savings will depend on system design and usage.

8 There may be engineering constraints not apparent before the survey or installation.