Buying a new boiler is a massive investment for any homeowner, but it can also be a pretty tricky process.

Get it right, and you can have peace of mind for many years to come, enjoying all the benefits that come with a high-performance, energy efficient boiler.

But get it wrong, and you’re in for a world of trouble, not to mention even more expenses down the road.

The $64,000 question is:

How do I know which boiler to buy?

At Plumbing Superstore, our customers often ask us exactly this. That’s why we decided to put together the ultimate boiler guide that will provide plenty of help with buying a new boiler.

Here’s what we’re going to cover.

Table of contents:

How a boiler works

Commercial boilers in a room

Before we dive into the various types of central heating boilers available on the UK market, we need to get a basic understanding of how boilers actually work.

Here’s the deal:

Your boiler works by burning a fuel source, such as natural gas, Liquefied Petroleum Gas (LPG) or electricity, in a combustion chamber to produce heat. This heat is then transferred to water or another fluid within the boiler through a heat exchanger.

As the water absorbs the heat, it undergoes a phase change, turning into steam if high temperatures and pressures are reached or simply becoming hot water in lower-pressure systems.


The steam or hot water generated in the boiler is then used for various purposes, such as heating buildings, producing electricity in power plants or driving industrial processes. To ensure safe operation, boilers are equipped with safety mechanisms, including pressure relief valves and temperature controls. In addition, they may employ advanced features like condensation to maximise energy efficiency.

All in all, boilers play a crucial role in providing heat and power for a wide range of applications, making them essential components in many industries and residential heating systems.

If you want to find out more about how central heating works in general, check out our guide.

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Boiler types in the UK

Gas central heating wet systems are currently the prevailing choice in the UK, although electric wet central heating systems are occasionally encountered. As most boiler guides will tell you, gas boilers are the preferred option for several reasons.

Here’s the scoop:

Firstly, the majority of UK residences are already connected to the gas network, offering a convenient heating solution.

Secondly, gas is more cost-effective than many alternative heating sources, resulting in savings on energy bills.

Thirdly, the UK has substantial gas reserves in the North Sea, ensuring a reliable domestic supply without the need for extensive gas imports.

Having said that, we should point out that fossil fuel boilers (gas, oil and LPG boilers) are set to be phased out after 2035 in line with the UK Government’s renewed commitment to achieving Net Zero by 2050 – although some exemptions will be in place to help households transition to renewable energy sources.

With that in mind, let’s delve into the different boiler types:

Combi boilers

Combi (combination) boilers efficiently provide both heating and hot water on demand from a single unit. Their compact size and straightforward maintenance make them popular, especially in smaller homes and flats where space is limited. Combi boilers are cost-effective, provide an immediate supply of hot water and are favoured in new builds.

Conventional boilers

Conventional boilers, also known as regular or heat-only boilers, operate in an open-vented system with external components like pumps and valves. They excel at heating rooms for extended periods but do not supply hot water, limiting their versatility.

Conventional boilers are simple in design and cost-effective. They work well with older radiators and are suitable for budget-conscious consumers.

System boilers

System boilers combine features of both conventional and combi boilers.

Check this out:

They have a substantial hot water cylinder but run in a sealed system. Ideal for larger homes with multiple bathrooms and high hot water demand, system boilers ensure consistent hot water flow to multiple outlets simultaneously. However, they are larger in size due to the hot water storage cylinder.

In case you’re wondering:

No, hot water doesn’t need to be continuously on for central heating, as properly insulated cylinders can maintain heat for hours or even a day after being heated by the boiler or immersion heater.

Condensing vs non condensing boiler: how are they different?

Potterton commercial boiler

If you’ve been thinking about getting a new boiler for a while, you’ve probably come across the terms ‘condensing’ and ‘non-condensing’.

But how do they differ? Let’s find out!

Condensing and non-condensing boilers differ primarily in how they handle the combustion process and utilise the heat produced. In effect, a condensing boiler is a far better energy-saving boiler, designed to maximise energy efficiency by extracting additional heat from the exhaust gases that would otherwise be lost.

The thing is:

In a condensing boiler, the combustion gases cool down to the point where water vapour in the exhaust condenses into liquid water. This phase change releases latent heat, which is captured and used to preheat the incoming cold water. The process significantly increases the boiler’s efficiency, often reaching mind-blowing efficiency levels of up to 99%.

What’s more:

Condensing boilers are typically more environmentally friendly and cost-effective in the long run because they consume less fuel and produce lower emissions. They are particularly suitable for modern heating systems, including underfloor heating, where lower water temperatures are used.

Non-condensing boilers, on the other hand, can’t capture this latent heat from the exhaust gases. So, they release the hot gases into the atmosphere, resulting in lower energy efficiency, typically around 70-80%. As a result, these boilers are often found in older heating systems that run at higher water temperatures, such as traditional radiator systems.

However, it is now illegal to install non-condensing boilers in the UK, as government regulations state that new boilers must have a minimum efficiency of at least 92%.

What type of boiler do I need?

Valiant boiler

At the end of the day, the best boiler type will vary depending on the size of your home and household and your specific needs. Combi boilers are many buyers’ go-to choice because they’re compact and easy to maintain. With a larger house, though, you might want to consider a system boiler.

In any event, when choosing a boiler, you should consider the following factors:

  • The size of your home
  • Your hot water & heating needs
  • Your existing heating system
  • Available space
  • Fuel availability in your area
  • Installation & maintenance
  • Sustainability

Keep in mind that different boiler types tend to have different lifespans.

Now that we’ve covered the various types of boilers, we need to discuss boiler sizes.

What size boiler do I need?

Boiler size or output is measured in kilowatts (kW). And, once you’ve made up your mind about the type of boiler you intend to buy, you’ll need to figure out what size you require.

In general, that will depend on the size of your home. Specifically, the key elements in the equation are:

  • Number of bedrooms
  • Number of bathrooms
  • Number of radiators

Keep in mind that the state of your insulation will also play a huge role. If you live in an older, poorly insulated property, you’ll need a boiler with a considerably larger output to maintain a similar temperature to the one you would in a new build with state-of-the-art insulation.

With that caveat in mind, here’s a breakdown of the most common scenarios for combi and system boilers.

Combi boiler size calculator

Combi boilers have to be more powerful than the alternatives because they have to heat the home while simultaneously producing hot water on demand.

BedroomsBathroomsRadiatorsRecommended boiler size
1-21Up to 1024-27kW

System or regular boiler size calculator

If you’re wondering why system boilers in particular have to be less powerful than combi boilers, there’s a simple answer:

System boilers have a separate hot water cylinder that takes care of your domestic hot water (DHW) needs.

BedroomsBathroomsRadiatorsRecommended boiler size
1-21Up to 109-18kW

With so many different boiler types and sizes currently available on the UK market, by this point you’re probably wondering how much buying a boiler is going to set you back, right?

That’s exactly what our boiler recommendations are going to zoom in on next.

What is the average price to replace a boiler system?

Imax Xtra 2 Condensing Boiler

As we’ve already mentioned on a couple of occasions in our boiler guide, buying a new boiler can be a very expensive proposition.

Just how expensive will ultimately depend on several factors, including:

  • Types of boilers
  • Size of boilers
  • Boiler models & brands
  • Installation costs
  • Removal of old boilers

Given the number of variables in question, it’s virtually impossible to calculate exactly how much your boiler replacement is going to cost.

No need to despair, though! Here are some estimates of your likely boiler replacement costs:

A new boiler by itself is likely to set you back anywhere between £500 and £3,000 plus VAT.

But that’s not all:

Your new boiler must be installed by a certified specialist. So, you’re looking at another £1,500 plus VAT in installation costs on average, once again depending on your boiler options.

In general, a like-for-like swap will be cheaper, unless you need to change the location.

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How to choose a boiler

Depiction of the energy crisis

Buying a new boiler can be quite challenging. We hope our boiler guide for buyers has given you a head start so that you know what factors to bear in mind.

The bottom line is:

Consider your budget, including installation costs and long-term operational expenses, to find a boiler that balances your needs and financial constraints.

Consulting with a qualified heating professional for up-to-date boiler advice can help you make an informed decision tailored to your specific circumstances.

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