Boiler Installation Guide: Everything You Need to Know

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Last updated: October 31, 2023

Your boiler is the heart of your central heating system – it keeps your home warm and cosy in winter and provides hot water for everything from running hot baths and showers to washing dishes. In short, it is an essential piece of equipment in any household. For this reason, when a boiler fails or needs replacing, it can be a very stressful time. However, there’s no need to worry as we’re here to help you make the sense of it all! In this boiler installation guide, we cover everything you need to know about installing a new boiler, including exactly what the process entails, what boiler type and size is right for your needs, how much you can expect to pay for installation, as well as what happens afterwards.

What Types of Boiler are There?

The first step in getting a new boiler installed is to find out what type of boiler you currently own. The brand doesn’t matter that much, however, the type does as it will tell you which models you should be looking for.

Combi Boilers

Combination or combi boilers are the most popular type of boiler in the UK, with over 50% of all dwellings using a combi unit. It’s no wonder why – compact, easy to install and able to heat your home and provide it with hot water, combi boilers seem to offer the best of all worlds.

Since a combi boiler is connected directly to the mains water supply of your home, it heats water directly, meaning there’s no need for a separate water tank, which is something a system boiler as well as a regular boiler needs. For this reason, combi boilers save a great deal of space, making them a No.1 choice for flats where space is at a premium. Another great thing about this type of boiler is that since it doesn’t have the storage tank, it heats water as and when you need it, so it’s very energy-efficient.

On the downside, combi boilers are not great for homes with two or more bathrooms as they will struggle to provide enough hot water.  Also, since a combi boiler relies on mains water pressure, your water flow may be sub-standard if you live in an area with poor water pressure. Finally, since a combi boiler doesn’t have a hot water tank, if it breaks down, you’ll be without hot water.

System Boilers

Similarly to a combi boiler, a system boiler is connected directly to the mains, however, unlike its counterpart, the system boiler doesn’t deliver hot water on demand; instead, it stores it in a cylinder. This means that additional space in the house or flat is needed for the hot water cylinder, but thankfully, this can usually fit in an airing cupboard or loft.

The best thing about system boilers is that they can easily meet higher demands for hot water. This makes them perfect for homes with two or three bathrooms. These boilers are also relatively easy to install and, importantly, they are compatible with solar thermal energy.

While not extremely bulky, a hot water cylinder does require some additional space so a system boiler may not be suitable for smaller homes or flats. Also, while you can run multiple showers simultaneously with a system boiler, once you’ve used up the hot water stored in the cylinder, you’ll have to wait a while until the boiler reheats it.

Conventional Boilers

Also known as the regular boilers, heat-only or traditional boilers, conventional boilers are the oldest and most traditional heating systems. This type of boiler comes with a cold water tank, hot water cylinder and an expansion vessel.

A conventional boiler is usually recommended for properties where a system or combi boiler installation is not possible. For instance, if you have multiple bathrooms, the mains pressure is too low, or if your pipework cannot cope with the mains pressure, a conventional boiler may be the only option.

Due to their complex nature, conventional boilers are quite difficult to install. They’re also not as energy-efficient as combination or system boilers. Furthermore, they take up a lot of space, so are unsuitable for smaller homes and often medium-sized properties as well.

Modern living room with kitchen

What’s the Best Boiler Size for You?

The next step is to figure out the right boiler size for your home. To ensure you always have enough hot water and you’re not overpaying for heating bills, it’s essential to choose a boiler that meets your needs.  How do you do this? By buying a unit with an output rating that meets the level of your heating and hot water requirements.

Since boiler size is measured in kilowatts (kW), you’ll want to get a model with the number of kW that’s just right for your needs – not too low as then then boiler won’t be able to meet your demands, but not too high as you’ll waste fuel and spend more money on energy bills. To work out which boiler size is the best for your property, pay attention to the following things:

  • The number of radiators
  • The number of bathrooms
  • The number of people in the household

Of course, there are other factors to take into account as well, including how well your house is insulated and whether or not you’re planning a home extension, but any gas engineer will already know that and will help you choose the suitable boiler size for your property. Nevertheless, if you want to calculate things yourself, here is a rough sizing boiler guide for different boiler types based on the number of radiators you have in your home.

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Is it Time For a New Boiler Installation?

Getting a new boiler is an investment so it’s worth double-checking if your heating engineer is able to repair your current boiler or if you indeed need a new appliance. Here are the five most common signs that it’s time for a new boiler installation.

Regular breakdowns

If your boiler is breaking down every few weeks or you’re often experiencing low pressure, it’s more than likely things are going to get worse over time. Instead of spending money on constant repairs, it may be more cost-effective to install a new boiler. Another reason to purchase a new unit is if your heating engineer is having a hard time finding replacement parts (typically the case for boilers older than 10 years).

Boiler giving off bad smells

Whilst carbon monoxide is odourless, gas companies add a scent to it to help you identify a gas leak. A healthy boiler won’t give off any type of smell, so if you notice an unusual or bad scent, including the eggy, sulphur odour coming from your boiler, open your windows and leave the house before contacting the Gas Emergency Services. Carbon monoxide can be extremely dangerous and it’s vital that you react the moment you notice the smell. Unusual smells such as an electrical burning odour can also point to other faults in the boiler.

Increasing energy bills

If your energy bills are slowly increasing in recent months and your supplier hasn’t raised your tariff, it’s highly likely your boiler is to blame. All boilers lose their efficiency with time, meaning they have to use more fuel to meet the same heating and hot water demands. Most modern boilers are much more energy-efficient than older models so getting a new boiler installed could help you save on heating bills. When purchasing a new unit, pay attention to the ErP or energy rating label, which starts from dark green (A) and goes to red (G). The most efficient units will be in the green zone.

Boiler making strange noises

If your boiler is making strange noises such as kettling, clunking or banging sounds, it may be time for a new boiler installation. While these types of noises could indicate a host of different problems, including minor and easily fixed ones such as a blockage in the system, and a serious and expensive to fix ones like a faulty pump, one thing is sure – if they’re getting worse over time, you need to contact a Gas Safe registered engineer so they can make sure the appliance is safe for use.

Your heating system is leaking

Even a small leak can create big problems and lead to serious damages to both your heating system and home. Furthermore, a leak in the system means that your heating is inefficient and you’re paying more on your heating bills. If you’ve recently had a leak and you’ve fixed it but your boiler is leaking or dripping again, it may be time for a new boiler installation. While a small, one-time leak can be due to a broken valve which can be replaced, bigger and/or more regular boiler leaks are typically caused by widespread corrosion.

How Much Does Boiler Installation Cost?

Boiler installation costs vary widely, depending on where you live, who you employ for the job, which boiler brand, type and model you choose, and how much work is required for installation. Of course, there are extra boiler parts to think about as well, such as filters, smart controls, etc.

For example, if you’re simply swapping your current boiler for a newer and better model, such as an old combi boiler for a new combi boiler, the installation will be straightforward and therefore should be inexpensive. Likewise, if you’re installing a new boiler in the same place as the existing unit, the fitting will be simple and quick. On the other hand, if you’re installing a new boiler type (e.g. removing an old conventional boiler and getting a new combination boiler), and you’re moving it to a new location, the installation cost will be much higher as it will involve extra work, parts and rerouting the pipework.

Roughly speaking, you can expect to pay anywhere from £1000 to £2500 for the installation of a new gas combination boiler, and £1500 to £3500 for the installation of a system or conventional boiler.

How to Compare Voiler Installation Quotes

Since the prices for boiler installations vary enormously, we highly suggest you compare installation quotes from multiple heating engineers before you make a deal. Aside from giving you the assurance you’re paying a fair price for your boiler installation, comparing multiple installation quotes will give you an opportunity to choose the right person for the job.

Here are the most important things to ask and check with your boiler installer:

  • If they are Gas Safe registered (ask to see their Gas Safe ID card!)
  • If you really need a new boiler installation or if the problem(s) can be fixed
  • Which boiler brands and models they recommend for your needs
  • The reliability  and longevity of the boiler and how much the parts cost
  • How long the boiler installation will take to complete
  • Check exactly what is covered by the warranty

What to Expect During the Installation

A good boiler installer will try their best to make the installation process as simple as possible. This includes having all equipment and parts necessary for the installation with them, adhering to all legislation, covering your carpets and other surfaces in order to protect them, and generally working efficiently and safely.

Before your installer begins, they will check your water pressure, how many radiators you have, the number of bathrooms in your house, as well as the positioning of your current boiler and boiler flue if you have them.

How long does a boiler installation take?

If you’re having a simple boiler swap (e.g. combi gas boiler for combi gas boiler), the installer should be done within a day. However, if you’re replacing your existing boiler with a different type (e.g. regular for combi), the installation will be more complex and can take up to three days. Generally speaking though, most modern boilers can be fitted within one day.

Can a plumber install a boiler?

While plumbers can be highly experienced in all aspects of plumbing, most are not qualified to install a gas boiler. Unless your plumber is Gas Safe registered, we don’t recommend hiring them to work on your boiler.

After your boiler has been installed

Once your installer of choice has fitted your new boiler, they should take you through how to safely use the controls. Your installer should also provide you with a Building Regulations Compliance certificate, after which you can register the installation with the manufacturer to validate your warranty. Bear in mind that most manufacturers request you to do this within 30 days of your boiler installation.

To keep your boiler running smoothly and your boiler warranty valid, you’ll need to have your boiler serviced annually by a Gas Safe registered engineer.


  1. Boiler Installation –  Worcester-Bosch
Boiler Installation Guides
Using a Gas Safe Engineer
Boiler Types
New Boiler Replacement and Installation Costs in 2023
Changing From a Conventional Boiler To a Combi Boiler
How Much Does Moving a Boiler Cost?
How To Hide a Boiler in Your Kitchen