Types of boilers – and which is right for your home

We all need a way to heat our homes and our water – and for most of us, that comes in the form of a gas boiler.

You probably don’t spend much time thinking about your boiler – unless it breaks down. It's often only when something goes wrong that we think about how our boiler runs day-to-day.

If you want to have year-round peace of mind when it comes to your heating, our boiler cover can help. But whether you have cover or not, you might want to know more about the types of boiler available and how they work. The more you know about your boiler, the better you'll be able to maintain it.

So, here we’ll look at the different types of boilers and how they run.

What is a gas boiler, and how does it work?

A boiler is a central heating unit that pumps hot water through the pipes around your home. It supplies the hot water to your taps, and heats up your radiators.

Usually, it’s connected to your mains gas, so there’s a constant supply of fuel. If you’re not on the gas grid, you might use oil or LPG (liquid petroleum gas) to fuel your boiler, or you might have an alternative source of heating, such as a heat pump.

While a gas boiler’s running, a valve connected to the boiler opens so that gas can enter. This is then burned to create hot flue gases, which heat the water that’s pumped through the boiler. That hot water’s then sent to your radiators, taps and showerheads – wherever it’s needed – using an electric pump.

Different types of boilers

The way a boiler works is fairly similar from model to model, but each boiler type works slightly differently. Certain boiler types will be more suitable for your home than others. Three popular boiler types are:

• Conventional
• Combi
• System

To help you get to grips with how each boiler type works, we've outlined how they run below:

Conventional boilers

Also known as heat-only boilers, a conventional boiler is a common way to heat your home – especially for larger households. This type of boiler comes with a separate cold water tank and hot water cylinder. It also needs a ‘feed and expansion tank’, which is usually kept in your loft. These can take up quite a lot of space, and the hot water cylinder needs to be kept insulated.

The cold water tank fills with water from the mains supply, which then goes through your boiler to be heated and sent to the right place. For example, the water will go from the boiler to the feed and expansion tank and then to the radiators for your central heating. Or your boiler will send water to the hot water cylinder, to provide hot water for your taps.

Conventional boilers are generally reliable, especially if they are looked after properly with an annual service. But they can also be fitted with a backup immersion heater so you can still have hot water if your boiler breaks down.

Combi boiler

A combi boiler is usually a smaller model that can provide heating and hot water without any separate water tanks or cylinders. A combi boiler usually has two separate heat exchangers (although some models only have one). One heat exchanger connects to the radiators, and the other connects to the hot water supply.

Combi boilers are usually more compact, energy-efficient, quick to install and cost-effective then conventional boilers. Because a combi doesn’t connect to separate water tanks, it’s on constant standby. When you need hot water, the boiler can instantly burn fuel so that the heat exchanger can heat the water.

This is great for smaller homes and families who don’t need as much hot water as larger households. But because they’re compact boilers, combis struggle to supply heat and hot water to multiple bathrooms.

System boilers

A system boiler is a bit like a cross between a conventional and a combi boiler. It provides heat and hot water using a hot water cylinder, but doesn’t need a feed and expansion tank.

The cold water comes directly from the mains supply and is then heated and stored in the hot water tank. The system will send hot water to your taps or radiators from the tank, when it’s needed.

What’s a condensing boiler?

All the types of boiler we’ve talked about above (conventional, combi and system) can also be condensing boilers.

Any new boiler you install now has to be condensing by law in the UK. This is because they’re much more energy efficient than non-condensing boilers. They stop water vapour escaping during the heating process, and instead use a heat exchanger to recycle that heat back into the system.

So condensing boilers not only waste less energy, but over time, they could save you money on your heating bills.

What’s a hydrogen-ready boiler?

A hydrogen (or hydrogen-ready) boiler burns hydrogen, which is cleaner than gas from fossil fuels. It works just like a natural gas boiler in how it heats your home, but it uses a greener fuel.

You can install a hydrogen-ready boiler now, and run it on natural gas from the mains. Then when hydrogen becomes available in Britain’s gas network in the future, an expert can quickly convert your boiler to run on hydrogen.

How to choose a boiler system

What type of boiler do I have?

You can generally tell which type of boiler you have just by looking at it, or by considering the type of property you live in. There are some clear distinctions:

• A combi boiler will usually have 5 or 6 copper pipes coming from the bottom and no external pump. You’ll usually find combis in smaller homes.
• A system boiler will usually have 3 or 4 copper pipes coming out of the boiler and will often be installed in larger properties with multiple bathrooms. There will also be a separate hot water cylinder.
• A regular boiler will often have two pipes coming out of the top and one coming out of the bottom of the boiler unit. These are usually found in larger homes with a loft, as they need a separate tank, cylinder and pump.

What size boiler do I need?

If you live in a smaller home which doesn’t need loads of hot water and heating, you might opt for a combi boiler. These take up the least amount of room, as they don’t need an external tank, and are the most efficient.

If you’ve got multiple bathrooms and higher demand for hot water, you might need a conventional boiler. They’re better suited to bigger homes because they need an external hot water cylinder and tank – but this means they can handle higher usage.

A system boiler’s good for homes with high demand but with slightly less storage space, as they don’t need a separate tank in the loft.

If you’re looking for a new boiler, you might find that choosing a newer version of the same type of boiler is the best way to go. This is because the pipework and tanks in your home will be set up in a certain way and replacing all these could be expensive. Our friends at CORGI HomeHeat can help with a new boiler that could save £840 a year on your bills.**

Taking care of your boiler

Hopefully now you understand a bit more about how different boilers work. If you’d like peace of mind when it comes to your heating, we serviced over 200,000 boilers in 2022 * and offer a range of simple boiler cover plans. They all include an annual boiler service and unlimited callouts. We’re here for you 24/7 – you can call us anytime on our emergency helpline or raise a claim online. So a Gas Safe engineer will always be on hand to help in an emergency– day or night.

*CORGI HomePlan are part of the OVO family, and as a group serviced 215,927 boilers in 2022.

**Energy Saving Trust latest figures suggest you could save £840 a year if you live in a detached home and replace a G-rated boiler with an A-rated one. https://energysavingtrust.org.uk/advice/boilers/