The difference between Combi, Heat Only & System boilers

Choosing the right boiler to suit your needs can be daunting, but knowing the difference between a Combi boiler, System boiler and a Heat Only boiler can help make the decision easier. There are many factors to help you choose which boiler is right for you, including whether you have a high demand for hot water or if you’re limited by storage space. 

For instance, if your home just has one main bathroom (with a shower or bath) and you're looking for a compact, efficient heating solution, then a Combi boiler could be for you. If you’ve got more space (and room for a hot water cylinder), then a System or Heat Only boiler might be more appropriate. 

Read on for all the differences between the three boilers so you can make the best choice for your home.

What type of boiler do I need for my home?

When it comes to boilers, there are three main types: Combi, Heat Only (sometimes known as a ‘conventional’ or ‘regular’ boiler), and System. If you have just bought a new home, your existing boiler has broken down, or the Combi boiler that you have is over 15 years old, then you may well be considering a replacement. 

A Combi boiler is a great option for many homes as it provides both heating and hot water on demand without the need for a water tank (which takes up valuable space). However, for larger houses where hot water is needed across multiple outlets (e.g. the kitchen and a bathroom) at the same time, a System or Heat Only boiler may be more suitable. Our guide to choosing a new boiler goes into more detail about the considerations you should make when buying a new model for your home. 

Understanding the three different types of boiler

Below, we’ve given you a snapshot view of each boiler type.

What is a Combi boiler?

A Combi boiler (or Combination boiler), is an all-in-one efficient compact unit which provides both hot water and heating to your home. Rather than taking water from a storage tank or cylinder, a Combi boiler takes water straight from your mains and heats it on demand, as and when you need it. They’re ideal for smaller homes or houses with limited storage space, and are one of the most efficient types of boiler. 

For more details about Combi boilers, check out our articles:

• What is a Combi boiler?

• Advantages of a Combi boiler

• How to find the best Combi boiler for your home 

You can also use our Combi boiler size guide to understand which size is the best fit for your home and your heating needs.

What is a System boiler? 

Unlike a Combi boiler, a System boiler takes water from a tank or cylinder to provide your home with hot water and central heating. System boilers are fully pressurised and are filled via a mains cold water connection. A heat pump inside the boiler circulates hot water to your home’s radiators to keep the temperature warm and comfortable. 

Just bear in mind that a water tank or cylinder does require extra storage space, meaning a System boiler tends to be more suitable to medium- and large-sized homes. 

What is a Heat Only boiler? 

Heat only boilers (also known as regular or conventional boilers), are the perfect option for larger homes. A Heat Only boiler requires a small expansion tank and larger cold water storage cistern in your loft space to provide heating and hot water to your home.

Like System boilers, Heat Only boilers take up a little more room compared to Combis. If you’ve got the space, they’re a worthwhile investment, lasting between 15 and 20 years. For more information about their typical lifespan, take a read of our article: How long will a Heat Only boiler last before it needs replacing?  

And for a better idea on the best size boiler for your home, check out our blog: What size boiler do I need?

Combi, System, and Heat Only boilers: A direct comparison

We’ve designed the table below to wrap up the main differences between a Combi, System and Heat Only boiler:

Are Combi, System and Heat Only boilers types of Condensing boilers?

All three of these boilers are types of condensing boilers, which are highly-efficient modern-day boilers that run at lower temperatures than older, more traditional boilers. A condensing boiler has a much larger heat exchanger. It’s designed to extract over 90% of the heat from the fuel it burns (which would normally be lost to the atmosphere through the flue) and recycle it back into the heating system.

• Combi boilers incorporate condensing technology and are designed to use less energy during the heating cycle. Head to Combi boiler vs Condensing boiler for a more in-depth look at why modern Combis are classed as condensing boilers. 

• A System Condensing boiler is designed to work with a water cylinder to provide hot water, as the cylinder itself doesn’t have the hot water heat exchanger. A System boiler contains an expansion vessel and pump allowing the boiler to be installed in a sealed system. 

• A Heat Only condensing boiler is the simplest type of boiler. It can be used on either an open vented system, which has a tank in the loft, or in a sealed system with the addition of an external pump and expansion vessel. A Heat Only boiler uses a water cylinder to provide your home with hot water. 

Find out more about condensing boilers and the advantages of using one to provide your home’s heating and hot water in our blog: What are the benefits of high efficiency Condensing boilers? 

Ready to choose a new boiler?

It can be a daunting decision trying to choose between a Combi, System and Heat Only boiler. Our tips and advice articles are packed with more advice to help you find the model that works for you and your home. Make sure you have a go with our interactive boiler finder, which takes into account information like your property type, number of bedrooms, and number of bathrooms to give you the most appropriate recommendation.