Combi boilers vs system boilers: what’s the difference?

Choosing a new boiler can feel overwhelming, especially when you’re faced with terms like combi boiler, system boiler and conventional boiler. While they all heat your home, each boiler works in different ways – so it’s important to understand which is best for your home.

Choosing between a combi boiler and a system boiler depends on how much hot water you need, how much space you have, and how many people live in your home. In this guide, we’ll explain how each boiler works, the key differences, and the advantages and disadvantages of both options, so you can make a confident decision before choosing your next boiler.

What is a combi boiler?

A combi (short for combination) boiler is an all-in-one heating system. It provides heating and hot water directly from the mains, without the need for a separate hot water cylinder or cold water tank. This means all parts of the boiler are housed neatly in one small area, often around the size of a kitchen cupboard.

When you turn on a hot tap, a combi boiler heats water instantly and delivers hot water on demand. This makes combi boilers compact, efficient and a popular choice for many UK homeowners.

Combi boilers are typically best suited to flats and small-medium sized houses that have:

• One or two bathrooms
• Low occupancy
• Low demand for hot water
• Limited storage space (since a combi boiler takes up little room)

What is a system boiler?

A system boiler works alongside a separate hot water cylinder, usually stored in an airing cupboard. Unlike a conventional boiler, it does not need a cold water tank in the loft as key components are built into the boiler itself.

System boilers can supply multiple taps at the same time without a noticeable drop in pressure. This is because the hot water is stored in a cylinder.

System boilers are typically best suited to larger homes with:

• More than two bathrooms
• Homes planning future upgrade such as solar thermal systems
• Higher hot water demand

If you’re unsure, a heating engineer can recommend the best type of boiler for your property.

Combi boiler vs system boiler: key differences

The main difference between a combi boiler and a system boiler is how hot water is delivered.

A combi boiler heats water instantly as you need it, while a system boiler stores hot water in a cylinder for later use. This affects performance, space requirements and suitability for larger households.

Advantages of a combi boiler

Combi boilers remain the most commonly installed boiler type in the UK due to their efficiency and compact design. According to Uswitch, around 80% of UK homes currently have a combi boiler.

Advantages of a combi boiler include:

• A space-saving all-in-one system
• No need for a hot water cylinder or cold water tank
• Hot water on demand
• Typically lower installation costs than system boilers
• High energy efficiency with minimal heat loss

Disadvantages of a combi boiler

While combi boilers are efficient and convenient, they are not suitable for every home.

Some of the disadvantages of a combi boiler include:

• Limited hot water flow when multiple taps are used at once
• Reliance on mains water pressure
• Not always suitable for larger households
• Potential performance issues during periods of high demand e.g. lower water pressure

Advantages of a system boiler

System boilers are designed to meet the needs of homes that use a lot of hot water throughout the day.

Advantages of a system boiler include:

• Able to supply multiple taps at the same time with consistent water pressure
• Compatible with renewable heating systems
• Reduced strain when hot water demand is high

Disadvantages of a system boiler

System boilers also have some drawbacks, particularly in relation to space and cost, such as:

• Water needs to be heated before use
• Space is needed to house a hot water cylinder
• Higher installation costs compared to combi boilers
• The possibility of stored hot water running out
• Some heat loss from stored water

Are combi boilers cheaper to run than system boilers?

Both combi and system boilers can be highly efficient when installed and used correctly. Modern combi and system boilers are typically A-rated for energy efficiency, so it really comes down to the size and demand of your property.

Combi boilers can be cheaper to run in smaller homes with lower hot water demand, while system boilers may be more efficient for larger households that regularly use multiple taps at once. In practice, running costs depend more on how hot water is used than on the boiler type itself.

What is a conventional (regular) boiler?

A conventional boiler uses both a hot water cylinder and a cold water storage tank, usually located in the loft. This is an older system design commonly found in traditional UK properties.

While conventional boilers can still be effective, many homeowners now replace them with combi or system boilers to improve efficiency, reduce space requirements and simplify their heating system.

So, which boiler is right for your home?

Choosing between a system boiler and a combi boiler depends on all of the factors we’ve discussed throughout this article, including the size of your home, available space and hot water usage.

A professional boiler survey is the best way to confirm which option is right for your property. 

Need help choosing the right boiler?

If you’re still unsure whether a combi or system boiler is best for your home, professional advice can help you avoid costly mistakes.

At Add Heat, our Gas Safe registered engineers assess your property, explain your options clearly and recommend the most efficient boiler for your needs. Whether you’re replacing an old system or planning a full upgrade, we’re here to help. Get in touch with us today.

In the meantime, you might find the following heating guides useful:

• Our annual boiler maintenance checklist for homeowners
• How to save up to £300 per year by adjusting your boiler flow temperature
• When is it time to upgrade your boiler?