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The Difference Between Combi, Heat Only & System Boilers

The Difference Between Combi, Heat Only & System Boilers

Choosing the right boiler to suit your needs can be daunting, but knowing the differences between a combi boiler, system boiler and a heat only boiler can help make the decision easier. There are many deciding 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. 

If your home just has one main bathroom and you're looking for a compact, efficient heating solution then a combi boiler could be for you. In this blog post, Ideal Heating shares the differences between combi, heat only and system boilers. 

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 boiler that you have is over 15 years old then you may well be considering a replacement. A combi boiler is a great option as it provides both heating and hot water on demand without the need for a water tank. However, some homes may require a system or heat only boiler. 

What Is A Combi Boiler?

combi boiler, or combination boiler, is the perfect heating solution for providing both hot water and heating to your home, all in one efficient compact unit. When it comes to a gas combi boiler there is no need for a water cylinder or tank, saving you money and space. This makes these boilers ideal for smaller homes. Want to know more? Read here to find out more about Ideal Heating’s combi boilers.

What Is A System Boiler? 

System boilers provide your home’s central heating while using water tanks or cylinders to store hot water. They are fully pressurised boilers and filled via a mains cold water connection, with a heat pump inside the boiler to circulate hot water to your home’s radiators to provide your home with the warmth and comfort it needs. 

If you have little space in your home for a water tank you may want to opt for a combi boiler as they have many advantages that are beneficial for smaller homes. 

What Is A Heat Only Boiler? 

Heat only boilers, also known as regular or conventional boilers, are the perfect option for larger homes. With quiet operation, a heat only boiler requires a small expansion tank and larger cold water storage cistern in your loft space to enable it to provide heating and hot water to your home. 

As with system boilers, heat only systems do take up a little more room compared to our trusty combi boilers. Unsure what size boiler your home requires? Check out our combi boiler size guide.

What Are The Differences Between A Combi, System And Heat Only Boiler?

combi boiler provides both heating and hot water instantly and is a type of condensing boiler. 

But, what is a condensing boiler? A condensing boiler is a high efficiency boiler which runs at a lower temperature than older, traditional style boilers. Combi boilers incorporate condensing technology and are therefore classed as a condensing boiler. Read more about combi boilers and condensing boilers. 

system condensing boiler is designed to work with a cylinder to provide hot water as it does not 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. 

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. 

See the difference between a combi, system and heat only boiler below: 

Combi Boiler

System Boiler

Heat Only Boiler

Property Size

Small homes

Large to medium-sized homes

Large homes

Storage Space

No storage space required

Extra storage space required

Extra storage space required





Hot Water Supply

Hot water is available on demand

Hot water is stored in the cylinder

Hot water is stored in the cylinder

Immersion Pump

Cannot be used in case of a boiler breakdown

Can be used for meeting hot water needs in case of a boiler breakdown

Can be used for meeting hot water needs in case of a boiler breakdown

Water Usage 

One hot water outlet should be used at a time

Hot water can be served to multiple outlets

Hot water can be served to multiple outlets

Choosing Your Combi Boiler  

It can be a daunting decision trying to choose between a combi, system and heat only boiler. Read our advice on upgrading your combi boiler to help you choose a model that works for you and your home. 

Take a look at our simple, easy-to-use Boiler Finder tool to help you choose the right boiler for your home.