Combi Boilers

Combination or combi boilers as they are commonly known are the most commonly fitted type of boiler in the UK accounting for approximately 70% of UK boiler sales. Combi boilers are now all condensing boilers.

Combi Boiler Controls installed

Combi boilers remove the need for a cold water storage cistern, feed and expansion tank and a hot water cylinder as the provide both the hot water and central heating directly.

Combination boilers tend to have a higher kilowatt rating than their heat only or system boiler counterparts. This is because a high capacity is required to generate the instantaneous hot water. The higher kilowatt rating is often more than is required to heat the property but thankfully modern boilers are able to adjust their performance to allow for this.

Combi boilers have a number of advantages.

Combi boiler explenation diagram

They do not heat stored water in a hot water cylinder unnecessarily. There is a constant supply of hot water without the fear of running out. A considerable space saving is achieved as a hot water cylinder is no longer required. Because there is no longer a cold water storage cistern or a feed and expansion tank, no stored water is required, removing a potential leak and freezing risk.

Combi boilers do have disadvantages.

If the boiler fails then there is no hot water and heating! If you have a hot water cylinder it is likely you will have an immersion heater as back up to provide hot water.

The maximum flow of hot water from a combi boiler is limited. In larger properties with multiple hot water outlets or where there is more than one thermostatic shower the combi boiler may struggle to provide the requisite amount of hot water. In this case you should have a detailed discussion with your engineer about the best route forward.


As there are pros and cons regarding combination boilers, it is important to explore every avenue with your engineer to make sure that your needs will be fulfilled by installing a combi boiler.

Frequently asked questions

Regarding Combi Boilers

Mains water is heated directly inside a combi boiler, because of this the water is not exposed to the atmosphere until someone turns on a tap. During heating calcium bi-carbonate that is present in water is changed to calcium carbonate. This in turn leads to the creation of carbon dioxide. When you view the water very closely it will look like millions of tiny bubbles. The bubbles are not to be confused with sediment as it is only cosmetic in its appearance. Should you fill a glass of water and let it stand, as it cools the water will clear. In areas of hard water and in systems of low flow rates it is more prevalent.

Combi boilers can be fitted in a wide variety of locations throughout your home or business premises. Obviously there are laws and regulations that have to be followed regarding placement of a combi boiler. A visit with your engineer will clarify the best locations to install your new combination boiler.

It is possible to replace a system boiler with a combi boiler in most situations. There are a lot of people who decide to replace the system boiler with a combination boiler for several reasons. Some like the extra space that a combi boiler creates. Others like the environmental aspect of saving fossil fuels because you only have to heat the water you actually require as opposed to heating a tank of water twice a day that you may never use. In general the cost of replacement is a little higher than like for like. Talk to an engineer who can walk you through not only the benefits but also the pit falls so you have a clear picture in mind which way you should go.

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