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Combi Boiler vs System Boiler

The decision on whether a combi boiler or system boiler is right for your home will likely be based mostly on your demand for hot water. If your household needs a lot of hot water, if you’re in a large property with multiple bathrooms for instance, a system boiler will be better suited to you. With one bathroom and less demand for hot water, a combi boiler is generally likely to be the one for you.

The reason for this is based on the fact that system boilers store hot water in a cylinder, so it can meet the increased demand for hot water. On the other hand, combi boilers heat water on demand and send it directly to the hot water outlet when you need it. Both types of boilers have advantages, but it is likely that one will be a better fit for your home than the other.

Let’s do a comparison of both boiler types so you can determine which one is right for you.


Combi Boiler vs System Boiler
Generic modern boiler in house interior.

What is a combi boiler?

Combi boilers are the most modern type of boiler that take up the least amount of space within the home. As the name suggests this boiler combines your hot water and central heating, with everything powered by the single unit itself. Combi boilers heat water straight from the main supply as and when you require it, so you will have hot water delivered to you instantly at a strong pressure. This style of boiler does not need a cylinder or any other parts as it doesn’t store water in tanks like other types of boilers.

Why should you get a combi boiler?

There are many benefits you can enjoy when you opt for a combi boiler in your home in comparison to other boiler systems, from lowering your carbon footprint to saving space around the house and more. We will explore these benefits further as well as other key advantages of combi boilers below.

Highly efficient

Combi boilers are designed to achieve an efficiency rating of at least 92%, which helps to lower both your carbon footprint and heating bills.

Small and compact

As you only require the boiler unit itself (no tanks or cylinders), they don’t take up a great deal of space. They are able to comfortably fit within a kitchen or airing cupboard, making them easier to hide and maximising your storage space.

Quick and easy installation

As there is no need for the installer to fit any tanks or cylinders, the installation process will be a lot faster and easier compared to system or conventional boilers which will save you time and money.

Instant hot water

The cylinder that comes part of a system boiler requires a high level of energy and can sometimes leave you waiting for it to refill before you are able to access hot water. Whereas combi boilers take their water supply directly from the mains, so you will have hot water on demand whenever you need it.

Mains water pressure

Combi boilers heat water from the mains so there should be a strong flow of water from your taps. However, this will depend on how strong the mains water supply is in your home.

What is a system boiler?

All system boilers are fitted with an unvented hot water cylinder, the cylinder is where the hot water for your home is stored until you need it. By storing water in this way, system boilers can meet higher demands in houses that have two bathrooms or more. The drawback to this, however, is that as soon as the water in the cylinder has been used up, you will have to wait for it to fill up again before you could use a lot of water, if you wanted to take a bath for example.

Why should you get a system boiler?

System boilers also come with several benefits you can enjoy within your home that we will break down for you below.

Simpler to install than standard boilers

Standard boilers are the oldest type of boiler on the market. Unlike system and combi boilers, they are not connected to the mains so require water tanks in the loft as well as needing quite extensive pipework too. Due to system boilers not needing as many external parts as conventional boilers, they won’t take as much time and will be easier to install, which will help save you time and money.

Fast response to hot water demand

With the addition of a pump, system boilers can quickly respond to heating and any demand for hot water.

Doesn’t need a cold water tank

There is no need for a cold water or feed expansion tank in your loft (this is needed when fitting a conventional boiler), which means system boilers take up a lot less space.

Solar thermal compatibility

If you have solar thermal in your home, or are thinking about having it, then the hot water tank or cylinder of a system boiler can be adapted to heat the water using solar energy, making for an easy transition to renewable energy.

What to consider when looking at combi and system boilers

Before you take the plunge on a combi or system boiler, there are a few things you should factor in and think about.

Combi boiler drawbacks

Can’t meet high demand for water

Heating water on demand is really useful until you want to use two different showers at the same time. The water pressure will then drop, and you might regret not going for a system boiler with a cylinder that can store water.

Unsuitable for properties with weak mains pressure

You should make sure your home has strong mains pressure before you decide to get a combi boiler. This is because if it is weak or inconsistent the water coming out of your taps will be the same.

Might not be compatible with old pipes

Older heating systems were designed for a supply of water that was significantly weaker than it is today and so your pipes may not be able to handle higher mains pressure.

System boiler drawbacks

Take up more space than a combi boiler

With a system boiler as well as the boiler unit itself, you will also need room to accommodate the cylinder.

Hot water cylinder insulation

If the cylinder is not well insulated, the water will start to cool down quickly. This means more energy will be required to heat it back up again which will push your heating bills up. So, be sure to consider insulation if you are going for a system boiler.

Size of the cylinder

How much hot water is available to your home before it needs refilling will be determined by the size of the tank. This should equal the demand of hot water in your home.

Hot water is not instantly supplied

When the tank runs out of water you will have to wait for it to fill up again before you are able to access more hot water.

Should you go for a combi boiler or system boiler?

The most important factor when choosing between these two boiler types is the hot water demands of your household. For smaller homes with less residents and one bathroom, a combi boiler makes the most sense for you. You will be supplied with hot water access instantly from a highly efficient boiler unit that should help you lower your carbon emissions and save you money on your energy bills.

As beneficial as combi boilers are for small homes, they are unable to provide a strong supply of water to more than one tap, bath, or shower at a time. With that in mind, if you have a bigger home with more residents and bathrooms then you should go for a system boiler.

Combi and system boiler installation

Prior to fitting a new boiler, the heating engineer should carry out a power flush. This will get rid of any debris, limescale, and sludge from the heating system to improve its efficiency.

Replacing a system boiler with a combi

Making the switch from a system boiler to a combi is a useful way to free up some extra space. Your installer will take out the cylinder and change the pipework so that the hot water can get directly to the taps. This job will take up to two days to complete.

Replacing a combi boiler with a system

If your combi boiler is not meeting your hot water demand then a system boiler is a good alternative. However, it could be that your combi boiler doesn’t have enough power so ensure it has the right output rating. Replacing the units is a fairly easy process, particularly if you are intending to have the system boiler fitted in the same place.

Swapping the combi boiler for a system one can usually be done in one day, but you will also need a cylinder and new pipework. It is important to check that there is enough space available for the hot water cylinder, as well as the hot and cold water piping, in a cupboard or loft space.


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