Combi boiler vs system boiler – Which is best?

system boilers vc combi boilers which is best?

Is a combi boiler better than a system boiler? Find out in this full comparison guide

Deciding which boiler to go for is a big decision and one which you’ll be living with for at least the next fifteen years, so you’ll want to get it right! 

The main differenece between combi boilers and system boilers is that a combi boiler doesn’t require a hot water cylinder. This means that they are great for when space is at a premium or there isn’t a high demand for hot water. On the other hand, if you have a large home, maybe a system boiler would be better for you.

So, in this article, we’re going to look at the difference between these two types of boilers along with their pros and cons to help you come to the right choice for your home.

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What is the difference between a system and combi boilers?

The best system boilers take cold water from the mains supply where it’s then heated by the heat exchanger which transfers the energy from the gas to the water. The hot water is then moved to a storage cylinder where it’s kept hot. The heating system is within the system, and that’s where this type of boiler gets its name from.  

When you need the hot water, it’s then pulled from the tank. A system boiler does need a cylinder to store the water which has been heated, but it doesn’t need a cold-water tank as required by conventional boilers.

Combi boiler vs system boiler

A combi boiler is a highly efficient single unit that provides hot water for the water supply and the heating. The combi takes water from the mains and heats it on demand whenever the tap or heating is turned on. That means that there’s no need for a storage cylinder.

The combination boiler gets its name because there’s the combination of two tasks, heating and hot water, all undertaken within the one boiler.

Because there’s no need to pre-heat the water and then store it, the combi boiler is very cost-effective.

Pros & cons of a system boiler

So, let’s have a look at some of the pros and cons of having a system boiler in your home

System boilers pros

  • If you need hot water from several outlets at the same time, then a system boiler can cope with that level of demand as the water is already heated and ready to be used.
  • The system boiler takes up less space than a traditional boiler because it takes water directly from the water supply and so doesn’t need a cold-water tank.
  • If you’re looking for hot water as soon as you turn the tap, then a system boiler can pretty much do that. That’s because a pump is built into the system, allowing the super-fast delivery of the already heated hot water.
  • System boilers are compatible with solar panels that can generate power to heat the water in the hot water tank.

System boiler cons

  • If you suddenly need a lot of hot water, then you may use all the stored water and then have to wait until it heats up more.
  • Because the water needs to heat up in advance, you’ll need to programme a timer for when you need the water. If your plans change, you may find yourself without a hot water supply.
  • If you don’t have much space at home, then you may struggle to find room for the hot water tank.
  • Once the hot water enters the cylinder, the temperature will begin to drop. This means that it’s essential to ensure that the cylinder is well insulated to prevent too much loss of heat, which then affects the boiler’s efficiency.
  • There is more to go wrong with a system boiler so that may mean higher maintenance costs compared to a combi boiler. Though you can reduce these costs by taking out a boiler cover plan.
  • A system boiler is unlikely to be suitable for a home where there is a water supply with low pressure, or an older radiator system fitted that may not cope with the higher pressure that comes with a system boiler.  


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Pros & cons of a combi boiler

Now let’s take a look at the pros and cons of installing a combi boiler.

Combi boiler pros

  • Combi boilers are the newest type of boiler and with that comes improved efficiency. In fact, there’s a requirement for all new combi installations to have an efficiency rating of at least 92%. The great news about that is that your energy bills will be lower, and you’ll reduce your carbon footprint.
  • If you need hot water on demand, then the combi boiler is a great fit. While you need to wait for the system boiler to heat and then store the water, the combi heats the water once you turn on the hot water tap.
  • Easy to install & with no need to fit hot water cylinders, the boiler installation is much quicker with a combi boiler saving time and money.
  • With just a single unit to find room for, combination boilers are a great choice for when space is tight.
  • As the water is coming from the mains, you’ll have high-pressure hot water, though that does depend on the water pressure coming into your home to begin with.

Combi boilers cons

  • A combi will struggle to cope with a high demand for hot water. So, if you have the washing machine on while someone has a shower, then the water may suddenly go cold for one of those outputs! If you have more than one bathroom, then a combi will probably not be able to meet your hot water needs.
  • A little patience is needed because once you turn on the hot water tap, the supply needs to be heated and then it’ll be sent to the tap. Now it is only a matter of a few seconds, but it’s not the speed of delivery that you’ll get from a system boiler.
  • A combi boiler needs water to come from the mains at a high pressure. If the pressure is low, then the water coming from the taps will also be at a low pressure
  • If your home has older plumbing, it may not be able to cope with the high-pressure water flowing from a combi boiler, so check if your new boiler will require new pipes.

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So is a combi boiler better than a system boiler?

So, as you can see, we can’t really say that either the system or the combi boiler is better overall. If we compare a system boiler vs combi boiler running costs, then it is likely that a combi boiler will be more efficient and so offer some savings.

However, if a combi isn’t able to provide the volume of hot water you need, then getting a combi boiler is pointless as it will not be able to heat your property.

The consumer organisation Which? says that it’s getting the wrong boiler which leads to unnecessarily high fuel bills. There isn’t much difference in the new boiler cost either.

What we can say with confidence is that one or the other will be a better option for your home. So that means:

A single person living in a flat will benefit from the space-saving combi boiler, which will be able to cope perfectly with one person’s hot water needs.

A three-bedroom home with one bathroom may still be a suitable contender for a combi boiler though this depends on whether there are times in the day when hot water is needed in several locations at the same time. In that situation, a system boiler will be a better option.

A house with more than three bedrooms and multiple bathrooms will need the pre-heated supply to cope with the demand for hot water.

Combi boiler vs system boiler


Deciding on the right replacement boiler for your home isn’t always an easy task but getting it wrong can mean that you’re not able to get the hot water and heating needed or that you pay more than you have to.

We’ve solved this problem for you by providing you with personalised recommendations & boiler deals through our fixed price quote service.

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Simply answer a few questions about your requirements, and we’ll provide you with a list of recommended boilers that will be a perfect match.

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