3 Worcester Bosch combi boilers showing the case and inside ready for install

Combination (Combi Boiler)

A combi / combination boiler is a single, compact unit that’s designed to provide both central heating and hot-water. Ideal for saving space, this boiler is designed to heat water immediately, rather than heating and storing. There is no need for a bulky hot water storage cylinder or cold-water storage tank, as usually found with Heat Only systems and System Boilers.

A Worcester Si compact combi boiler installed in a kitchen cupboard with no pipework on show

Advantages of a Combi Boiler

  • Space Saving - A Combi Boiler can be fitted into a standard-sized kitchen cupboard
  • No Extra Water - No large water storage cylinders or tanks meaning less risk of water damage in your home
  • No Frozen Pipes - As there are fewer pieces to the boiler, you have a much lower risk of your pipes freezing and being left without hot water
  • Save Time and Money - A combi boiler system is typically quicker and easier to install than standard boilers.

Close up combi boiler system face plate showing controls on a Worcester Bosch

Possible Disadvantages of a Combi System

  • Possible Pressure Drop - If you have a very large, busy household with multiple bathrooms, you could lose water pressure
  • One Unit - All of the heating takes place within one central unit, meaning when the hot water is turned on, the central heating will be paused

How water passes through the system within a domestic property. Showing water running from a pressurised cylinder to a shower, hand basin and separate circuit for radiators.

System Boiler

With a System boiler, the majority of the components required for generating heating and hot water are built into the boiler. 

The only external part required is a cylinder that stores hot water. Water is supplied directly from the mains.

A System boiler eliminate the need for a tank in the loft, that would otherwise be needed with a Regular Heat Only boiler. 

A hot white standalone bath with hot steamy water from the water fall taps

Advantages of a System Boiler

  • Great for high hot water demand - A system boiler uses a large tank of stored water meaning you can use multiple hot water sources simultaneously without a reduction in pressure. This makes it ideal for busy family homes and homes with more than one bathroom.
  • Stronger Water Pressure - As the water supply is delivered directly from the mains, the water coming out of your taps will be at a higher pressure than Regular Heat Only boilers, which rely on gravity to send water down the pipes.
  • No need for a cold water feed tank - Save space in your loft as System boilers don’t require a cold water tank as they can be used with pressurised cylinders.

A pre-plumbed pressured cylinder in an airing cupboard with radiator with lock shields

Disadvantages of System Boiler

  • Takes up room - System boilers don’t need a cold water tank they still take up more room than a combi boiler. This is because they require a hot water cylinder.
  • Hot water is not unlimited - The amount of hot water you have is limited to the size of cylinder you have installed. When this runs out you will have to wait for the cylinder to recover and heat the new incoming water.
  • Heat Loss - A hot water tank can lose heat, causing it to be inefficient especially if the tank is not insulated correctly.

A diagram of how a heat only boiler / regular boiler works as part of a domestic system

Heat Only / Regular / Conventional


Conventional or Heat Only boilers tend to be found in older properties but they can still be a great boiler choice for modern homes that have a high demand for hot water or a large number of bathrooms. As these systems require a hot water cylinder and cold water tank they may not be suitable for smaller properties. It’s important to keep in mind that with a conventional boiler you will not be able to get instant hot water. Instead you will need to program your thermostat to heat the water in advance ready for use later on.

A new chrome shower head with water

Advantages Of A Heat Only Boiler

  • Maintain Pressure - They still work well even if your mains pressure is low and you will still be able to get hot water even if someone is running the shower and the kitchen taps at the same time
  • Simple Design - They’re easy to maintain as they have a simple design

An energy efficient chart showing the current energy rate at 50 with the potential of the home being 80 resulting in a bill reduction and money saved.

Disadvantages Of A Heat Only Boiler

  • Take up space - Cold water storage tanks are generally situated in the loft. They can be used to supply the hot and cold water taps on the bath. They can also act as a Feed and Expansion tank (F+E) for the heating system
  • Not as efficient - Conventional boilers aren’t as efficient as combi boilers – there will be heat loss as hot water is stored, usually in a copper cylinder
  • All gone - Your hot water can completely run out and will take time to reheat