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What’s the difference between combi, system and regular boilers?

Choosing the right kind of boiler to suit your needs and lifestyle can be confusing. This article helps to break down combi, system and regular boilers to explain the main system types, how they work, their pros and cons, and what type of home they are most suited to.

combi system regular boilers explained


Combi boilers

A combi boiler is both a water heater and central heating boiler combined in a single compact unit.

Combi boilers account for more than half of the sales of domestic boilers in the UK as they are incredibly cost-effective and energy efficient, which helps to lower energy bills.

Unlike other boilers, a combi boiler heats water directly from the mains when a hot water tap is turned on. This means an external hot water cylinder or cold water storage tank is not required, freeing up space in your home.

Using a combi boiler also means that there’s no need for you to deal with noisy pumps as you are not constantly heating stored water.

Combi boilers are ideally suited to small homes with one bathroom and flats as they can only produce a certain amount of water at a time, which could be a problem in a larger home with multiple bathrooms where there is likely to be an overlap with using the hot water. They struggle to generate large amounts of hot water at the same time and this can often lead to a drop in water pressure.


  • Compact in size making it an ideal choice for smaller properties
  • A good solution when there is little to no loft space
  • Instant access to hot water – no waiting for the tank to heat
  • No cold or hot water tank required freeing up living and loft space
  • Hot water delivered at mains pressure meaning you could get a more powerful shower


  • Not ideal for larger households or homes with several bathrooms (see system boilers)
  • Water pressure can be affected when multiple outlets are used at once
  • No access to stored hot water if an issue with the boiler arises
  • Often a short delay in heating up
  • Takes longer to fill baths

Combi boilers are available in both Gas and Oil.


System boilers

System boilers require a hot water cylinder for storing heated water, but don’t rely on water supplied by a cold-water storage tank in the loft like regular boilers do.

Much like combi boilers, system boilers are ideal in homes with little to no loft space or those with loft conversions, but they are more suited to larger homes and commercial properties because they can supply hot water to more than one tap at the same time.

All major heating and hot water system components are built into the boiler itself, making system boilers relatively fast and easy to install.

System boilers are also compatible with solar water heating systems, which deliver environmental benefits as well as lower energy bills.


  • Ideal for homes with more than one bathroom
  • Constant supply of hot water to any number of taps at the same time
  • No need for a loft tank
  • Economical to run
  • Compatible with solar panels
  • Built in components means installation often faster and looks neater


  • Less efficient than combi boilers as there is unavoidable heat loss as hot water is stored
  • Requires more space than a combi boiler
  • Only heats water in advance
  • Hot water supply can run out and takes time to reheat

System boilers are available in both Gas and Oil.


Regular boilers

Regular boilers, often known as traditional, conventional or heat only boilers, are typically found in older properties and are ideally suited to homes that have a traditional heating and hot water system.

Regular boilers differ from system boilers because they also rely on a cold water storage tank to feed the hot water cylinder as well as a tank to maintain the water level of the central heating system (both of which are commonly found in the loft).

If a property has an older radiator system, it might not be able to cope with the higher water pressure delivered by system and combi boilers, so it is often best to replace an existing boiler with a regular boiler.

Like System boilers, regular boilers are also compatible with solar panel water heating systems which generate lower carbon footprint and lower energy costs.


  • Access to hot water from multiple taps at a time
  • Ideal for larger homes with two or more bathrooms
  • Good option in areas where water pressure is low
  • Compatible with solar water heating systems


  • Operates at a slightly lower pressure than combi and system boilers
  • Space is required for both cold and hot water storage tanks required
  • Less efficient than combi boilers as there is unavoidable heat loss as hot water is stored
  • Hot water isn’t heated on demand so can run out and takes time to reheat

Regular boilers are available in both Gas and Oil.


For more information or advice about the type of boiler most suitable for you, please feel free to contact us >

Find our more about new boiler installations >


Reference: https://www.worcester-bosch.co.uk/products/boilers/explained

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SGS Boiler Installations, Heating and Electrical Services Salisbury
Call:01722 331066