This type supplies instantaneous hot water.
Combi’s do not work with storage tanks.
All the main system components are found within the case.
80% of boilers installed in the UK are combi’s


  • Lower running costs – no wasted hot water.
  • Space saving – no need for other tanks
  • Cheaper to maintain – service and maintenance costs are reduced.
  • Hot water on demand – supplies instantaneous hot water. 
  • Competitively priced as all system components are housed inside the boiler case
  • Available on an extended payment plan


  • Potential low hot water flow rate – slow fill up times
  • Reliant on one source of hot water – no immersion heater back up
  • Serves the nearest draw off first – can lead to poor flow.

Notice the lack of storage tanks in the diagram. 
Smart controls and / or other measures are required to ensure conformity with the Boiler Plus Regulations 2018.

Worcester Combi Boiler
Combi boiler diagram
Ideal System Boiler
System boiler diagram


System boilers rely on other storage tanks to work.
They incorporate many of the same components as Combi’s
These are generally situated elsewhere in the central heating system.
The heating pump and expansion vessel  are incorporated within the boiler case. 
System types often work with unvented cylinders


  • Better for properties with a high hot water demand.
  • The majority of system components are inside the boiler case.
  • This type are a benefit in flats.
  • Available on an extended payment plan


  • This type relies on good incoming water flow rates.
  • More space is required to site your boiler and cylinder.

The expansion vessel manages increasing system pressures.
The user is required to keep the system topped up
This is similar to how a combi works.


Regular or ‘heat only’ types work with a storage cylinder
They provide heating and hot water.
A traditional British system found in many house types.
Often replaced by combi boilers.
Suitable for like for like swap.
Similar storage set up to a Fire & Back Boiler Unit (BBU).
If you have a BBU you will need to change to another boiler type.


  • Less reliant on suitable incoming water flow rates.
  • Less expensive to buy than other types of boiler.
  • Likely to work better on older ‘open-vented’ system.
  • Available on an extended payment plan


  • Less components covered by the manufacturer’s warranty.
  • Slower heat up than sealed system variants.
  • More chance for system corrosion.

Note the difference between the combi set up and this regular schematic.
There are many benefits to sealing your heating system.
We recommend sealing the system where possible.

Ideal Logic Heat Only
Regular boiler diagram

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