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Choosing a New Boiler

Posted by SDR Admin 203
Last updated 13th July 2022
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    • Combi Boilers

      How they work: Combi’s work as sealed systems, providing hot water for both the taps and central heating system, heating the water directly from the mains as and when it is needed — meaning there is no need for a hot water storage cylinder, or a cistern in the roof space.


      • Compact
      • Quick to Install
      • Provide heat for your radiators and hot running water on demand
      • No water storage cylinder required
      • Less space used up
      • Ideal for smaller properties


      • It’s a priority system, so it only satisfactorily deals with one heating need at a time
      • While fine for small families with one bathroom, larger families will experience poor flow rates when multiple outlets are used at once.
      • Performance is also dependant on the diameter of the pipe entering the property: if it’s less than 22mm, then a combi is a bad choice.
    • System Boilers

      How they work: System boilers are fitted to sealed heating systems, but unlike combi’s work on the principle of storing hot water in a cylinder, so they can feed several outlets at once at mains pressure. There’s no need for a cistern in the loft and the expansion vessel is built in.


      • Ideal for larger homes
      • Major components built in (i.e. expansion vessel and pump)
      • Installation is quicker, cheaper and neater
      • Flow rates are usually high as water is delivered at mains pressure
      • Hot water is instantaneous.


      • Will run out of hot water if overused
      • Some installers claim they are more complex
      • Prone to problems more than regular boilers (such as pressure loss)
    • Conventional Boilers

      How they work: Conventional boilers are now largely bought as replacements for homes with an open-vented system (i.e. supplied by means of a feed and expansion cistern in the roof space, which is open to the air). Like system boilers, they work on the principle of stored water and require a separate hot water cylinder.


      • The water out of the taps will be at a good flow rate (not to be confused with pressure)
      • Hot water can be supplied instantaneously
      • This is the ideal setup for a ‘power’ shower, which requires a cold water feed from the cistern and a separate electric pump.


      • They’re more expensive to install
      • Needs more components and pipework
      • Takes up more space
      • They can suffer from low pressure if the cistern is not located high enough, meaning additional shower boosters may be required
      • Hot water can run out

      Note: Sealed or Open? In a sealed circuit, the system is filled to approx 1 bar pressure and then sealed. Unlike an open-vented system, there is no cistern, so an expansion vessel handles excess water. Sealed systems are more efficient.

    • SEDBUK

      SEDBUK is no longer used as a rating because it was updated to meet a new EU Law. We still recommend checking your older boiler to see if you could be saving money on your heating. It may be time to upgrade!

    • ERP

      Very similar to SEDBUK, but more comprehensive as a guide. Take a look below.