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Condensing boilers attempts to recover the 20% heat loss in conventional boilers by deliberately inducing condensation. The conventional boilers will see the flue gas release reach 180 degrees, and the condensing boiler will use this heat to heat more water through a secondary heat exchanger, taking the flue gas heat down to 55 degrees. This is achieved by adding a second heat exchanger (usually stainless steel or aluminium) on the flue. This second heat exchanger makes the condensing boilers bigger and more expensive than conventional boilers (typically double the cost), although efficiency rates can be as high as 90%, meaning that the extra capital outlay is usually recovered within 5 to 7 years. In other respects the condensing boiler works in the same way as a conventional boiler. The condensing boilers also need a drain-away to be installed on the flue to remove the condensation and often these soak-aways are external pipes. These pipes are susceptible to freezing in harsh winters (which is especially the case this winter).


The advantages of this system are that:

  • It provides mains pressure hot water
  • It is more efficient and cheaper to run
  • Can be used with renewable systems

The disadvantages are that:

  • They cost more
  • Are a pressurised system that require professional installation
  • Have high annual maintenance costs
  • Have external pipe-work prone to freezing
  • They take a long time to re-heat the water tank

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