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Why isn't my brand new condensing boiler condensing?


Many customers buy brand new condensing boilers and expect to see instant reductions on their gas bills, but are shocked to learn that their lovely shiny brand new condensing boiler rarely condenses. It will only be condensing as the system is heating up and cooling down. This means that they are not getting the potential energy saving benefits of having a condensing boiler. 

But why exactly isn’t their boiler condensing?  

Water vapour is produced as a by-product of natural gas being burned. A condensing boiler works by extracting additional heat from the waste gases and condensing this water vapour back into water liquid and so recovering its latent heat through vaporisation. 

For a condensing boiler to condense, the return temperature back to the boiler has to be 55 degrees and below, as for natural gas this is the dew point of water. Many boilers and systems are installed and set up incorrectly meaning that the return temperature is coming back above 55 degrees and the boiler stops condensing. 

What causes a boiler to stop condensing? 

There are a number of reasons a boiler will stop condensing:

  • incorrect set flow temperature
  • incorrect pipe sizing
  • Incorrect pump sizing
  • Incorrect pump setting
  • Incorrect boiler sizing
  • Incorrect flow rates
  • Incorrect velocity
  • No hydronic separation when it’s needed
  • Incorrect temperature drop across the boiler (delta T)
  • Incorrect temperature drop across the radiators (delta T)

Systems that have cylinders and are set up in traditional Y-Plans and S-Plans rarely condense as the boiler has been set with higher flow temperatures to allow for quicker re heat times for the cylinder. The down side of this is when the boiler runs on heating mode the flow is still set higher and so the return will come back above 55 degrees. You can lower the flow temperature to ensure the return comes back below 55 degrees, but doing this means longer recovery times for your hot water cylinder. This is where priority hot water comes in when coupled with an Intergas boiler.  See our blog on priority hot water for more information on this. If you would like us to set up your system to achieve its full potential in energy saving and have your boiler condense more, please get in contact with us.

MD Plumbing and Heating are experts in central heating system design and installation. We will ensure the correct size boiler, pump, rads and pipework are used to achieve maximum efficiency out of your boiler and heating system, and save you money on your gas bills.