Water that is heated through a combi boiler is heated directly from the mains water supply so it’s the first time coming into atmospheric pressure. This means that during heating, the calcium bi-carbonate present in the water temperature increases and changes to calcium carbonate.
This enables the creation of Carbon Dioxide which close up is seen as millions of bubbles, this is not dirty water or sediment as it looks , it's merely cosmetic.
If you do a test and fill a clear glass with some hot water and let it cool it will slowly clear. Like magic, this phenomenon is more apparent in hard water areas and from boiler models with perhaps slower and/or lower flow rates.
Being a heating engineer in York I come across this often due to the hard water. To overcome this I usually install a calcium reducer which stops this from happening.
Thanks for reading
Steve - The Boiler Doctor
