if you have a combi there is only one way water can get cross contaminated and by your post you do not have that problem you would be reporting something else...it is simply carbon dioxide in the water from when the boiler heats the water ...more common in hard water areas
corgi bloke, July 2008
you have a valve which you open to top the boiler up, at other end of filling loop fit a non return valve,you now have to open the valve every time you top up with water
