The water from combi boilers is heated directly from the mains - so the only time the water is released to the atmosphere is when it comes out of the tap. This means that during heating the calcium bi-carbonate, present in the water, changes to calcium carbonate. This creates carbon dioxide which close up is seen as millions of bubbles. This is purely an aesthetic affect and does not affect the quality or taste of the water. If you run some hot water into a glass and let it cool it will slowly clear. This phenomenon is more commonly seen in hard water areas and from models with somewhat slower and/or lower flow rates.