Why is the hot water produced by my combi boiler sometimes cloudy?

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 causes the creation of Carbon Dioxide which close up is seen as millions of bubbles, this is not sediment it's merely cosmetic. If you run some hot water into a glass and let it cool it will slowly clear. This phenomenon is more apparent in hard water areas and from models with perhaps slower and/or lower flow rates.