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

Question

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

Accepted Answer

The water from oil 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. During heating, the calcium bi-carbonate present in the water changes to calcium carbonate, which then is seen as millions of bubbles. If you let the hot water cool down in a glass for example, you will see it slowly clear.