Why doesnt water evaporate from central heating?

[Harry Bloomfield]

It does - indeed if you could see inside your heat exchanger you'd see small bubbles forming in localised hot spots (like you do in a kettle or pan) where sufficient energy has enabled water molecules to break free from their liquid suspension and become gaseous.

The bubbles are localised boiling, the bubbles soon disappear as the rise up into the cooler water above. Cavitation on a propeller or impeller is another form of boiling, where the impeller manages to create a vacuum in the water. If cavitation is severe enough, it can seriously erode a propeller or impeller.