On 14 Feb, 22:18, Peter Parry wrote:

Air. *The water in the reservoir is cold and has a lot of dissolved
air. * In the pipe it is under pressure so the air remains dissolved.
When it comes out of the tap the pressure immediately falls and the
air comes out of solution and turns back into gas bubbles - exactly
the same as opening a bottle of fizzy pop.

So, sparkling water at no extra cost?

Cheers
Richard