On 18/11/2007 16:13, John wrote:

I must ask why you think this,

I /know/ it because I have recently had to drain my hot water cylinder
to fit a side flange for a shower.

If the main stoptap into the house is turned
off as the OP stated and a hot tap is opened then the cylinder would
eventually empty (apart from the dregs in the bottom), unless my
understanding of gravity is wrong!

Given that the hot cylinder fills at the bottom, and empties from the
top I suggest you try emptying one ... if it hasn't been fitted with a
draincock near the inlet you'll need a siphon tube.