fred wrote:
In article , Tim Downie
writes
John Rumm wrote:

I think there is another misconception here as well. The only time a
pressurised water cylinder is at explosion risk is going to be when
it has the possibility of being heated to over the boiling point of
the water content. Then there is the risk of the explosive
production of steam should it rupture.
For a cylinder heated via a water flow from a boiler, the chances of
super heating it seem minimal.

You're forgeting Sods law. The only time the safety valve is likely
to get stressed is when your CH boiler packs up on Christmas Eve and
you turn on the immersion element that's not been used in years. A
sticky stat in that and a sticking safety valve can then lead to an
explosive situation. A vented cylinder needs another level of
system failure before it can blow.
So, just to summarise, for an unvented cylinder to be at risk of over
pressure failure following oveheating:

a) The immerser must be in use and the stat fail
and
b) The manditory overheat stat must fail
and
c) The manditory over pressure relief valve must fail

Still scared?

Not terribly, no. Never was. But, all those things can fail silently and
the first you might know of their failure is when your cylinder explodes.
Of course it's rare and the Americans and continentals have been using
unvented electrically heated cylinders for years and we don't hear of
massive numbers of incidents.

It just cannont be denied that an open vented cylinder is intrisically less
dangerous than a pressurised one. If I had one or lived next door to one
I'd sleep easier knowing that somebody competant who understood the dangers
had periodically checked the safety relief valve.

Tim