On Tue, 02 Dec 2008 17:13:55 +0000, 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?
Don't forget

d) The manadatory backup over pressure device /over temperature device.



--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html