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Aidan
 
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skilso wrote:
Recently a plumber


A sudden thought about this. Is there an open vent pipe from the top of
the cylinder, which terminates over the cold water storage tank?

It might be that sometimes part of the heater and the thermostat are
not immersed in the water. This could explain the melting and why the
cylinder is cooler lower down. It shouldn't be possible for the heater
to become un-immersed, but there are some lunatic installations around.

If there's no open vent, the water level may drop as air accumulates
at the top of the cylinder.

You should be able to find a diagram of the correct cylinder
installation on the web or in a DIY book, to compare with your set-up.