In an earlier contribution to this discussion,
wrote:

You obviously know your stuff. Thanks a million. It has indeed purged
itself and we now have hot water once more. Thanks for the last tip
too; I'll keep it in mind for future mishaps.

May I pick your brain a little more? Because of the awkward situation
of F&E tank I cannot access it in such a way as to be able to reach
the base to check depth. Is there a way to monitor this from outside?
I'm thinking of something like 2 'U' tubes fastened together in such
a way that one is inverted over the rim of the tank, reaching almost
to the bottom, connected to the other which extends below the tank
base and rises again to the rim. If I then syphon some water from
the tank it should find its own level and provide a water gauge. Does
that sound feasible?

If I'm pushing my luck with that question, please ignore it. I'll just
have to experiment.

Tony.

It doesn't actually need much water in it - as long as there's enough to
cover the outlet which feeds the heating system. [The level rises, of
course, when the system is hot - so there needs to be plenty of room for
expansion before it gets anywhere near the overflow].

I can't see into my tank very easily. I usually check that it's healthy by
reaching over the top and pushing the ball down. If water flows in when I
push the ball, but immediately stops when I let go of it, that shows that
everything is ok. If it carries on running, it suggests that the level was
too low - and that the ball valve was stuck.

If you really want a visual check, could you mount a mirror somewhere where
it would enable you to see into the tank? If you want a sight tube, all you
really need is a hole in the side of the tank near the bottom, and a
right-angled tank connecter in it - with a length of rigid transparent tube
extending up from the connector. Or you might be able to rig up an
electrical alarm of some sort - using a float connected by a piece of string
to a micro-switch - in such as way that if the level dropped too much, the
string would operate the switch.
--
Cheers,
Set Square
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