You're only left with a pressure head height from the
last bend in the pipe down to the bath tap. I think this could be your
problem.

Agreed

You say that the hot water vent pipe (overflow you're calling it) is also
on
this part of the pipework, and that it drops its water level when the bath
tap is turned on. I'm not surprised by this because the head of water
above
the bath tap will be the height of the water in the vent pipe, which
should
become level with the water in the cold water storage tank. The pipework
then does this dance all over the place and will take ages to fill up
again,
especially if the bath tap is still turned on.


This was put in 30 years ago and it was a solution based on use of space and
you can't go wrong with a solid fuel back boiler if the electricity goes
off. It does here in the winter quite often. It was not based on the
dynamics of a gravity system.

Is there room in the loft for a hot water cylinder? Even if it lies on
its
side? Lifting the hot tank in to the loft and dropping all the pipework
down from there will cure all your ills in this plumbing system.

Max size of loft opening is 400mm and the useable height to the highest
point is less then 1.5 metres. Friend still wants to use back boiler in wood
stove. So need say 1 metre head above hot tank for it to work properly so
little chance of that!

Further she has to run off boiling water quite often as the water cannot be
used for a bath or shower, it is a one person household so no real use of
water as the bath taps do not work and she is a shower type person so uses
an electric shower!!!

My suggestion is to move the cold water storage tank above the bathroom and
the hot water cylinder to an airing cupboard next to the bath room. This
means no return trip to the loft for the hot water and to take the boiler
out the stove and replace it with a fire brick. I was rebuffed as it would
be loosing the advantage of the back boiler!!!!!!!!!!

So that was Thursday, Friday and Sunday lost but who wants the time off!

Eddie