John Stumbles wrote:


Any thoughts?

1) Big house, any secondary circulating pumps hiding anywhere? However,
that wouldn't really explain the "......overflow runs intermittently:
for a few minutes at a time then stops again. "

2) Cold water main connected to a mixing valve somewhere? That wouldn't
explain the "......overflow runs intermittently: for a few minutes at a
time then stops again " either.

3) (I like this one, 'cos it's so daft it's plausible). There is a
slight inverted 'U' in the open vent pipe. Air collects in the
inverted 'U' and pushes the water level down as it accumulates or
expands. The water is displaced up the open vent and up the cold feed.
The water levels in the tank and the overflow aren't the same because
of the air-lock. When the air reaches the bottom of the 'U', some air
escapes up the open vent and water levels drop. Process then repeats.
There may be some momentary water hammer evident when you open a tap as
air escapes.

That does explain the "......overflow runs intermittently: for a few
minutes at a time then stops again " bit, but I'm at a loss to explain
where the air is coming from. I can only think there's some big-time
electrolysis happening with a defective immersion heater. Old consumer
unit (no RCCBs)?