hot water tank in loft overflowing

[sahd]

It is still overflowing, a tap in the handbasin in the wet room was isolated and seemed to cure the problem. A non-return valve (might be wrong word) was fitted but the problem is still there after re-installing the tap into the system, my plumber is completely bewildered and suggests it might be the washing machine and that I should contact a washing machine specialist, any thoughts on this? Please I need HELP

[gucciman]

have you been up into the loft to observe the tank and see exactly whats happening? is the tank filling up from the ball valve mains water inlet? is it filling up from the open vent?
how long is it overflowing for?
is it only overflowing when the boiler is heating the hot water?

[sahd]

the water is coming from the open vent - not the ball valve.
It continues to overflow "till hot water is run off - which lowers the level again. It will then slowly (about 6 hours if the water level is taken back to the correct level in the tank) fill and overflow again. It is a steady heavy trickle from the overflow rather than pouring out.

[gucciman]

presuming that yours is a fully pumped system and not gravity hot water i would be suspecting that your system could be sucking in air, the air takes up volume within the pipework thus forcing the water up the open vent.
this is only a guess as ive only been a trainee heating engineer for 2 years and no-one fits the old style systems anymore and i'm a gas engineer rather than a plumber.
from what ive learned so far the main reason for sucking in air is something to do with the relationship between the pump and the open-vent.
the only other reason i know of for the open vent discharging water is poor circulation of water thru the boiler so the water is over heating and thus expanding up the open vent, this could be due to a faulty pump, thermostat or blocked heat exchanger.
perhaps a non-return valve could be fitted to the washing machine supply pipework too.
theres quite a few guys on here who know a lot about plumbing and should give you a better answer than me, gasman will probably have a good answer for you too.