Overflowing central heating vent pipe

[Builder2011]

Hello and thank you for reading.

Almost sure I know the reason but checking for all possibilities. Any ideas appreciated.

Gas central heating system unused for a year. Vented system. Seven radiators. Cold water tank in loft. Header tank in first floor bathroom.

Two radiators disconnected and another with a stuck valve so closed.

Boiler serviced and system switched on. Boiler set to level 1 of 6.

Hot water, quite a lot, constantly flowing from vent pipe into header tank as soon as boiler switched on.

When boiler thermostat (level 1) switches boiler off, hot water still flowing through vent pipe into header tank.

Any ideas as to cause appreciated.

Thank you for reading.

 

[PrenticeBoyofDerry]

If water is discharging through the F&E vent pipe, I would expect a thermostatic failure.

 

[denso13]

Possible pipework blockage forcing water up the vent.

 

[Builder2011]

Thank you PrenticeBoyofDerry.

Thank you denso13.

The radiators heat up well. The hot water heats up fine.

 

[Builder2011]

The system has been flushed out after running through with X800 equivalent.

Vent still overflowing a little, probably the same as before.

Two radiators disconnected, other five connected.

Thank you for reading.