Header tank overflowing

[gasman24]

Wondering if anyone can suggest why and offer a solution......
I have been working on a baxi back boiler, gravity hot water, pumped central heating.
Everytime you turn the c.heating on the tank is filling up from the cold feed connection raising the water level in the F&E tank and then pushing it over the overflow like an open tap.
Once the system has heated up it turns to just a drip.
I have held a glass of water over the open vent on fire up and it has taken the lot which proves its sucking air down the vent.
Would anyone be able to offer a suggestion as to why its happening????

 

[keego]

Has the pump been changed recently, If it has then it might be on the wrong way, or wired up wrong so it's pumping backwards.

 

[gasman24]

nope i changed the pump myself 2 years ago its pumping back into the boiler (return pipe)
thanks for that though
any more suggestions??? this ones really giving me a headache!!!!

 

[ollski]

Surely if it's sucking air down the vent and pumping up through the feed that would mean that pump is between the feed and vent in which case wouldn't it be possible to tee the feed into the 22mm vent pipe?

 

[gasman24]

Sorry I don`t understand what u are trying to say???
The system is:
gravity hot water
2 x 28mm pipes
one is flow to the cylinder with vent pipe to F&E tank
one is return to the boiler with cold feed from F&E tank.

pumped heating
2 x pipes for heating
22mm flow with double check valve
22mm return with grundfoss 15-50 pump