Header Tank

[Russb45]

When my boiler heats the hot water it is forcing water up to the header tank.

[raysparx]

if the water is getting very hot it may be that you have a faulty thermostat the header tank and expansion pipe are there to supply a feed to the cylinder and a release for steam if the water temp rises to much otherwise the tank would explode,I can't think what else it could be Good Luck

[stanleyman]

You need to give more info about the system.