How can i fix my Central heating system?

your post don't really make sense.
but going on what your saying is water is pumping over the vent pipe into the tank.

so the water in the tank will be hot.
whats the con't of the water in the tank black/sludge
or rusty coloured water ?

any of these will indicate a restriction/blockage around the bottom of the vent/cold feed area.

hi-spec plumbing & heating (essex)
August 2010