ideal classic boiler problem, please help

[nk1]

i have an ideal classic boiler (getting on a bit now like me) when it is running it has started to pump water out of the vent pipe in to the header tank, this water is fairly warm and causing some steam, dont want that in my loft. this started last year, when i lowered the temprature on the stat it stoped the water flow and all was well, unfortunately this year it seems it will not stop unless i turn the boiler off.
any ideas what is causing the problem, and what will it take to fix it
thanks

 

[hi-spec plumbing heating]

blockage, sludge in system.
pump speed high etc.
as it's getting worse i would put it down to crud in system , check the bottom of the f/e tank.

 

[nk1]

thanks for your help, turned flow on pump down, cured problem.
thanks again
nick