The S.D. boiler in my new flat won't operate because the pressure is way too high. I have tried bleeding a radiator and using the relief valve which will lower the pressure while the mains supply is shut off, but as soon as I turn the mains back on up goes the pressure again. I've tried turning the mains on just a little bit but that doesn't work.

Barry Latchford, August 2007

is the filling loop letting by,make sure both valves are shut under boiler, your boiler has a built in filling loop.
if that fails the dhw heat exchanger is letting by to the primary circult.

hi-spec plumbing, August 2007