>(if there is one)
now you need to check all your pipework, radiator valves for leaks. it must be a visable leak to loose pressure in 30 mins.
check all your pipework under floorboards, do you have any heating pipes in solid floors, as concrete will corrode copper pipe.
look for any signs of green on your pipes thats a sign of water.
hi-spec plumbing, July 2007