s30 is just no demand for heat from external controls ie programmer or roomstat.
boiler shouldnt run without pump spinning but that would give you rapid pressure rise if it suddenly failed 0-3 bar in a second, but like i said it shouldnt allow boiler to run so you wouldnt be able to put it on until rectified so,
other option is expansion vessel but generally from cold to rise over 3 bar would take around 5 mins.
basically as your sealed heating system full of water heats up, water expands and the expansion vessel is where expansion is allowed to take place, should this have no air charge or be full of water then there is no room for expansion and pressure will rise beyond 3 bar and pressure relief valve will open to protect boiler and let excess water out to try and lower pressure.
clear as mud?
ste
November 2009
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