You obviously have a leak somewhere on the heating circuit , if it is not coming out of the pressure saftey valve then the leak is not visable on all pipework in view then it must be somewhere under the floors.
noel, heating engineer, November 2009
Check for leaks on radiators. Verify that when heating the pressure does not go up and open the pressure release valve. If it does try to release the pressure from the boiler and put new air into the pressure vessel via a small valve. (This valve looks like a bike tire valve and you use a bike pump to pump it to about 0.75 bar)
The pressure vessel contanes a rubber ballon like item that is filled with air. The system water when it is heated expands. The air in the pressure vessel is then compressed. The air is more compressable than the water and therefore the pressure in the system does not increase. If the air buble fails then when the water in the system is heated the pressure would increase, resulting in an explosion. To prevent this a pressure release valve is fitted which opens up at 3 bar. Dumps the water to prevent the explosion your heating is ok till it cools down and then the pressure is to low.
