There are a few things that could be happening here, first, is there any water dripping from the discharge pipe outside?, this could be due to a faulty pressure valve, next what is the pressure in the pressure vessel, this needs to be 1 bar usually and is there water in the vessel which could indicate a ruptured diaphram. Next some of the rad valves could be leaking around the gland nuts but this would dry up when the heating is on so you would never find it.
I would turn off the flow and return to the boiler and raise the pressure to 1 -1.5 bar (make sure boiler is turned of electically) and see if the pressure then drops, if not then the problem is in the heating system, if it does drop the the problem is one of the first 2 causes.
Mike Jeffery, November 2009
