"Pressure gone from system."
Something must be happening to the system to make it lose pressure. One would suggest a leak but where? You will need ot go hunting.
A radiator? A radiator valve? One of the joints in the pipework? The pressure relief valve on the boiler (that will be the pipe venting outside)? The boiler itself (Boiler, heat exchanger, pump, diverter valve, etc.)
Good Luck
March 2016
howtomendit.com for free repair help, information and advice.
