my combi boiler has taken to dumping the reservoir of water.by this i mean the water that is put into the system via a flexi pipe and reads off the pressure guage on the front(hope u understand what i mean)
i keep refilling it and it will last fora day or two then the whole lot is dumped again.can anyone enlighten me as to why
from the what you have said here it sounds like the dump valve. Do u have a 15mm pipe going straight through the wall to outside. (check the wall to see if its wet)If so then your filling loop is passing and when the pressure builds up (most boilers dump at 3mbar) it will dump the water outside. If it is then isolate the filling loop from the boiler when u have filled it up and see if that sorts the problem.
If its a flexi hose then buy 2 x 15mm caps, unscrew the flexi hose a fit the caps on both pipes (the cold water supply and the non-return valve)
If the boiler still releases the pressure via the pressure relief valve then there's a problem with either water mixing in the boiler. Could be a hole in the plate heat-ex or the seals on the diverter valve.
The pressure vessel isn't working correctly, if the boiler still dumps the water daily, then my guess is that it will the pressure vessel. The easiest way to check this is to fill the boiler to 1.0 bar on the pressure gauge. Turn off the cold supply to the boiler and run the heating as normal for a day (you wont be able to get hot water) check the pressure gauge throughout the day, when the heating is on. The gauge dial will rise abit normally about 0.5 but shouldn't be anymore than 1.0 bar, if it rises anymore than this then there's a problem with the pressure vessel. Either it needs re-pressurising or the diaphragm has split.