Assuming the automatic air vent, if fitted, and the pressure release valve are ok. Assuming the filling loop is disconnected. If the leak is not within the boiler, it must be somewhere else. How quickly does the pressure fall.
Either you have a leak within the pipes to the radiators or one of the rads themselves. Try isolating as much of the circuit as possible and then retest. You may need to fit temporary isolation valves. Do not turn on CH but test cold. Be prepared for water spillage. draw a map of the circuit and plan carefully what you are going to do.
if the pressure is dropping, it could be a few things,
A leak on a radiator valve, pipework, fitting or boiler.
If the pressure rises before dropping, then it's either the plate heat-ex or the expansion vessel.
Check the pressure when the heating is on, if it rises more that 0.5 - 1.0 bar then it's probably the expansion vessel at fault. If it doesn't rise and the pressure relief valves isn't passing, then it's a leak somewhere.
yes it is a vented system .the pressure drops when the heating has been on and it drops quite quickly .i keep having to refill the boiler every time .I changed the diverter because i thought it was that was causing the drop in pressure
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