Boiler pressure relief keeps running

Hi
I have a vaillant turbomax boiler the pressure was high so released some of the pressure which blows the water outside. But the water would not stop running and pressure went down to nothing. Have topped up from the filling loop but slowly runs back out again, back to nought. Obviously the pressure relief value is not shutting fully. Can anyone give me any advice.

Thanks Pete

Looks like your prv wont reseat needs changing,pump up your expansion vessel as well,look at FAQ above will show you how

Is the prv an exspensive part and is it possible to do myself

try opening and shutting it a few times,make sure it clicks back

I have turned it back and forth a few times and it has slowed down but the water still slowly runs away and drops to 0.

renewal then

PRV s once opened do not always seal again properly due to dirt being trapped on the sealing face. The easiest option is to fit a new one.
Before you do, you need to deal with the reason why the PRV opened.

The water pressure should be around 1bar when heating is off and rise to about 2bar when heating is on. A pressure of 3bar opens the PRV.
The most common cause of the pressure rising to 3bar is a faulty expansion vessel. The vessel has a flexible diaphragm. It is charged with air when first installed and then water pressure is applied, the pressure applied is such that the diaphragm does not move. So basically the whole vessel contains air only at this stage.
When the heating is on and the water expands the extra volume pushes against the diaphragm and compresses the air, which increases in pressure.
This being shown by the water pressure gauge. The compressed air acts like a spring and when heating is off, the volume of water goes down again the air expands and pressure returns to that previously set.
The worst problem would be a ruptured diaphragm. So no air to compress.
Second worst problem which is most common is loss of air charge over time. This means your starting with a vessel that's partially full with water leaving a much reduced volume of air. and the pressure rises far too high, but the PRV saves the day by opening.
Solution here is to re-charge vessel, at the same time ensuring the excess water can be discharged out of the system.

Is replacing the prv an easy job to do and what do I have to do exactly