Saunier Duval F30E with a leaking Reduced Pressure Zone Valve

Any help is appreciated.
I came home from work last night and the CH was not working. The water
pressure was at 0.5 bar and there was water on the floor below the boiler.
I refilled the system to 1.0 bar and the boiler fired up. I then had a
constant drip of boiling hot water coming out of the reduced pressure zone
valve as the heating was running. When the heating went off it turned into
more of a trickle and the pressure went down to zero. I had to look in the
manual to find out what the leaking part was. It is definitely not the
pressure release valve as this discharges outside.
For my second repair of the night I got the foot pump out the van and tried
the expansion vessel. The air pressure was at zero and I inflated it to 0.5
bar as per specs in the manual. Water pissed out of the RPZV as I added the
air and then pissed out water again when I repressurised the water side. All
seemed well and the boiler worked fine until I went to bed. This morning the
pressure was at zero again and the floor was wet through.
The only two things that are different with my combi to normal installations
1) it is not used for HW (it will be next year)
2) it has not been connected to the main cold water supply for the 2 years
it has been installed. It was filled up 2 years ago when installed and has
never lost any pressure.
So what is the faulty part, the expansion vessel or the RPZV?
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I open the manual and searched for the RPZV (thinking it must be the name for some common component but SD have given it a fancy one).
There is little info on it. Guessing, and looking at the photos and the description for its replacement it might be some sort of valve that prevents too much pressure being put into the primary circuit?
Anyway if it's leaking then a repair or replacement is called for. You might find that it's leaking due to a bad O-ring.
If the expansion vessel is at fault then pressure will rise on heating and the discharge pipe open up, which it isn't. The RPZV is not a synonym for the Pressure Releif Discharge Valve,
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Ed Sirett
I wondered (had suggested to me) if it was some sort of back flow device as there is no filling loop on the SD. The F30E has a "filling tap" built in to the boiler.
The water comes out of a hole in the bottom of the valve not at a joint (if that makes any difference?)
Pressure is definately not rising. I have put a plastic bag on the discharge pipe and this has no water in it. I have no idea if it did do before the first failure of the boiler and before I repressurised the air in the expansion vessel.
The good news is I have refilled the boiler with water and it has worked for over 48 hours this time. The pressure has dropped from 1.0 bar to 0.8 bar during the 48 hours but at least I am warm.
I might phone SD to see what the RPZV actually does. All the plumers I have asked said they have no idea why a combi would have one (apart from the check valve idea from one of them).
The only other info on the RPZV in the manual is "on install there may be a small discharge from the RPZV"
Thanks for your time Ed.
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