Potterton Performa 28i Losing pressure - PRV and AAV?

Hi my first post so excuse any errors!
I have a Potterton Performa 28i wall mounted combi boiler in a 2 bed flat with 5 rads all TRV fitted. The flat is some 40 years old and originally had warm air changed to GCH.
Appox 4.5 years ago I had the floor mounted conventional glow worm boiler changed to the combi. One of the reasons was because the system was always full of air no matter how much I bled the rads plus the boiler was some years old.
When the combi was installed we had issues with noise and burbling in the boiler. Potterton said it was air and changed the expansion vessel for a vented one I think? Apologies, some years back but remember the engineer saying it was a vented vessel I think? All was fine after that until now.
The boiler is losing pressure rapidly. It will fall to 0 overnight. If we have the CH on it stays at 0.5 to 1.0 but as soon as CH is switched off it falls and stays at 0.
Yesterday I repressurised to 2.0 and switched the CH on and checked the overflow. The overflow was leaking water quite rapidly. I assume it is a PRV fault?
I wanted to check which PRV I needed and on opening the front boiler casing I found the AAV on top of the pump body weeping water.
I need help with the following:
1. Do you recommend me changing the PRV and if so how to drain down?
2. Why would the AAV be weeping and what should I do?
3. Any other checks or points I have missed.

Thanks in advance.

Neilson said:

Potterton said it was air and changed the expansion vessel for a vented one I think?

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no it's not a vented one, you haven't let the air out of it have you?

Neilson said:

1. Do you recommend me changing the PRV and if so how to drain down?
2. Why would the AAV be weeping and what should I do?
3. Any other checks or points I have missed
Thanks in advance.

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1)no check the expansion vessels' precharge
2)although it could be faulty its more than likely venting off excess pressure due to damaged or undercharged expansion vessel. recharge your system to 1 bar turn on the CH and watch the pressure gauge if it rises above 3 bar it will vent off via the PRV and will indicate a faulty/undercharged expansion vessel
3)see above

Matt

Matt, many thanks for your reply.

The expansion vessel was changed some 4 years ago when the boiler was new and I haven't touch it.

Yesterday as soon as I had recharged the system to 2.0 bar I was loosing water from PRV overflow pipe outside so assumed the prv was stuck or faulty?

Watched the pressure after recharging to 2.0 bar and with the CH on it still drops to about 1.0 bar and never rises.

When the CH is off it drops to 0.

How do I check the expansion precharge?

Thanks again.

Checked the pressure this afternoon and the gauge has dropped from 2.0 to 0.5 even with the heating on.

Anyone else have a second opinion please.

recharge your system to 1 bar

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I had recharged the system to 2.0 bar

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Doesn't really help when someone tries helping and you disregard their instructions, however, if you lose pressure through the PRV at 2bar then I would suggest it needs changing. It may be worth changing the AAV at the same time as yours has wept - although it may be ok it's not worth the risk of having to do it later.
To drain the system you can use the PRV as you will be changing it anyway. Whilst the system is empty check the expansion vessel pressure.

Mick thanks for your reply.
I did miss the 1.0 bar but the pressure never rises more than 0.5 bar with the CH on and drops to zero within a few hours if off or quicker.
Can you tell me the best way to check the expansion vessel pressure.
Thanks.