Ravenheat woes :(

I have a ravenheat combi (yes i know they are c**p but it was here when i bought the house) i've lived with it for a year and i think its about 7 years old
i replaced the diverter valve last year and a dhw temp sensor and since it has been great. now since putting it into ch and dhw mode it has been a pain, the dhw works great, always fires and runs, hot water temp fine. the central heating is the problem, it loses pressure very quickly, i take it to 1.5 bar and turn on the ch, it rises to 3 bar fairly quickly within 24 hours it is at such a low pressure that the boiler will not fire when the stat calls for heat. (dhw still controls boiler ok at this point) I have attached a plastic bottle to the outside pipe and it catches a litre of water within 30 mins of the boiler running. Im assuming this means that the pressure vessel has no air pressure in it. It is almost impossible do get at, i have managed to depress the pin in the centre of the air valve and had only air hiss and no water.
I can see no way of attaching a air pipe to repreassurise it without either removing the pressure vessel or taking the boiler off the wall! (who designed this pos) Is it possible to do away with the internal pv and install an secondary pv? attached to the same take off point or elsewhere in the system? or do i just get a man in and have the whole boiler off the wall?

suggestions please
and does my diagnosis sound correct?

Cheers Nick

The expansion vessel is doable with a bit of effort, one of these helps.



Flexible shraeder adaptor.

Alternatively an external vessel can be fitted on the heating return pipe as close as is practicable.

At £8.34 for tha addaptor you described (ive ordered a michelin one for truck tyres) it can stay in place once installed, ill try that first.
Thanks fot the advice

just so im prepared, should i need a new expansion vessel, ive been looking at the zilmet range. I believe this should be plumbed into the ch return to the boiler and they come pre charged. afaik the ravenheat csi85 has an 8 litre vessel, do you replace 8 with 8 or would going to 12 be of any advantage?

Just had a job like this about 2 weeks ago on a ravenheat 780. I simply [with the aid of some blu tack] replaced the schrader valve core. I found that the regular Regin stirrup pump could be attached easier than that particular extension. BTW the connection on mine works the opposite way to those found on car foot pumps.

It's always better, if space allows to add a slightly bigger vessel IMO. Boilers are designed to be compact and the fact that an 8 litre vessel is usually enough for 8-10rads max is often overlooked.

Until you get it sorted and afterwards, reducing the boiler stat temp will put a lot less strain on your vessel