Jaguar combi pressure problem

[stevejohna]

A friend of mine has a problem with his Jaguar combi. Basically it was moved by some one I do not consider competent, about 3 months ago, and ever since he has been having problems.
He asked me to have a look ( I am an electro/mechanical engineer but by no means an expert on heating systems). What I found was a cold system pressure of 3.5 bar, with hot pressures in excess of 5.5 bar. Needless to say the relief was lifting. I found the filling loop still connected and fully open, constantly topping the system up.
I closed the loop and dropped the sytem pressure to 2.8 bar by bleeding radiators. We then switched the heating off and watched the pressure drop back down to 0.5 bar with the system still reasonably warm. At the time I put this down to air in the system so we topped back up to 1.5 bar cold and bled back down to 1.0. The hot pressure then peaked at 3 bar but when the system cooled (after I had to leave ) it dropped back to zero.
I dont think there is a leak because it will hold a hot pressure of 3 bar.
I am thinking maybe it is expansion related but I cant get back to have another look until tuesday so I have no idea whether this boiler even has an expansion vessel.
I am starting to suspect that when it was moved the guy who did it came across this problem and left the loop open deliberately to keep the system running, as it faults below 0.8 bar.
Any help or advice is very much appreciated.

Cheers,
Steve

 

[chrishutt]

Yes, sounds like problem with expansion vessel (big red thing at back of boiler). Either it's just lost it's air charge (use bicycle pump to restore air pressure to a little under 1 bar - I think) or more likely the diaphragm has failed and you need a new vessel. Easiest and cheapest to fit generic vessel (8 litre capacity) external to boiler rather than official spare part.

 

[stevejohna]

Thanks for the reply.
Is the air fill point easilly accessible and should it be filled with no water pressure in the system?
If the diaphragm is split am i right in thinking that I should get water out of the air filling point?
Also, if fitting an external expansion vessel, does it matter whether it goes on the feed or return side?

 

[gasgeezer]

if the exp. vess. is split then you will get water out the schrader valve to repressure the vessell isolate and drain boiler leave prv open do not over press. or it can split if you fit ext. exp. vess. it can be on either return or flow otherwise boiler off wall to fit proper part.

 

[chrishutt]

I wish gasgeezeer would use punctuation . At least full stops at the end of sentences - you use them correctly on abbreviations so you can do it.

As for exp. vessel location, the return pipe is usually preferred because you are less likely to get excessive temperatures. As you said, pressurise it without any water pressure, but you don't have to drain, just release pressure.