As I suspected it is a "aerjec" which is confirmed by the small print on your diagram (made by "Myson" from the mid Eighties). If you compare figure 4, in your diagram, with your existing pipe lay out in the airing cupboard, you will see what I mean about it being installed incorrectly. Obviously for this reason it is not guaranteed that it is doing it`s job correctly and there fore could be the reason for the air in your system. I would therefore advise you to consider re-installing it correctly, unfortunately this will mean quite a bit of pipe work alterations. I would do away with this unit and alter the pipe work to the "H" configuration regarding the feed and vent. The boiler would have automatically bled its self on refill,
There does not seem to be to much of a problem with sludge by looking at the photo of the head tank. The sediment that is there is usually caused by the expansion of water going up to the into the header tank via the cold water feed and then re-entering the system as the water cools. This will cause some of the inhibitor to enter the header tank causing the water to change coluor. There does not seem to be any problem with pumping over.
Sorry spraggo, I forgot to mention I just googled aerjec on the internet and got the diagram from a PDF file.
I have no idea if the one installed in my parents' airing cupboard is a real aerjec or a knock-off. It has no identifying marks or writing on it that I could see (and it's tucked right at the back of the cupboard), but I do agree it's best to remove it.
An RGI who came out to do a quote for fitting a couple of double rads recommended that the air-thingy be removed and the pipework re-done, but never turned up on the agreed date and calls to his answerphone went unreplied. Another RGI was called in to do a quote and the he missed a couple of appointments and was never heard from again.
I guess they had more important jobs to do at the time but it would have been nice to just say they couldn't do the job.
That was why my father and I did the DIY install of the rads and drained/refilled the system.
It's good to hear you don't think the system is pumping over, I guess they just need to keep an eye on it and keep checking for air in the rads? With regards to the sediment in the tank, will it need to be cleaned out again in a few years time and fresh inhibitor added?
Thanks
Paul