The last rad. of the upstairs circuit is a drop-feed to the lower floor (solid floor!). Rad. valves are bog-standard, c/h pump is in the return to boiler, system vented, all other rads. properly balanced, system degunged and inhibited so water runs clear. No air collects in rads.
This last rad. persistently fails after a few days. I can clear it by shutting the rad. c/h return and allowing a fast drain from the hot feed (luckily I fitted a combined valve/drain-c*ck some years ago). After about 2 pints of the hot feed have run off there is a rattling noise in the local feed pipe (air bubbles I guess). After this I can return things to normal and this rad. will work for 2-3 days before failing again. I'm getting bored with going through this palaver :-(
My guess is that I have an air leak in the hot feed pipe run between the last rad. upstairs and the drop fed rad. The next move will be to shift upstairs furniture, take up some floorboards and inspect this pipe run. But before I set about all this I'm hoping wise fellas here will advise:
Is this a sensible diagnosis or have I missed something obvious?
Would 35-year old soldered joints go leaky in this way - i.e air getting in but, as far as I can see, no water coming out?
If all this is true I guess I will need to drain the system and remake the soldered joints?
Thanks.