That's good, you have not got a blocked or dry F&E.
I can't see anything odd (I am a householder not a pro but have done these)
looks like you have 28mm gravity feed to cylinder, the 15mm rising from the upper cylinder pipe should go off to the F&E
The Tee you mention would normally have 22mm left side (part of circulation); 15mm upwards (to F&E); 22mm right side circulation continues
the 15mm I see in the boiler pic might be the F&E pipe, in which case there may not be a Tee. If it is in fact 22mm it might be the vent pipe that is bent over the F&E pipe
If the boiler is very old it might have sludge at the bottom, interfering with the gravity flow a gravity HW circulation has very little flow so sludge can build up there (OOI a few weeks ago I converted my old mum's from gravity HW to pumped, and it dislodged a considerable amount of black and brown sediment, even though I had previously given it a chemical clean and fitted a Magnaclean)
Did you get sediment out of the drain cock?
It never hurts to give an old open system a chemical clean, as they usually have a lot of sediment in them. it will only cost £15 for the cleaner and £15 for some new inhibitor. If you can do basic plumbing and can afford £100 you can fit a Magnaclean as well, I guarantee you will be amazed and delighted to see how much black sediment it collects. However I don't know if the problem is caused by Sediment or something else. I normally use X400 but X800 will I understand work OK and only takes a matter of hours rather than weeks.
Let's see if any of the pro's can spot anything else.
p.s. feel both the 28mm pipes between boiler and cylinder and see if they are both too hot to hold.
It looks like you have a gate valve which is to restrict flow to cylinder and stop it taking all the flow (when you have radiators on). If you open it, count the turns from current position to fully open so you can put it back after finishing tinkering.
p.p.s. when you have finished, put thick Climaflex lagging on all those hot pipes, they are pouring money away in wasted heat
top one should be very hot.
if it aint, then your boiler may be underfired, or your boiler stat is turned too low, ...especially prevalent if you have kids/grandchildren....
fekk knows...
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