It most often occurs at the Tee where the 15mm f&E pipe joins the 22mm or 28mm circulating pipes near the pump or cylinder.
It is often a blockage of black magnetite that needs to be cut out and a compression tee fitted in its place (sometimes with new pipe stubs). If you are lucky enough to have a compression tee there you can undo it and poke it clear. You will need a chemical flush to get out the rest of the much that is floating round. If you have the money handy this is a good time to fit a magnaclean which traps the black circulating in the water.
If it is black magnetite in the blockage you can sometimes feel it with a magnet held against the copper pipes, as the magnet will be attracted to it.
I have heard you can sometimes dislodge the blockage with an Aquavac on the Vent pipe.
Thanks for your suggestions JD.
As an alternative could I cut the 15" pipe just above the tee (to replace the tee would not be easy due to lack of access) and poke around with something suitable?
Next thing after the feed should be the pump, where you could get access. A thin drain-cleaning spring can clear it.
SOmetimes the Aquavac (or the mains) works becasue the blockage is jellylike or just "sludgy".
going back to the origanal question its best to check if its blocked before you do anything else open a drain of cock and drag some water through if the f + e tank ballcock runs the cold feed isnt blocked
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