All my radiators are piping hot when the oil-fired heating system is on but I have no hot water. Unfortunately, the system is all or nothing, you cannot switch between heating, water or both. The hot water pipe in to my hot water tank is also piping hot, but the pipe out at the base is cool. I have tried turning all the radiators off and run the boiler in case it is a pump issue, but this has had no effect. The electric heater in the tank works fine.
I had my friendly plumber fit an auto-bleed valve on the top of a length of 22 mm copper pipe, rising from the hot inlet to the indirect cylinder, to just above the top of the 210 litre tank. This allows manual priming (bleeding out of air), and if only gently closed, finger-tight, operates as an auto-bleed valve. I also had a similar finger-tight type bleeder fitted to the air-bleed position on my tallest upstairs radiator.
I surmise your problem is an air-lock on the return pipe from the hot-tank. Before getting a valve fitted, try bleeding the cylinder, cold, using a LARGE STILLSON-WRENCH at the upper, inlet to the cylinder's heating coil.
Tanks are not very strong, so use 2 wrenches if there are 2 hexes. Try at the outlet if that doesn't let air out;- tighten coupling when just water comes out.
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