glowworm fuelsaver 25-30 no hot water

[brickmaker]

i have a threeway valve which has a lever with switches between hot water and central heating

the central heating works fine , however when the lever is on hotwater , we get banging noises from the boiler and the cylinder remains cold although the pipe leading to the top of the cylinder are hot
the cylinder is indirect with a coil and has a thermostate around the outside

when the cental heating is on the radiators are hot but the cylinder is still cold so no hot water this way either

can any one help

 

[fitz1]

Possibilities are flow circ is hot due to heat transference from heating flow and either ur valve is faulty or u have a blockage in ur coil, maybe air.