central heating not heating cylinder

[dodgerconway]

hHre's one that has me flummoxed and up until now ive always figured out my central heating problems.
The rads heat up all over the house but for some reason the pipe that enters the cylinder and that should heat the water when the central heating is never heats up. Its cold going into the cylinder even when the heat is on full.
Any solution would be gratefully received.

 

[gas4you]

DHW zone valve broke and stuck shut, or sludge blockage, or air lock. Last 2 could be in the return or flow.

 

[dodgerconway]

thank you, where would i find the DHW valve?

 

[twgas]

usually on pipe between pump and cylinder