I have fitted a replacement radiator, drained heating system, refilled ok - I think!. Now got hot water to rads but no hot water to tank, all pipes from boiler getting hot? Old system conventional boiler not combi, condening, pressurised etc?
must be an airlock at the cylinder
vent the air from the flow pipe on the hot water cylinder - it should have a 1" nut or (hopefully) a vertical bleed point where you can do this
if you dont know where this pipe it is probably about halfway up the side of your cylinder with another pipe of similar size directly below it