Common return

[gangeox]

With the programmer calling for central heating only, what stops the returning water from the last radiator flowing up the return from the cylinder?

By the way, its a fully pumped Y plan system

 

[chrishutt]

Perhaps you don't realise that the heating pipes that heat the cylinder do so by means of a coil inside the cylinder. When on CH only The motorised valve closes off the circuit to the cylinder so there is no through route, therefore no flow.