potterton EP3000 programmer?

You sound to have an 'S' plan system (two zone valves rather than a diverter valve)
You could check that the heating zone valve is in the auto rather than manual position, otherwise it sounds like the heating zone valve is passing.

It's possibly a wiring fault, but unlikely.

Consider converting to a 'Y' plan, (diveter rather than zones, less components, less to go wrong.

Carefree
July 2010