I have a Sundial Y-plan configuration. The 3 port midposition valve is a Honeywell V4073A and have a Worcester Greenstar 18ri.
After replacing an older 3 port valve which had become stuck with the Honeywell V4073A , I found that the boiler was no longer switching off when in heating only mode and the room thermostat was satisfied.
When the thermostat was demanding heat (pin 5/white live), I was seeing 230V on the orange wire from the 3-port valve (when disconnected from the wiring centre). When temperature was reached at the thermostat (pin 5/white 0V), I found that the orange wire from the 3 port valve was reading 48 volts! As the orange wire is connected to pin 8 and therefore the boiler live (L/R on pump overrun configurations), the boiler continues to fire.
Measured during Hot water was off, pin 7/grey live.
If I switch HEATING off, then WATER on, then water off on the programmer, then the boiler does switch off. The orange wire then reads 0V.