Hi all you guys, I've really found this thread useful, helped me fix the problems out here in Mallorca at my friend's house.
Two points I need to share.
Water Pressure : I think those points are barking up the wrong tree. The valve is help shut by THE SPRING in the solenoid. The solenoid just pulls the spring.
Back EMF (as we call it in UK) has not had a mention. On DC solenoids one would fit a diode and capacitor close to it solenoid.
I'm not sure about AC solenoids, but there are voltage dependant resistors and even two 50V( say) zeners anode to anode across along with a .01uf capacitor should shift and spikes. The programmer should have that protection anyway, but that can be a significant distance away from the valve.
Do keep this thread alive.
Warmest regards, Derek