My boiler is connected to a sun-water-heating system. After the sun goes down this system start to draw heat from my boiler. 
Today I'm manually closing the valve connecting my boiler to the system. 

I'll like to build system based in Arduino that will operate a solenoid that will open and close the valve. 

The solenoid will probable by normally-closed and I'll open it only when sun is up which is a few hours every day. 

I remember reading somewhere that solenoid should be operate for short time, is that true?
Can I leave the current on for say 6 hours to keep the solenoid open?
Is there a better way?