The thing to remember about these watervalves is that "0" voltage=full heat. This is a failsafe incase of a climate control electrical failure. They plan on the worst scenario and try to make the situation as safe as possible. By looking at it this way, they figure full heat will keep the windshield clear on a cold damp day. If this happens in the summer time, I guess they figure you can roll down the windows and open the sunroof.
At any rate, if you operate the car with the electrical connector disconnected, you should have your full heat. If it does go to full heat, it means the system has failed in such a way that there is constant 12 volts going to the water valve, maybe a bad interior temp sensor, that would be my first guess, second guess would be the control unit (control panel) itself. I wouldn't replace anything before getting a proper diagnosis done.