I have been having a problem with my mixing valve. I have lost hot water for the second time in two weeks. I called in a plumber and they fixed it in minutes by turning the screw on the mixing valve counter clockwise. I have been trying all night with no luck. The other thing that the plumber did was remove any air from my pipes. Tried that too. Not sure if I did this right. How do I reset the water tank and mixing valve? How do I correctly remove all air from my lines?