I have a tankless water heater system in my house - house and system are 2 yrs. old. When the water control leaver is upright water on the shower it is at its hottest - 140 degrees, however when I turn the leaver counter clockwise to reduce temp. of water it stays about the same temp. Whats my problem? This started 2 days ago. I am assuming the problem is in the cold water supply to the shower.