while checking out a slow leak in a cold water line, i found that there was a 1/2" copper cold water line T'd into a 1/2" hot water line that runs to my kitchen sink. There was also an in line gate valve to control this cold water flow (which no longer works). I am guessing this was a method to reduce the temperature of the hot water in the kitchen sink if it too hot straight from the hot water heater. Is this a common thing to do? I'd never seen it before.