Yesterday my 8 year old 50 gal AO Smith gas water heater began leaking out of the top of the unit around the exhaust flue. My home is also 8 years old, it is a ranch with the heater in the basement. I was surprised at the way it was plumbed. Instead of a drain valve sticking out of the side it instead has a 3/4" copper pipe connected to a tee, one side of the tee goes down and a shutoff valve is mounted. The other side of the tee reduces to 1/2" has a shutoff and goes up to the ceiling and follows the same route as all of the supply lines. They all go thru a finished portion of the basement so I have no way of knowing where this particular line ends. I replaced the water heater and left this line unconnected. I have hot water at all fixtures. My question is what is this line for? Why would the plumber run a line from the drain valve? Thanks in advance for your help.