As I understand the codes, the T&P relief valve needs to drain somewhere safe, and it can't have anything in its discharge line that could block it, like a valve. Our water heater's T&P valve just drains onto the basement floor, eventually leading to the same drain as the furnace, A/C, and water softener. You'll have to check your local building codes--in our last house, the A/C condenser just drained to the outside of the house, but where we live now, all that water has to go into the sanitary sewer.