Faucet or hot water heater? I don't know in what respects you may be asking but in a hot water heater, the only difference is that the cold water goes in and hot water goes out. The cold water line has a shut off valve. They are marked on the top of the heater, usually cold on right and hot on left and red washer on hot and blue washer on cold. Materials are usually the same except some codes require copper for a specific distance before turning to CPVC. For a faucet, the hot is on the left and the cold is on the right. Any particular reason for the question?