Originally Posted by tmmar
If I remember correctly, you just have to close the one valve that lets the cold water into the heater. Once that is closed, nothing should go into the heater. If that is not working, the valve may be defective.
If you don't close the hot water valve, then antifreeze can back up through the bypass line and hot water line into the heater if it's ever the path of least resistance (say, all faucets are turned off while anti-freeze is being pumped).
Further reason you need to close all three valves: the hot water, the Coldwater, and the bypass valve.