As a firefighter and inspector here in the NW, I don't readily see a reason for the 5' rule, nor have I ever actually seen one that high, except for a few in a basement with the gas line coming in from the ceiling. As far as worring about a gas fume cloud 5' off the ground, if you have that much gas in a room or a house, you better know what you are doing before you go into that room and turn off the gas! My new GE gas water heater doesn't say anything about 5' either. Good comparison about the other gas appliances, why single out water heaters? Any more thoughts?