What is the name of this valve on water heaters?
And why do they have it cut off in some of the houses
that I inspect? (vacant homes)
I can always tell …by very low pressure on the
hot water side of a faucet.
http://i187.photobucket.com/albums/x296/dawlita/Antonio080-1.jpg
It’s a Ball Valve to isolate water from entering the tank, I do not know why they have it turned off…is the Tank empty?
Home winterized?
is the power still on? have you seen any goofy looking guys hanging around? what does the tpr look like? 
http://dsc.discovery.com/videos/mythbusters-water-heater-evolution-high-speed.html