I am trying to turn off the cold feed to my hot tank so I can do some work on the system. A shut off valve was installed in the line some 12 years ago when a new hot water tank was installed. I have not turned that valve during that period but today it appears odd. The valve only turns through an angle of about 20 degrees and seems to reach a very definite stop. This is not at all what I expected as normally valves I have met before will turn off through multiple turns. Is it jammed, broken or do valves like this exist?
I'm taking a guess that it's a gate valve (red circular top) and yes they do get jammed! You are much better off turning the cold supply off at the mains and then opening all the hot taps to drain down the whole tank rather than risk mucking about with these nasty valves.
Darn nusiance gate valves seizing up, replace the things with full bore lever valves. http://www.screwfix.com/p/full-bore-lever-ball-valve-22mm/30584