Cannot shut off hot water from heater
First off, I'm very much a newb to DIY home repair. So...
I need to adjust the temperature valves for my showers. I've read that I need to shut the water off to do this.
In my basement, coming out of the water heater, is a pipe which carries the water upstairs. It has a valve (not a ball valve, the ordinary "twist" kind). I turn this all the way clockwise until it seems to be as far in as it can go...but it doesn't seem to turn the hot water off. Hot water still comes to the sink and shower upstairs.
I unscrewed and re-screwed to make sure I was letting loose any build up, but it seems to be the same.
These are old pipes (old house). Any clues on how I can shut the water off?
BTW - there is one other pipe and valve coming out of the heater. This one goes to the first floor, to the kitchen and a 2nd bathroom. The same problem exists there as well. I turn the knob closed...but the hot water remains, even after running for some time.
Any help is greatly appreciated!