Ball Cock
Is your overflow running constantly?(the small pipe poking out of your house) Chances are the ball valve is at fault. Whether it's on your combinational cylinder, toilet or on a tank in your loft the procedure is virtually the same.
Replacing
Turn off water supply to the ball valve (cold mains).
Drain off some of the water to lower the level of water(in the tank, cistern or what ever your replacing the valve on).
Undo both nuts holding it in place and on the pipework. There will be water coming out of the pipe, so have something to catch it ready.
Remove old valve (check the condition of the washer on the connecting pipework, replace if necessary)and replace with the new.
Reconnect in reverse order.
Fill and check for leaks.
Job Done!
Why not just replace part of the valve?
Overflow pipe "overflowing" constantly can be caused by the ball/float filling with water due to a hole. Also check that the ball valve arm isn't stuck either on the side of the container or due to it seizing up over time.
Water hammer, the noise created when the valve is getting close to shutting can be fixed by checking the "guts" of the vlave. It could be dirt or a faulty component. (Can also be due to pipework not being clipped properly.)
Check these things first and replace either component if at fault, its even cheaper than replacing the full valve.
1/2 inch ball valve part 1
1/2 inch ball valve part 2
