When using the hot water the pipes start to knock, what is causing this and how to correct it? The pipes knock shortly after the hot water is turned on, and the knocking will continue until the hot water is turnrd off. It is a hard constant knocking.

When using the hot water the pipes start to knock, what is causing this and how to correct it?
Define "knock". Does the pipe bang when you shut the hot water off? Does it
Knock once or does it keep it up? Does the pipe rattle or just make a clicking sound in the wall? Give us more details please. Tom

Pipe can make knocking noise from several reasons. Some of the most common ones are:

1. Thermal Expension. Install Thermal Expension Tank.
2. Pipes not attached to the framing. Reattach pipes.
3. Pressure too high. Lower water pressure.

Also,it is a good practice to install Water Hammer Arrestors throughout the house. They absorb shocks caused by solenoid valves. We usually install one on each water supply directly behind washing machine valves.