I was told there should be at least one foot of copper or steel pipe
between the heater and PEX or CPVC pipes. Looking on the web, all I see
are opinions, no facts. Does the code have anything to say about it?
What about the manufacturers of these plastic pipes?

I do see where the heat from the heater could cause failure of these
pipes, so I'd rather just use the metal pipes, but I'm looking to find
out what is supposed to be done.

I'm looking for FACTS and REQUIREMENTS, not opinions!