CPVC by draft hood

Looking for codes specifics on thew 18 inch copper rule or no plastic pipe by the gas heater draft hood.
anyone know? Been looking

It should be in the installation instructions as well.

Do you know the specifics for code

Found it…
Charlotte Pipe states:
When connecting to a gas water heater, CPVC should not be located within 6" of the
flue or used to make the direct connection to the hot water side. A metal section of
pipe and a CPVC-to-brass threaded transition fitting should be used. This reduces
the potential for damage to the CPVC pipe that may result from excessive radiant
heat from the flue.

Here was an attempt at answer too: