D3309 Grey Pipe, is it safe? Is it ok for hot water? Should it be used for drinking water?
I'm "assuming" you are referring to Grey pipe that carries the ASTM D3309 markings. (i.e. pb polybytylene pipe) then the answer would be yes, it is would be safe for both hot water and drinking water. You might see various markings on the pipe similar to :
NSF-PW .062
ASTM D-3309 C-1-S
SDR-11
That said... be aware there have been "thousands" of lawsuits due to pipe ruptures and failures of polybytylene pipe. In addition to your initial concerns... you should also consider where you plan to use the pipe, how much you plan to install and if it will be accessible after the installation. Plenty have homeowners have been disappointed due to flooding when this pipe bursts inside their walls and ceilings. I personally wouldn't use it in any new construction and would limit its use to an area that wouldn't be damaged by flooding if it busted.
ASTM (American Society for Testing Materials) Polybutylene (PB) plastic pipe and tubing ASTM D 3309 standard test method for polybutylene plastic hot- and cold-water distribution systems.
Polybutylene (PB) is a flexible plastic piping material, typically gray or black in color, used in an estimated six to ten million hot and cold water plumbing systems in residential site built and manufactured housing between the late 1970’s and 1996. It was not used in drain, waste, and vent applications. All of the model plumbing codes, used for adopting either local or state plumbing codes in most areas of the country, included PB at one time or another as an approved material for use in residential construction. In addition, PB was included as an approved material in manufactured housing. The product was listed by NSF International as meeting an ASTM standard relating to performance (D3309) and as meeting ANSI/NSF 61, the health effects standard. PB also underwent long term hydrostatic testing for a Plastic Pipe Institute (PPI) design basis listing.