Drywall construction around gas water heater vent?

[Belmondo]

What are the guidelines for constructing drywall around a vent pipe? My tank WH is in a closet next to the brick flue, but the inside is unfinished and I'd like to clean it up a bit. Googling came up with a reference to 6" from flammable material being the rule. That makes it tight to place a stud on the brick where I need it, with 3" clearance from the 3" ventpipe. Can I pack the space between brick and drywall with fiberglass? What about the drywall itself, can it come right up to the vent, or cut it a couple of inches clear and use a wall plate of some kind that I've seen.

While I'm giving the WH some attention, I've noticed it has no sprinkler head in the closet, as I've sometimes seen in utility areas. Is this not a requirement, just a good idea, or unnecessary for a gas water heater rather than a boiler?

[jadnashua]

While the codes are changing, very few locations require sprinklers in any single family homes, anywhere in the USA. Not saying that they may not be a good idea, but that few places have them, or require them.