Alright, according to my tables of you have a total of 109k btu then you'll need an opening of 18" square or 5" round duct equivalent (if you bring the air in from outside)
Andy G brings up a good point about checking with your local AHJ (Authority Having Jurisdiction or local inspection office). I'm using the Canadian Gas Assoc codebook which is the same codes as the AGA, but these are just MINIMUM guidelines, and your local codes may be more strict. They may want you to have your tank mounted on a platform in case of a major leak, they may want you to chage the way your T&P relief piping is piped (it goes out through the wall to the outside I assume? If so how high above the ground is it?)
It may be a PITA to ask the local inspectors, but it's a gas appliance your dealing with, not electric, so it's better to be safe than sorry.
Thanks for the exercise. I've been working exclusively on Industrial/ Commercial these last few years and this was a good little one to sink my teeth into.
...this room when done will be either bedroom or bonus room.
Sorry to burst your bubble but the code states that "The water heater, unless of the direct vent
type, shall not
be installed in a bathroom
, or any enclosure
where sleeping accomodation is provided"