While Chapter 5 of the Uniform Plumbing Code is titled “Water Heaters,” it also addresses combustion air, chimneys, different types of gas vent materials, exhaust gas venting, etc.
In the 2000 edition of the UPC, Chapter 5 was 9 pages with 3 sizing tables. The same chapter in the 2015 printing has 25 pages of text and 26 pages of sizing tables.