I know that this is more information than you requested, but I figured since it was “related” that you might be able to use it. hope that this helps.
**The International Residential Code **
M2005 WATER HEATERS M2005.1 General.
Water heaters shall be installed in accordance with the manufacturer’s installation instructions and the requirements of this code. Water heaters installed in an attic shall conform to the requirements of Section M1305.1.3. Gas fired water heaters shall conform to the requirements in Chapter 24. Domestic electric water heaters shall conform to UL174 or UL 1453. Commercial electric water heaters shall conform to UL1453. Oiled-fired water heaters shall conform to UL732.
1305.1.3 Appliances in attics.
Attics containing appliances requiring access shall be provided with an opening and a clear and unobstructed passageway large enough to allow removal of the largest appliance, but not less than 30 inches (762 mm) high and 22 inches (559 mm) wide and not more than 20 feet (6096 mm) in length when measured along the centerline of the passageway from the opening to the appliance. The passageway shall have continuous solid flooring in accordance with Chapter 5 not less than 24 inches (610 mm) wide. A level service space at least 30 inches (762 mm) deep and 30 inches (762 mm) wide shall be present along all sides of the appliance where access is required. The clear access opening dimensions shall be a minimum of 20 inches by 30 inches (508 mm by 762 mm), where such dimensions are large enough to allow removal of the largest appliance.
Exception: The passageway and level service space are not required where the appliance is capable of being serviced and removed through the required opening.
P2801 GENERAL P2801.1 Required.
Each dwelling shall have an approved automatic water heater or other type of domestic water-heating system sufficient to supply hot water to plumbing fixtures and appliances intended for bathing, washing or culinary purposes. Storage tanks shall be constructed of noncorrosive metal or shall be lined with noncorrosive material.
Water heaters shall be installed in accordance with this chapter and Chapters 20 and 24.
Water heaters and storage tanks shall be located and connected so as to provide access for observation, maintenance, servicing and replacement.
P2801.4 Prohibited locations.
Water heaters shall be located in accordance with Chapter 20.
Direct-vent water heaters.
Appliances installed in a dedicated enclosure in which all combustible air is taken directly from the outdoors, in accordance with Section R703. Access to such enclosure shall be through a solid door, weather stripped in accordance with the exterior door air leakage requirements of the International Energy Conservation Code and equipped with an approved self-closing device.
P2801.5 Required pan.
Where water heaters or hot water storage tanks are installed in locations where leakage of the tanks or connections will cause damage, the tank or water heater shall be installed in a galvanized steel pan having a minimum thickness of 24 gage (0.016 inch) (0.4 mm) or other pans for such use. Listed pans shall comply with CSA LC3.
P2801.5.1 Pan size and drain.
The pan shall be not less than 1.5 inches (38 mm) deep and shall be of sufficient size and shape to receive all dripping and condensate from the tank or water heater. The pan shall be drained by an ***indirect waste pipe ***having a minimum diameter of 3/4 inch (19 mm) or the outlet diameter of the relief valve, whichever is larger.
P2801.5.2 Pan drain termination.
The pan drain shall extend full-size and terminate over a suitably located indirect waste receptor or shall extend to the exterior of the building and terminate not less than 6 inches (152 mm) and ***not more than 24 inches ***(610 mm) above the adjacent ground surface.