Engineers and designers should consider all local codes when specifying approval requirements. For an in depth explanation, reference the National Board of Boiler and Pressure Vessel Inspectors Guide In general, water heater pressure vessels require ASME if the heat input exceeds 200,000 Btu/Hr (58.6kW) or if the storage capacity exceeds 120 gallons. Many water heater manufacturers choose to label and rate their tank 119 gallon to avoid any confusion. However, a tank rated at 120 gallons does not require ASME as code is only required for tanks that exceed 120.