“Miniature boiler” defined.

“Miniature boiler” means a power boiler or high-pressure, high-temperature boiler that does not exceed the following limits: 1. An inside diameter of the shell of 16 inches (410 millimeters); 2. Except for electric boilers, a heating surface of 20 square feet (1.9 square meters); 3. A gross volume, not including casing and insulation, of 5 cubic feet (140 liters); and 4. A maximum allowable working pressure of 100 PSIG.