(1) boilers in buildings occupied solely for residence purposes with accommodations for not more than five families;
(2) railroad locomotives operated by railroad companies for transportation purposes;
(3) air tanks installed on the right-of-way of railroads and used directly in the operation of trains;
(4) boilers and pressure vessels under the direct jurisdiction of the United States;
(5) unfired pressure vessels having an internal or external working pressure not exceeding 15 p.s.i.g. with no limit on size;
(6) pressure vessels used for storage of compressed air not exceeding five cubic feet in volume and equipped with an ASME code stamped safety valve set at a maximum of 100 p.s.i.g.;
(7) pressure vessels having an inside diameter not exceeding six inches;
(8) every vessel that contains water under pressure, including those containing air that serves only as a cushion, whose design pressure does not exceed 300 p.s.i.g. and whose design temperature does not exceed 210 degrees Fahrenheit;
(9) boiler or pressure vessels located on farms used solely for agricultural or horticultural purposes; for purposes of this section, boilers used for mint oil extraction are considered used for agricultural or horticultural purposes, provided that the owner or lessee complies with the inspection requirements contained in section 326B.958;
(10) tanks or cylinders used for storage or transfer of liquefied petroleum gases;
(11) unfired pressure vessels in petroleum refineries;
(12) an air tank or pressure vessel which is an integral part of a passenger motor bus, truck, or trailer;
(13) hot water heating and other hot liquid boilers not exceeding a heat input of 750,000 BTU per hour;
(14) hot water supply boilers (water heaters) not exceeding a heat input of 500,000 BTU per hour, a water temperature of 210 degrees Fahrenheit, a nominal water capacity of 120 gallons, or a pressure of 160 p.s.i.g.;
(15) a laundry and dry cleaning press not exceeding five cubic feet of steam volume;
(16) pressure vessels operated full of water or other liquid not materially more hazardous than water, if the vessel's contents' temperature does not exceed 140 degrees Fahrenheit or a pressure of 200 p.s.i.g.;
(17) steam powered turbines at papermaking facilities which are powered by steam generated by municipal steam district facilities at a remote location; and
(18) manually fired boilers for model locomotive, boat, tractor, stationary engine, or antique motor vehicles constructed or maintained only as a hobby for exhibition, educational or historical purposes and not for commercial use, if the boilers have an inside diameter of 12 inches or less, or a grate area of two square feet or less, and are equipped with an ASME stamped safety valve of adequate size, a water level indicator, and a pressure gauge.
(b) An engineer's license is not required for hot water supply boilers.
(c) An engineer's license is not required for boilers, steam cookers, steam kettles, steam sterilizers or other steam generators not exceeding 100,000 BTU per hour input, 25 kilowatt, 2-1/2 horsepower and a pressure of 15 p.s.i.g.
(d) Electric boilers not exceeding a maximum working pressure of 50 p.s.i.g., maximum of 30 kilowatt input or three horsepower rating shall be inspected as pressure vessels and shall not require an engineer license to operate.