Such words do not, however, include:(a) Steam boilers used for heating buildings other than public buildings contemplated by the Public Buildings Safety Act (chapter S-3) or industrial establishments contemplated by the Industrial and Commercial Establishments Act (chapter E-15);
(b) Boilers with safety-valves set at a pressure of not more than fifteen pounds per square inch, with a capacity not exceeding seventy-five horsepower and a heating surface not exceeding one thousand one hundred and twenty-five square feet;
(c) Boilers used elsewhere than at buildings under construction, with safety-valves set at a pressure of over fifteen pounds per square inch, but with a capacity not exceeding fifteen horsepower and a heating surface not exceeding two hundred and twenty-five square feet;