Section 7. The preceding section shall not apply to boilers of railroad locomotives, motor vehicles or steam fire engines brought into the commonwealth for temporary use in times of emergency, nor to boilers used in private residences, nor to those used for heating purposes which carry pressures not exceeding fifteen pounds to the square inch and have less than four square feet of grate surface, nor to boilers of not more than three horse power. The said section shall not apply to boilers under the jurisdiction of the United States nor to those used exclusively for horticultural or agricultural purposes.