Section 21. The owner or user of a boiler required by this chapter to be inspected shall prepare the boiler for inspection as directed by the inspector. The inspector shall, if requested, give the owner or user at least fourteen days’ notice to prepare a boiler for inspection; provided, that no notice shall be required of an external inspection under steam, nor if the boiler is being installed or has not been inspected within one year and a certificate of inspection issued. If, in the judgment of an inspector of the division, any such boiler or its appurtenances is in a defective or dangerous condition, he may immediately forbid the operation of the boiler, whether or not it is under the inspection of an insurance company; and no person shall again operate such boiler, or cause it to be operated, until a certificate of inspection has been issued by an inspector of the division.