D.C. Mun. Regs. tit. 17, r. 17-302
The provisions of this chapter shall not apply to persons actually engaged in the performance of work under contract with the Federal government, or work done by personnel employed by the Federal government during the course of their regular government employment.
The provisions of this chapter shall not apply to a person who installs, maintains, or repairs refrigerating or air conditioning equipment, machinery, or devices in a laboratory for experimental and developmental purposes if the laboratory obtains an exemption certificate or letter from the Director clearly setting forth the fact that the work shall be exempt.
An employee of the District of Columbia whose name appears on the regular roll of examined licensees may be licensed without the payment of a fee if that person carries on no work authorized by that license other than for the District of Columbia. A license issued under this subsection shall be marked "District of Columbia Government Work Only."
The provisions of this chapter shall not apply to any person licensed as a steam or operating engineer under the laws of the District of Columbia if he or she is performing refrigeration or air conditioning maintenance or repair work along with his or her other routine duties as a steam or operating engineer for the person regularly employing him or her as a steam or operating engineer.
No license under this chapter shall be required of a person, firm, or corporation for the installation, maintenance, repair, or replacement of a self-contained unit system containing not more than two pounds (2 lbs.) of refrigerant; Provided, that if an electrical or plumbing permit is required by the D.C. Electrical Code or D.C. Plumbing Code for work involved in connection with the installation, maintenance, repair, or replacement of a self-contained unit system, the electrical or plumbing work may be performed only under the authority of the required electrical or plumbing permit.
No license of any kind authorized by this chapter shall be required of any person, firm, or corporation by reason of that person, firm, or corporation being engaged in the installation, maintenance, repair, or replacement of refrigeration or air conditioning equipment in motor vehicles or railroad rolling stock.
The provisions of this chapter shall not apply to any public utility, as defined in D.C. Code § 43-203 (1981), which is under the jurisdiction of the Public Service Commission of the District of Columbia, when that utility is engaged or employed in the business of installing, maintaining, repairing, and replacing refrigeration and air conditioning equipment as part of the performance of work for which the utility holds a franchise to engage in rendering a public utility service.
D.C. Mun. Regs. tit. 17, r. 17-302