Any water works or sewer department of a city or town, organized by general law or special act of the legislature and financed principally by water or sewer rentals, may, by unanimous vote of the body charged with the administration thereof, whether the local governing body, water board, or a board of water or sewer commissioners, establish a capital reserve fund for said department for the purposes as provided in N.H. Rev. Stat. § 35:1. Such reserve shall be established only from surplus from water or sewer rentals and no part thereof shall be made from appropriations by said city or town.