(6) For the purposes of this subsection: (A) The term “energy efficiency improvement” includes a solar heating system, a solar heating and cooling system, or a combined solar heating and cooling system, and the application of a residential energy conservation measure. (B) The term “solar heating” has the meaning given such term in section 3(1) of the Solar Heating and Cooling Demonstration Act of 1974 ( 42 U.S.C. 5502(1) ) and, in addition, includes a passive system based on conductive, convective, or radiant energy transfer. (C) The terms “solar heating and cooling” and “combined solar heating and cooling” have the meaning given such terms in section 3(2) of the Solar Heating and Cooling Demonstration Act of 1974 ( 42 U.S.C. 5502(2) ) and, in addition, include a passive system based on conductive, convective, or radiant energy transfer. (D) The term “passive system” includes window and skylight glazing, thermal floors, walls, and roofs, movable insulation panels (when in conjunction with glazing), portions of a residential structure that serve as solar furnaces so as to add heat to the structure, double-pane window insulation, and such other energy-related components as are determined by the Secretary to enhance the natural transfer of energy for the purpose of heating or heating and cooling a residence. (E) The term “residential energy conservation measure” means— (i) caulking and weatherstripping of all exterior doors and windows; (ii) furnace efficiency modifications limited to— (I) replacement burners, boilers, or furnaces designed to reduce the firing rate or to achieve a reduction in the amount of fuel consumed as a result of increased combustion efficiency, (II) devices for modifying flue openings which will increase the efficiency of the heating system, and (III) electrical or mechanical furnace ignition systems which replace standing gas pilot lights; (iii) clock thermostats; (iv) ceiling, attic, wall, and floor insulation; (v) water heater insulation; (vi) storm windows and doors; (vii) heat pumps; and (viii) such other energy conservation measures as the Secretary may identify for the purposes of this subparagraph.