Using a series of identically sized, single row, single circuit refrigerant coil modules, fin/tube refrigerant coils of different nominal air conditioning tonnages are constructed by arranging different numbers of the identically sized module in accordion-pleated orientations, with each modular coil having the same depth in the direction of intended air flow across the coil. Compared to conventional "A" coils used on the indoor side of air conditioning circuits, these accordion-pleated modular coils are more compact in the air flow direction, provide more coil surface area, permit lower coil face velocities with higher fin density, and significantly reduce the overall coil manufacturing costs since only one size of coil slab needs to fabricated and inventoried to later assemble refrigerant coils of widely varying nominal air conditioning tonnages. A specially designed one piece molded condensate drain pan structure may be releasably snap-fitted onto the underside of the modular refrigerant coil, and is provided on opposite sides with support structures that are configured to slidably receive an opposed pair of interior rail members within the housing of an indoor air conditioning unit as the connected drain pan structure and modular coil are operatively inserted into the housing through an access opening therein. Accordingly, the drain pan may be secured to the modular coil, and the coil/drain pan assembly mounted within the air conditioning unit housing, without the conventional necessity of using sheet metal screws to effect these connections.