Abstract: Interior space customizable, re-customizable, modular, hydronic system, comprising: one or more modular base heating and/or cooling units, each adapted for residing between studs of the room's wall and selectively comprising one of a frame adapted to be completely recessed between studs, partially (e.g., approximately ½-way) recessed between the studs, and not recessed at all but still substantially located between the studs, each modular base unit adapted for successive repeated linear implementation with other modular base units of like composition to meet greater or lesser heating and/or cooling needs of the interior space; and one or more contiguous face cover members, and optional edge members, adapted to be secured to corresponding modular base unit(s), wherein the face cover members and optional edge cover members each comprise peripheral edges adapted to achieve, singularly or in combination, an aesthetically coherent face, and optional aesthetically coherent edge cover members, for the system.