Trimmable heater apparatus, comprising: a heater mat which extends along a longitudinal length (L) and comprises a plurality of heater mat segments , wherein each of the heater mat segments extends along a respective portion of the longitudinal length (L) of the heater mat , and wherein each of the heater mat segments comprises a segment heater element , and wherein the respective segment heater elements are separated longitudinally from each other by respective trimmable distances (D); a power bus comprising a first power conductor and a second power conductor which span all of the trimmable distances (D) between the segment heater elements to extend in juxtaposed relation with at least a portion of each of the heater mat segments , wherein each of the respective segment heater elements is connected electrically to the first and second power conductors at respective segment connection locations in the power bus that are in juxtaposed relation with the respective heater mat segments which comprise the respective segment heater elements such that any two of the plurality of heater mat segments are severable from each other in the trimmable distance (D) between the segment heater elements of said two heater mat segments in a manner that does not affect the electrical connections of the segment heater element in at least one of the two heater mat segments to the first and second power conductors in the power bus ; and a displaceable cover sheet overlaying the heater mat and the power bus , wherein the displaceable cover sheet is displaceable in a manner that exposes ends of the first and second power conductors . a heater mat which extends along a longitudinal length (L) and comprises a plurality of heater mat segments , wherein each of the heater mat segments extends along a respective portion of the longitudinal length (L) of the heater mat , and wherein each of the heater mat segments comprises a segment heater element , and wherein the respective segment heater elements are separated longitudinally from each other by respective trimmable distances (D); a power bus comprising a first power conductor and a second power conductor which span all of the trimmable distances (D) between the segment heater elements to extend in juxtaposed relation with at least a portion of each of the heater mat segments , wherein each of the respective segment heater elements is connected electrically to the first and second power conductors at respective segment connection locations in the power bus that are in juxtaposed relation with the respective heater mat segments which comprise the respective segment heater elements such that any two of the plurality of heater mat segments are severable from each other in the trimmable distance (D) between the segment heater elements of said two heater mat segments in a manner that does not affect the electrical connections of the segment heater element in at least one of the two heater mat segments to the first and second power conductors in the power bus ; and a displaceable cover sheet overlaying the heater mat and the power bus , wherein the displaceable cover sheet is displaceable in a manner that exposes ends of the first and second power conductors . <EOS>