FIG. 1 is a front view of a heater showing my new design;

FIG. 2 is a rear view thereof;

FIG. 3 is a left side view thereof;

FIG. 4 is a right view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is a perspective view thereof.

The portions of the heater shown in broken line form no part of the claimed design.