FIG. 1 is a perspective view of the boiler, particularly showing the inventive design thereof;
FIG. 2 is a front view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a back view thereof.
The broken-line disclosure in the views forms no part of the claimed design.