The ornamental design for the outdoor unit for water heater, as shown and described.
FIG. 1 is a front perspective view of the front, right, and top sides of an outdoor unit for water heater showing our new design, wherein the portions formed of a fan, a fan guard, and an exhaust opening have colors darker than other parts;
FIG. 2 is a rear perspective view of the rear, left, and top sides thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a right side view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines shown represent portions of the outdoor unit for water heater and form no part of the claimed design.