I claim the ornamental design for an installation unit in a heating or cooling system, as shown and described.
FIG. 1 is a perspective view of an installation unit in a heating or cooling system embodying my new design;
FIG. 2 is a right end elevational view thereof;
FIG. 3 is a left end elevational view thereof;
FIG. 4 is a front elevational view thereof, the rear elevational view being a mirror image thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken line showing of parts of the drawings is included for the purpose of illustrating use and environment and forms no part of the claimed design.