The ornamental design for T800 thermostat, as shown and described.
FIG. 1 is an exploded perspective view of a T800 thermostat, showing my new design;
FIG. 2 is a front elevational view of the invention;
FIG. 3 is a left-side elevational view of the invention;
FIG. 4 is a right-side elevational view of the invention;
FIG. 5 is a top plan view of the invention; and,
FIG. 6 is a bottom plan view of the invention.
The broken lines showing a number in FIGS. 1 and 2 are included for the purpose of illustrating the invention in use, and form no part of the claimed design.