The sensor should be mounted on a wall which best represents the heat load on the building (i.e. a north facing wall for most buildings and a south facing wall for buildings with a large south facing glass area). The sensor should not be installed near heat sources such as exhaust vents or window openings.
The outdoor sensor can either be mounted directly to an outside wall or onto a standard electrical box. Wiring can enter the sensor either through the bottom or the back of the plastic enclosure.