Temperature sensor is mounted Flush in a wall for unnoticeable installation. Can be painted. Diameter is 1.5 inches, requiring a 1 inch hole for mounting. Should not protrude more than 1/32 inch. Sensor is then home run back to a TT module Sensor. (APR-8061) then into the APR-8870 Communicating Thermostate. Maximum cable length between TT module and remote sensor 300 feet.