most thermostats are simply a normally open or normally closed contact. thus they are non-polarized. most heating/cooling systems use a low voltage ac at a very low current to the thermostats switch. not dangerous, but always assume faulty grounds and other possible installation errors. replacement is easy, remove old, install new.
some new systems use much highly integrated devices to sense and return feedback information to the main hvac controller, these require that you replace thermostat units with same or exact replacement type.
hapticz, March 2007
