A communication system comprises a remote switching module having two signal terminals connected by just two conductors to the signal terminals of a local decoding module. Power is directly provided only to the local decoding module, which applies DC voltage of a first level across the conductors. The remote switching module has a voltage regulator which provides power of a second voltage level lower than the first level for operating a control unit in the switching module. The control unit controls a variable impedance having higher and lower impedance levels and which is connected across the switching module's signal terminals. The decoding module can detect the different impedance levels by sensing the current flow on the conductors. The switching module communicates with the decoding module by modulating the time intervals between changes in impedance levels. The system has particular utility for retrofitting cooling to a heating systems where preexisting thermostat wiring has only two conductors, say when an air conditioning function is added to a previously heat only system. The system is also useful when installing electronic thermostats on heating and cooling systems originally controlled by a mechanical thermostat.