Abstract: A programmable thermostat has two way communication with a utility using a power carrier line signal across distributed low voltage power lines in a building downstream of an HVAC system transformer. The two way PLC communication includes transmission to the utility of response information regarding the response taken by the thermostat to a power “shed” command, and preferably also includes sensed temperature and/or temperature set point information. The HVAC system transformer includes an integral capacitive bypass path for PLC data transmission across the windings of the transformer. The thermostat positions its temperature sensor to avoid heat given off by the PLC receiver/transmitter and its electrical circuits as well as the thermostat microprocessor and the electrical power circuits therefore.