The present invention is a system of wireless, battery operated temperature sensing devices which transmit ambient or other sensed temperature information to a programmable thermostat. The compact construction and multi-level programming of the temperature sensing devices for only periodic transmission of temperature information to the programmable thermostat permits years of operation with power supplied by small, inexpensive batteries. The temperature sensing devices are therefore easily installed at any location within about 100-200 feet from the programmable thermostat. The present invention also comprises a method of retrofit for the programmable thermostat of U.S. Pat. No. 5,449,319, whereby a compact receiver device is connected to available connections for input of remotely transmitted information and the programming of the programmable thermostat is adapted to receive, store, display and control environmental control devices according to the input temperatures received from the temperature sensing devices.