Honeywell RTH9580 Wi-Fi User Manual page 13

Installing your thermostat
Important!
The Wi-Fi thermostat requires a C wire to operate. The C, or common, wire
brings 24 VAC power to the thermostat. Many older mechanical or battery operated
thermostats do not require a C wire. If you don't have a C wire, try:
• Looking for an unused wire that is pushed into the wall. Connect that wire to C and
check that it is connected to the 24 VAC common at your heating/cooling system.
Check the video section at wifithermostat.com
Note:
Not all heating/cooling systems label the 24 VAC common C. Check your system
manual or contact the manufacturer to find out which terminal is the 24 VAC common.
View the Alternate Wiring videos at wifithermostat.com/videos
Wiring
conventional
For
conditioner), see page 12. See "Glossary" on page 73 for further definition.
heat pump
For a
definition.
heating/cooling systems (natural gas, oil or electric furnace, air
system, see page 14. See "Glossary" on page 73 for further
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