Home Automation RC-80BZ - Manual (Page 5)

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Common wire is required in “cool only” applications
Figure 6 - Single stage cool only thermostat
* Common wire is required to use the display backlight in the RC-80ZB. Also use
if heat, cool, or fan relay cannot supply 15mA to power thermostat, without activating.
Figure 7 - Single stage heat/cool thermostat