Why does my aux heat run when the system is in cooling mode on my heat pump?
For heat pump systems, this is caused by a wire being incorrectly attached to the W terminal on the UWP. Verify there is no wire connected to the W terminal, as the W terminal should not be used for heat pump applications. (Page 10)
From the official Honeywell T3 Pro Programmable Thermostat manual, page 10
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