What does it mean when 'Cool On' or 'Heat On' is flashing in the display?
When 'Cool On' or 'Heat On' is flashing in the display, the compressor minimum-off timer is active, which prevents the compressor from restarting too early after a shutdown. You should wait up to five minutes for the cooling or heating system to turn on. When the minimum-off timer expires, 'Cool On' or 'Heat On' will appear solidly in the display and the compressor and fan will turn on. (Pages 14, 22)
From the official Honeywell TH5110D Home Programmable Thermostat manual, page 22
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