Is it safe to operate the cooling system when it is cold outside?
No, you should not operate the cooling system when the outdoor temperature is below 50°F (10°C), as this can cause possible compressor damage. This warning applies both when testing the system after installation and during normal operation. Refer to your compressor manufacturer's instructions for further information. (Page 6, 29)
From the official Honeywell CT3355 Home Programmable Thermostat manual, page 6
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