Most manufacturers recommend at least three consecutive days of temperatures above 65 degrees to start up your AC in the springtime. And not to run your AC if it is below 60 degrees as it can cause damage to the compressor.
Additionally there must be power to your compressor for at least 3 hours before starting it. This allows an electric coil to warm the compressor, vaporizing the liquid refrigerant. Liquid refrigerant in an operating compressor will damage it.
If Ecovent detects a very low vent temperature it will disable your Ecovent system to prevent unsafe conditions for your compressor, which could happen if you turn your AC on prematurely. Please change your mode to Heat and enable the system to clear this alert.