In EcoBee click on "schedule" and create a comfort setting "Air out" with the fan on and minimal heat and maximum cooling temps to turn off the furnace and turn on the fan to circulate air.
If
Carbon monoxide warning
This Trigger fires every time the Nest Protect you specify detects warning carbon monoxide levels.
Then
Set thermostat comfort profile until next transition
This Action will set the thermostat into a hold using the settings from the specified comfort profile. This comfort profile hold will remain in effect until the next next comfort profile transition in the program.
ID PxAFim6c
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