nest Learning Installation & Configuration Manual page 23

Forced Air HVAC Systems - Fan Activation
Depending on your fuel source selection, you'll need to determine when
the fan activates for your customer's heating system. Some systems will
activate the fan automatically when the thermostat calls for heat. Some
systems will rely on the thermostat to activate the fan.
The default setting for fan control on the W1 wire is ​ D on't activate
because most forced air gas heating systems will activate the fan
automatically when the furnace (W1) is activated.
If you test the furnace and the fan doesn't activate, change this option to
Activate​ . The thermostat will then call for both the furnace (W1) and fan
(G) when heating.
22