This video goes over how to automatically turn on your fan when your furnace kicks on with your Nest Thermostat. This can happen if your furnace isn't correctly communicating with the Nest to turn on the fan when it heats up, and you can have the Nest automatically turn it on.
Note: Make sure your fan is working by turning on the fan only mode. If the fan does not turn on, there may be a problem with the fan or the wiring. Always be careful, if your furnace is running without your fan on, that can be an extreme fire hazard.
More information here: http://support-assets.nest.com/images...