Nest Thermostat with Humidifier

Hey there, I replaced my old thermostat (an Emerson 1F95-1291) with a Nest thermostat. I've got everything working - including the humidifier (just putting it in the * plug) worked great - tested it and it runs. However, I'm uncertain if it should be run with heat or if it can just run with the fan.

How would I determine which is necessary? If it depends on the furnace, it's a Trane xv95.

Thanks!

Nest Thermostat with Humidifier