To turn up the heat in our boots without bulky socks, we slip in Therm-ic's Insole Heat Kit. We trim the footbeds to perfectly fit inside our boots, shove our feet in, and press the power button. The rechargeable C-Pack 1700 Bluetooth batteries power the ultra-fine heating elements for 17 hours of constant heat and let us conveniently control the warmth level from the Thermic Heat Control app on our phone.