Do you have a tropical species that you need to keep at the right temperature, or do you want to heat your colony in the winter because it is too cold in the house?
 A heat mat can help with this. Don't place the mat completely under the nest, but place it slightly below it so the ants can determine which part of the nest is most comfortable.
 If you notice that it is getting too warm in the nest, slide the heat mat slightly away from under the nest.