Ventilation is the number key to keeping a crawl space comfortable. Open the vents in the spring and summer, close the vents in the fall and winter to help moderate the temperatures and humidity.
The best way to heat and cool a crawl space would be to branch off one of the ten or eight inch duct lines from a rarely used area into the crawl space.
Generally, the crawl space under a residence is not heated or cooled. If you are experiencing water line freezing, insulation of the water lines and the outer walls normally is enough.
