Carpenter ants have a valuable role in ecosystems—they burrow into and nest inside dead trees and expedite the process of decay—but in one's home, they should be killed. While their expressed purpose is beneficial, carpenter ants easily achieve pest status when they invade and damage wood in houses. Carpenter ants are usually between 1/4- and 1/2-of-an-inch long and black. While they can nest in moist or dry wood, they prefer moist wood; therefore, their nests are more likely to be found in wood that is moistened by water leaks of any sort and they are especially common behind dishwashers or inside hollow porch columns.
There is often no external sign of damage by carpenter ants. If, however, you do find damaged wood, locating the nest should be your first priority. This, however, is not easy. According to the University of Kentucky, which cites other studies, carpenter ants rely on scent trails to locate food and to guide them from satellite colonies to parent colonies. While this makes locating the nest difficult, you can observe the ants' trailing behavior and follow them back to their nests.
You can expedite the process of following them to their nest by feeding them late at night. If you see carpenter ants, feed them small bits of diluted honey from the backs (read: non-sticky sides) of tape. Once they're fed, the ants will return to their nests. This process may require some patience on your part; rest assured, the ants will eventually make their way into their nests.
Once you have located the nest, you have options. If the nest is inside, you can drill small holes into the wall and insert boric acid or some other powder insecticide to kill the ants. You should not spray liquids randomly into your walls, where it may come in contact with wires or circuits. Carpenter ants in your home may have nests outside. If you follow the ants and their nests are outside, you can spray or drench the nest liberally.
If you cannot locate the ants' nest, you may have to call a professional exterminator. Ensure that the exterminator that you call has a good reputation.