No householder wants to see an infestation of ants coming out of the heating vents. As soon as it happens, you need to take action to remove the ants and then be sure they don't return. This is a two-step process: finding where the ants enter the house and stopping them coming in, as well as eliminating those already inside.
Removing The ants
Clean the rooms where the ants have been very thoroughly with soap and water and a good cleaner. Remove any debris, such as food scraps, from the rooms; cleaning takes care of the dirt and removes the scent trails ants use for their routes. Clean once more with a solution of 10 parts water to one part vinegar.
Unscrew the heating vent covers. Use a vacuum with attachments to remove all the dust and particles from the inside of the vents, then wipe the vents clean with a cloth that's been soaked in soapy water.
Mix 1 cup water with ½ teaspoon borax in a mixing bowl and sweeten with approximately 1 tablespoon of sugar. Place a little of this mixture in empty soda bottle caps and put two of the caps inside each of the vents; the ants drink the solution, go back to their nest, but the borax is poisonous, and they die.
Spray orange oil down the vents; this will help to repel ants. Screw the covers back on the vents.
Keeping The Ants Out
Check the exterior of the house for all areas where ants can enter, even small cracks. Apply petroleum jelly to these areas. The petroleum jelly will deter the ants from entering as they won't be able to grip enough to move. Apply duct tape to larger cracks.
Spread diatomaceous earth around the areas where ants can enter the house, such as vents to the outdoors or cracks in basement plaster. Keep the diatomaceous earth restricted to areas where it can't be accessed by children or pets, and be careful spreading it, as it's a very fine powder. It kills ants by removing all the moisture from their bodies, quite literally desiccating them.
Clean the area around the outside vents or wherever ants can enter. Mix more of the borax solution and place it there for the ants to take back to their nest . If you find many dead ants close to the solution, then it's too strong; lessen the amount of borax.