Noquacks hit the nail on the head. Boric acid. I mix it with slightly watered down honey or syrup to get their attention. You want the ants to get it, not stick to it. They will take it back to the nest where others will enjoy the treat. Boric acid works from the inside out. It's abrasive to the ants exoskeletons and kills them.
Never, ever, ever spray, as ants will split and form another colony. I can't stand the thought of them in the house. You can't use the boric acid alone in the a/c vents as it is powder form, but try to get it as close as possible to the nest. Your goal is to kill the queen. I found it very satisfying one night when I had to get up to use the bathroom and found queenie staggering around on the bathroom floor. I put her out of her misery.
Every two months I'll place the boric acid around the water pipes that come up through the floor or anywhere else they might try to get in. A ants needs will change come fall. The will take the sugar it's all they can find, but they also go through a starch phase. I'll then mix it with corn meal.
Boric acid is only poisonous if taken internally or inhaled in large quantities, but it's still toxic, so take into consideration children and animals. It's cheap and can be purchased in some grocery stores. It will not kill fire ants.
It took about 5 days until I saw the last one. Three months later I figured out where they were coming in. They came in next to a tiny hole at a water spigot, climbed the copper pipe and disappeared into the subfloor. I moved a piece of wood near the copper pipe they were using and there had to be a million dead ants that fell off the copper pipe.
Best of luck to you!