Heating ducts are used for heating and air conditioning in homes; the ducts deliver and remove air from throughout a home, working from a main heating or air conditioning unit, usually found in the basement. Cats are curious by nature, and if your cat or kitten manages to make its way into the heating duct in your home, even though it may not be in any immediate danger, you do want to get it out as quickly as you can.
Call it. If the cat is older and you have had it for a longer period of time, you will probably have a better chance of coaxing it out simply by calling its name. A younger cat may not respond as well and depending on how young it is, may not even recognize its own name yet.
Shake a container of cat food or the cat food bowl. Even at a very young age, most cats are aware of what it sounds like to have cat food poured into their bowl, and simply by shaking the bowl around to make noise, you may be able to lure it out. You can also try placing a plate of wet cat food (which smells much more potent than dry food) near an opened vent from the heating duct and leave the room; the smell should lead it out when it gets hungry.
Crumple a piece of paper up or use one of its toys to make noise with, such as a bell or squeaky toy. Anything that makes noise will attract its attention, and since cats are usually quite playful, this may be enough to get it to come out on its own.
Locate the section of the heating duct the cat is in, and tap -- firmly but not too hard -- to the side of it. Make the noise in the direction you do not want it to go, and it should frighten the cat enough to make it run out the other way.
Remove some of the duct pipe if you are unable to get the cat out using other methods. It may be as simple as removing a grill or two yourself, or you may need to call in a professional HVAC company to come out and help.