This is covered in the Overheating Problems article, but I'll touch on it here. If you find that you don't have enough heat coming through the vents, it could be because your engine’s thermostat is stuck open. If this happens, your engine will run much cooler than it should and you won't have good heat at the vents.
Don't just replace the thermostat if you don't have good heat. Look for other causes first. One way to check if your thermostat is working correctly is to feel the hoses going to the radiator. When the engine is at operating temperature, the top hose should be hot and the bottom hose should be cool, or cooler than the top hose. If both hoses are cool to the touch with the engine at operating temperature, your thermostat could be stuck open. The only way to really find out is to remove it and inspect it. If you find it's stuck open, replace it, bleed the cooling system, and recheck your heat output. Don't skip bleeding the cooling system. Remember the number-one cause of no heat is air in the cooling system. One of the main causes of air in the cooling system is not bleeding the air out after servicing the cooling system. I recommend original equipment thermostats for best results.