The basis on how a balloon works is that warmer air rises in presence of cooler air. Named "principe d'Archimède" - this is because hot air is lighter than cool air, as it has less mass per unit of volume. Hot air balloons are in fact an ingenious application of basic scientific principles.
A modern hot air balloon has to be very large, as it takes such a large amount of heated air to lift off the ground. For example, you need almost 77,000 cubic feet of heated air to lift 1100 pounds of weight! To maintain the balloon in the air and allow rising, hot air needs to be propelled upwards into the balloon envelope using a powerful propane burner.