The hot air that makes a balloon round and gives it the lift required to carry all that weight is not just warm – it's hot. The temeprature at the crown – thats the highest point in a balloon – is around 100 degrees C, the same as the boiling point for water!
So, where does all this heat come from? The burners. They work on a similar principle to a gas stove, although a hot air balloon burner system uses liquid propane which is vapuorised in stainless steel heating coils before it is ignited at the jets by a pilot flame.
Just as balloons come in different sizes burners do too. A bigger balloon will require a greater power to lift it. The pilot uses the burner to heat the throughout the balloon flight and part of the trill in ballooning is the loud woosh of the burner.