What is burned to heat a hot air balloon?

Hot air balloons are typically heated using propane, which is burned in a burner system. The propane gas is ignited to produce a flame that heats the air inside the balloon's envelope, causing it to become less dense than the cooler air outside. This difference in density allows the balloon to rise and float in the atmosphere. The pilot can control the altitude by adjusting the flame to maintain the desired temperature of the air inside the envelope.