A hot air balloon is a bit like a bubble in water. It rises in air when the mass of relatively cool surrounding air displaced by the balloon is greater than the mass of the balloon and the gondola that hangs below it. There is a big opening at the bottom of the balloon where a propane burner that heats the inside air is located. Air is free to move in or out of the balloon through the opening. The heated air inside is less dense than the surrounding cool air. The balloon has to have a volume great enough so that air heated to some maximum temperature has a low enough density to allow it to float. What is the buoyancy? What is the volume? do we use archimedes principle?