What key design element was necessary for early hot air balloons due to the continuous need for heat?

Answer

A circular opening at the bottom

Because the pilot had to continuously feed the fire beneath the envelope to maintain lift, early balloons featured a circular opening at the bottom, distinguishing them from sealed gas-filled craft.

What key design element was necessary for early hot air balloons due to the continuous need for heat?
inventionaviationflightballoonMontgolfier brother