Convection at Christmas

This decoration clearly works by convection. The candles heat the air, which expands, becomes less dense than the surrounding cold air and floats upwards. It was pointed out to me that more the condles burn down the faster the rotor turns. One hypothesis was that the wick is bigger on a burned down candle so there is a bigger flame and more thermal energy fuelling the convection. This could be tested by taking part burned candles and propping them up to the height of the unburned candles at the start. If the hypothesis is true then the blade should turn faster. Another hypothesis is that the same thermal energy is released but that the extra distance means the resultant force onn the hot air due to the density differences is able to accelerate the air to a higher speed, The air will thus have more momentum and thus a greater rate of change of momentum on hitting the blades. In other words, a bigger force on the blades so a greater torque and faster turning.