Convection: Convection in Air

He places a joss sticks into the first hole. The smoke of the joss sticks then went towards the direction of the candle flame. As the smoke from the joss sticks are hot air, they are already expanded and rises up. As they are in the box, they rises up and reaches the top of the box. When hot air touches the top, they are being pushed to the side by the recently warmed air, which is the candle flame in this case.

Convection in Radiators
Radiators heats up the room by giving off heat by radiation and convection. The radiator warms the air that is around it and the warm air rises, which will pull the colder air towards the radiator, and the radiator will heat the air and the cycle restarts again.

The reason why radiators are usually placed on the floor is because hot air rises, cold air sinks.