You may have seen freestanding patio heaters that have just one dome at the top, whilst other models have 3 domes stacked on top of one another.
The single dome heaters are generally far cheaper than the triple dome heater, but they are not as powerful.
Triple dome heaters are designed to reflect the heat from the heater in 3 different directions rather than just one. Single dome heaters, on the other hand, can only reflect the heat downwards to an immediate area below the heater. This means that the triple dome heaters have a far larger range in comparison to the single dome heaters and those who want to heat a larger area of their garden prefer them.