Thermosiphons are so effective that the Model T Ford used a thermosiphon for the radiator, as in no mechanical pump on the engine to circulate coolant. A number of pre-WW2 aircraft used/attempted it as well.

I accidentally made a thermosiphon in my house when I installed a circulator pump to get the hot water to my upstairs bathroom faster. When you use the upstairs bathroom in my house there's an extra light switch which turns on a tiny pump in my basement. It circulates hot water in a loop so when you turn on the tap you'll have hot water almost immediately instead of waiting a few minutes for the tap to warm up, but you could also turn off the pump to save energy. Turns out the pump was completely unnecessary. That system would circulate like crazy all day long with or without the pump due to the thermosiphoning effect. I installed a spring check in the system to snap shut when the pump was off to prevent that and save power.