Thermosyphon warming process

[Andy Bennett]

'so if the water enters the top hot and exits the bottom at room temperature (like the early photos) its running at 100% effectiveness.'

But I would suggest that in the earlier pictures it is more likely to be running at closer to zero efficiency, where there is no flow, rather heat is being conducted through the static water rather than there actually being any flow at all. What I don't know is what temperature is required to actually produce the flow without a pump to assist.