On Monday, 3 August 2020 17:50:53 UTC+1, Nick Odell wrote:
I am thinking about constructing a tube to suck hot air from near the
ceiling of a high-ceiling room and reintroduce it and mix it with the
colder air at floor level. I have seen these things at work in other
buildings but I do not know what they are called.

destratification duct

https://www.tombling.com/ducting/destrat.htm

destratification fans are usually used, but adding a duct improves efficiency

Owain