The greenhouse vents example reminds me a lot of a float valve (or ballcock). It's a simple mechanism that used buoyancy to either open or close valves as wate

The greenhouse vents example reminds me a lot of a float valve (or ballcock). It's a simple mechanism that used buoyancy to either open or close valves as water level changes in a container. https://en.wikipedia.org/wiki/Ballcock https://en.wikipedia.org/wiki/Ballcock