Fluid dynamics is indeed a complex subject, I found this on my travels, it seems to be one of the more approachable and comprehensive guides: https://www.mcgillairflow.com/pdf/productlit/ductSysDesign_g...

Aside: paraffin wax is used in actuators, e.g. automatic green house window openers, on account of its impressive expansion. The MVHR I have also uses it for automatic "summer" bypass of the heat recovery path.