How to get HVAC systems to flow?

Think back to first principles:

Hot air rises. Cold air falls.
Furnaces and heat pumps work with more efficiently when there is a greater differential of air temperature coming in to the furnace/heat pump
In hot weather - where does heat come from?
In cold weather - where is heat lost to?
Pressure equalizes - if you take air away from a place - other air will flow in
- Air pressure equalization

If you have a house with a single heating and cooling system then it needs to be adjusted to flow differently when it is hot versus when it is cold.

In my 3 storey house the top floor is the one gets really hot. I had to install a separate heat pump on my third floor to better balance my home.

My ground floor is like an ice skating rink in winter. The flow of warm air is not enough and the cold air intake is in the ceiling which means the cold air on the floor just sits there not being circulated.

That lead to a series of experiments to fix this problem.

There is no vent into or out of my ground floor bathroom. Not a big deal in summer but in winter it’s like an icebox.

It needs airflow.