how can I set the mixing air between two thermal zones?

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Here is the description for the Air Wall Zone Mixing measure. It was a very basic proof of concept to look at using zone air mixing vs. conductive heat transfer between zones for models where the zone boundary doesn't represent a physical wall. It has a very simple formula that is by no means CFD, and it provides a user multiplier that can be used to do sensitivity analysis on more or less mixing between zones.

This measure replaces conductive heat transfer with zone mixing wherever air walls are used on matched surfaces or sub-surfaces for walls. A user argument is exposed for a coefficient that represents a target air changes per hour (ACH) for a room where the zone volume/the air wall surface area is the same as its zone height. As the room gets deeper the additional airflow per unit of depth decreases. If two zones have different mixing estimates, the lower will be used. If a smaller portion of an inter-zone wall is an air wall, that will also decrease zone mixing airflow. A construction will be hard assigned to the matched surfaces using the air wall, and the then boundary condition will be changed to adiabatic. This will avoid including both air mixing and conductive transfer across zones. Zone mixing objects will also be made for sub-surface air walls, but they can't be made adiabatic unless their base surface also is. A warning will be issued if that happens

The modeler description provides formula and some examples, as well as a PDF guide in the "resources" directory.

flow rate = zone mixing coefficient * zone volume/sqrt(zone volume / (air wall area * zone height).

You should get info messages that look like this

Add zone mixing between Thermal Zone: Space 101 and Thermal Zone: Space 104 with flowrate of 141.42 cfm

If there is enough heating load in the unconditioned zone B, then the amount of air mixing may only lower the temperature a small portion of the way to the temperature of zone A. Keep in mind there is mixing in both directions, so the hot air from B is brought into A while air from A is brought into B. If you system is autosized then it may get bigger to deal with the additional load on it.