Automatic Fan between Two Rooms

[techiejohn]

I have a single HVAC system with 2 zones.

The upstairs zone controls 2 bedrooms, with the thermostat in one of them.

Because of the location of the rooms, either one is too warm or too cold.

I thought the best way to fix this would be to duct a pass through between the rooms with a fan.

However I'd like it to be automatic. That is, I'd like the system to turn on if there's some temperature difference between the rooms.

Is there some "differential" thermostat that could be mounted in the two rooms that would allow the fan to turn on when there's a temperature difference?

Can anyone suggest a solution? Since it's not needed, I would prefer that the main HVAC system not turn on to equalize the two rooms.

[Freon]

Where is the return for the upstairs... upstairs in the hall or downstairs? Assuming no returns in either bedroom, move the thermostat into the hall toward the return air opening. Make sure there is at least 1" clearance under each bedroom door and the flooring material. The design is for the air exiting the bedrooms to mix in the hall where the mixed air will register an average temperature for the rooms. One room will always be different than the other but by adjusting the supply register to each room you can better balance the temperatures. More air to a southern exposed room in the summer (AC) and more air to a Northern exposed room in the winter (heat).