If you have any issue with air flowing back through the heat pump or (heat pump air flowing back up into the furnace) you can get backflow dampers that will only allow air to flow in one direction. they are gravity operated and you do not have to remember to manually set a gate when switching back and forth or using both systems. easy to find for the small round duct, would probably be a special order for the rectangular duct. Installing these would allow air to only flow away from the heat pump. and or furnace. with a backflow device you could rout the duct down to the horizontal duct and save some space in the cabinet (also remove some elbows and reduce friction loss in the ducting.
remember air will always take the easiest path out of the system, with the air from the furnace entering the side of the main duct it does not have anything to direct it one way or the other. even though you have ducted over to the vertical portion of the main duct, gravity may still try to send some of the air down and back to the heat pump. you will be able to see this by placing thin paper or yarn by the heat pump inlet when the furnace is running, any backfed air should move the yarn or paper.
Either way if you can you might want to test the system if you can before you cover it all up.
P.S. your project is looking great, I love seeing peoples hard work coming to life. Keep up the good work and posting your progress.
Dave
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