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Ducted mini split plenum design

jp381 | Posted in Mechanicals on

Hi all, I am in the process of replacing a gas furnace with a 12k fujitsu low-static ducted mini split to feed the second story of my house. The first story has a single wall mounted ductless minisplit. The existing ductwork around the furnace air handler has one branch that comes directly out of the top of the air handler, a second branch that comes off an elbow, and then the elbow that leads to a reducing trunk with additional branches.  This is obviously not good.  

I am considering two options the duct work around the new mini split air handler is significantly better than the other. 

Option #1 is to keep the ducts where they are and attach the air handler to a plenum with three outlets in a line. Two six inch circles to feed the two ducts that are located before the trunk and then an 8×15 elbow to feed the trunk.  

Option #2, which I’m pretty sure is better, will be significantly more work. This option is to move the two ducts that currently require take offs near the air handler/plenum to the opposite side of the room so that they can be fed from the main branch. 

My question is how much better is option 2 given that it will be significantly more effort? I am using a low static air handler to feed 6 ducts with several long runs, so I don’t have much margin for inefficient fittings. And yes, I plan to make sure everything is ultimately sized properly with Manual D. 

Thanks for any guidance!

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  1. greenright | | #1

    How about you install the ducted air handler and see where you at flow- wise? as long as your.indoor coil has sufficient flow for 20-ish degrees delta t you might use simple cheap manual dampers to modify the air flow as needed…

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