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Two minisplits vs one with damper for great room

CarsonZone5B | Posted in General Questions on

Hello GBA, I could use some advising and gba forums seem to almost always pop up in my technical house searches.  

I’m an owner/builder trying to dial in my hvac a bit.  Getting responses from hvac contractors here locally has proven a bit difficult. 

I want to heat/cool the house with minisplits but the wife does not like the head units so I need to put in concealed duct units.  This will be a timberframe home with no real ceiling/attic space for ducts.  It’s too bad they don’t just have a unit designed to be plugged right into a soffit, stair, under a cabinet, etc.  My thought was to put the concealed duct indoor unit under the stairs, with return coming in through a grill built into the stair risers and the supply coming out 2-3 grills in the wall of the stairs aimed at the floor of the living room and kitchen.  It’s a great room about 25′ tall, so I was thinking this may heat the floors and the upstairs loft, and cooler air coming down the stairs would be sucked in, heated, and blown out over the floors forming a convection loop.  Does this seem logical?

The issue is these registers aimed at the bottom floors seem like a terrible way to cool the great room, although it will be a pretty tight house.  I was thinking I could put a separate unit upstairs in a dropped ceiling above the bathroom and blow cold air out over the ceiling.  This, however, adds a bit to the cost and takes up more room so I was wondering if it would make more sense to just run another duct going up through the wall next to the stairs and blow the cold air from the unit under the stairs to the ceiling from there, using an electric damper to shut off the “heating” grills when cooling and vise versa.  I’ve read that I need to keep my ducted minisplit runs quite short though, and this would be a straight vertical run going up (which I assume is bad for cold air) about 17′ and I couldn’t find information on using dampers/zoning with ducted minisplits.  Finding much information on ducted minisplit installs at all is difficult.  

Any advice greatly appreciated,

GBA Prime

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