Feedback on this Minisplit System Plan
I’m considering a ducted horizontal mini-split on our 2nd floor (within the thermal envelope) to heat/cool four rooms (about 600 square feet total). The return would placed outside of these rooms, in the upstairs hallway.
Downstairs, I would place a ductless wall-mounted mini-split to cool/heat an open floor plan (700 square feet).
Would there be any concerns with this design?
The downstairs ductless unit would, in part, be heating/cooling the upstairs hallway. I’m curious if having the return on our upstairs unit pulling air from the downstairs would pose any challenges?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
This is exactly the setup I have and it works great. For smaller ducted mini splits, door undercuts should be sufficient to provide a return air path. I would consider installing a remote thermostat in the master bedroom (if there is one) instead of relying on the sensor in the return side of the air handler. This will allow you to control temperature based on the temperature in the room, not the hallway.
See if you can install the ducted unit where you can use a gravity drain for condensate instead of relying on the pump. This reduces a point of failure, but it’s ok if you can’t. Try to do a Manual J for proper sizing.
In my home I only need to run the downstairs unit for heat as it keeps the upstairs bedroom about 3 degrees cooler than downstairs. In the summer I only need to run the upstairs unit as it keeps the downstairs 2 degrees warmer. Obviously your results will vary.
Thanks, Kyle. This is helpful feedback. Do you ever need to run the systems at the same time, and if so, does it cause any issues? Also, any recommendations on keeping your attic ducts in the thermal envelope? I've been reading http://ductsinside.org, and it seems like the inverted soffits would be my best option (https://ductsinside.files.wordpress.com/2020/10/inverted-soffits-fact-sheet1.pdf).
I have only run both on really cold days while my wife was also working from home upstairs. No issues. The only slight concern is what I mentioned previously about thermostat placement. As warm air rises upstairs the temperature in the hallway could be warmer than the bedrooms with the doors shut. Placing the thermostat in one of the bedrooms should address this.
I ran my duct work in regular soffits above the bedroom doors. The air handler was mounted vertically in a closet. I think inverted soffits would be difficult to air seal. Is this a new or existing home? Where are you going to put the air handler?
Thanks, Kyle. Again, very helpful.
This is an existing home, and my closet space upstairs is tight, so I'm trying to avoid eliminating this space. If I kept it horizontal, I could put a traditional bulkhead above our stairs, and into the rooms. (However, I'm still willing to explore an inverted soffit in the attic w/ an air barrier. In this instance, the unit would be in the attic as well.)
Either way, if I create bulkheads or an air sealed tunnels in my attic, it seems like the same amount work, but I'd enjoy hearing a different perspectives.
Also keep in mind that an HVAC tech would always prefer to install into the attic, but would charge more to service it. So in the end the only thing you end up with is just more operating cost.
Well sealed and insulated your reverse soffit can work, essentially you are making the equivalent of a plenum truss. Moving the attic insulation, sealing around ceiling joists and wiring plus working in the attic, I think this is much more work than bulkheads.
Above the staircase is a great spot for a 2nd floor mini ducted unit. You can even simplify the install a bit if you change the unit to bottom intake and install a large filter grill directly against the underside of the unit. This saves you having to build a return plenum. There are also duct boards available for most units that let you connect standard round ducting which makes a home run type setup pretty simple to do.
Thanks, @Akos. For aesthetic purposes, I'm willing to put in a little extra work to make a reverse soffit, and depending upon the location of the install, I might be able to create an access panel in my 2nd floor ceiling for servicing. (Thanks for the tips regarding standard bulkheads as well.)
A quick glance at the the install diagrams for a mid-static, 9K BTU system, it seems like the reverse soffit should be at least 5' wide x 1' high (http://mylinkdrive.com/viewPdf?srcUrl=http://enter.mehvac.com.s3.amazonaws.com/DAMRoot/Original/10005\SEZ-KD09-18NA_Install_KB79K740H02_1-09.pdf). I would defer to an installer for exact dimensions (inclusive of ducting needs). Any tips in this respect would be much appreciated.
5' x 1' should be enough for the unit but you also have to look at the ducting. The reverse soffits also have to contain all your ducting to each room, so I would size based on that as well.
The SEZ is a low static unit, but that is fine for a simple install like this. Still have to watch your ducting a bit though, these units only do around 0.2" wg. Besides the ducting, you generally have to go with a filter about 2x the size based on furnace rule of thumb or go with a 4" filter instead of a standard 1". The nice part about these is they are pretty much silent.
The Mits drawing shows the access door off to the side. A much better one is a larger access panel that is about the size of the indoor unit plus a bit extra width on the side where the refrigerant connections are directly under the unit. I also like to install the return filter grill directly into the access door.
It might be worth while to post a sketch of the upstairs floor plan.
Thanks, Kyle. Here's my 2nd story floor plan. (It's drawn to scale, and my dimensions are +/- . 5"). Per @Akos above, I would likely put the unit above my hallway to create an easy access panel for servicing.
You could drop the ceiling in the hallway to house the air handler and all of the ducting.
If you go the reverse soffit route, figure out ahead of time what registers you are going to use (ceiling?) and account for the length of these and other interconnecting fittings (elbows) to determine the soffit height you need.
With the layout show, the simplest would be a vertically mounted air handler in the closet hallway and a couple of short bulkheads to each room with registers right above the doors.
Depending on the actual size of the unit you are looking to install, you can probably mount the air handler against the closet wall towards the staircase and have the return on the bottom there. This way you only loose about 1' of closet width.
This is much less work than the reverse soffit in the attic.
If this is out of the question, the next best thing would be to put the air handler in the same spot, run the ducts up to the attic and to each room. Encapsulate each run in the attic including duct boot in spray foam. One of the smaller two part spray foam kits would cover the entire install. Make sure to commission the system and check flow rates before covering them in SPF.