Mini split with transfer fans or ducted unit
I’m building a small well insulated and sealed house in Kentucky Zone 4. I’m trying to decide if my HVAC plan will work. I want to put in a 12k mini split in the living room area. I would have an efficient bath fan pulling from the open area and dumping into the two bedrooms and utility room. I planned on having open vents from the crawl space to the rooms above as a passthrough. I’m going to have a ERV pulling from the crawl, bath, and kitchen. Dropping fresh air back into bedrooms and living space. I will have a dehumidifier pulling from the bath, living area, and crawl space. Dropping back into the crawl space. Cool calc calls for Heating of 9555, Cooling of 7347, and CFM of 216.
Walls are r23+r6 zipR. Ceiling will be R60 with raised heel trusses. Crawl will be sealed and insulated probably R10-15 ish. Windows are Alpen R9 tilt turn. House is 36×28 1008 SF. 2 bedroom 1 bath, open living/kitchen.
We will keep the bedrooms and utility room closed 24/7 due to cats. I also planned on insulating the bedrooms for sound. My main question is will we have problems temperature variation, or will the air get mixed enough with the dehumidifier and transfer fans? I really don’t like the ducted mini split options, they are much less effecient and cost more. They also don’t ramp down quite as well. I have also read on here that transfer fans don’t work. I was just hoping with a tight house and a low load that it might be possible.
Thanks for any advice. I’m going to add a floor plan for you to see the layout.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
I think it would be fair to say that every time this topic comes up, opinion divides between people who do the math and point out it won't work, and those who want to do it and wish it would.
Like a lot of subjects -- phase change storage, thermal solar, geothermal and my favorite, "thermal mass" come to mind.
Undercut the doors and locate ERV registers to mix air and even out delta tees. The ERV will basically make a compartmentalized floor plan behave as an open floor plan with regard to the mini split. This works in tight well insulated low load houses.
What Malcolm said—forget the transfer fans. I was sure it would work too. Good luck.
I would go with a fully ducted unit on a new build. Overheating the bedrooms would be my primary concern with a single ductless. I would do the math on whether running a bath fan plus a ductless unit is really more efficient then a ducted unit. There are so many variables that determine exactly where you end up on the performance curve. You could even argue for a small amount of over sizing here like a Fujitsu 18RLFCD.
Thanks for the replies.
I used an online calculator comparing a 12k 30 seer 13 hspf to a 12k 20 seer 13 hspf. It showed a difference of about $30. Much less than I was expecting. Running the fan at full steam 130 cfm for a year would cost $8 per year.
So for comfort sake it seems like the extra few dollars a month is probably worth it. My next concern is installation. I’m going to be building as much of this house as possible. I’ve installed a ductless minisplit. That was easy.
How can I make the ducted unit DIY friendly? My plan for ducting would be to have a return/filter in the hall, backside of the small bedroom closet. Drop down into crawl, minisplit, then supply ducting. Can you really find sheet metal places that would make the custom filter box return plenum and the supply side box? I seem to get blank looks when I ask for the uncommon stuff around here.
Thanks again for confirming what I was worried about on the temperature variation.
The slim ducted units are small enough that you can mount it to the ceiling or into the back of a closet.
Some like the Fujitsu and Carrerir/Medea units can be mounted vertically, so they only take up about 3'x1' of floor space.
The simplest return is to covert the unit to bottom intake (which would be side intake if installed vertically) and mount a larger return air filter grill right against the unit over the intake.
For supply plenum the best is the pre-made ones such as this:
The other option for a plenum for these is to take a length of 8x12 rectangular duct, cut it to 3' and cap the ends. On the 8x36 side, cut an opening for the outlet of the slim unit. From there you can use standard takeoffs mounted to this duct to feed your zones. See attached, the unit is right in the corner.
You can do the same thing for the return. The other option is to use a return cold air boot backwards. That is: drywallwall->filter frame->cold air boot down to crawl->stack eblow->mini split. Since the widest boot is 24", you'll have to make a cover adapter to connect to the ducted unit intake but that can be a simple piece of sheet metal with a 8x24 cutout.
Thanks for the very detailed explanation. We might have enough space to put it in the dropped ceiling of the hall. The rest of the space will be 9’
I DIYd mine with duct fittings from Menards
I’ll have to check out Menards’. Thanks