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Review my minisplit design, please

Schwaby | Posted in General Questions on

Hello GBA community.

I really appreciate all I’ve learned from this site over the last several months in my quest to solve my residential HVAC puzzle.
I’ve got a house built in about 1960.  Sits on a slab.  Climate Zone 4A, Northeast Kansas, USA (66049).  Design Temp 4 degrees F.  House is a 78′ (E/W) x 30′ (N/S) rectangle = 2,340 SF.  Deduct from that a 6’x23′ porch = 138 SF and deduct another 20’x20′ garage = 400 SF.  That’s 1,800 SF of conditioned space.  House is 2×4 framed with some amount of pink batt insulation in all the walls.  There’s an unconditioned attic.  The ceiling has 2×8′ framing that’s pretty well filled with some sort of blown in material (fiberglass or cellulose or something?)  The blown-in material is anywhere from 8″ to 12″ in depth throughout the attic.  The garage is uninsulated, unconditioned, and very drafty.  Our doors could use some better weather stripping.  Our windows are original.  Single pane plus a storm.  The office space and the mudroom are the two coldest rooms in the house.  There’s a whole-house-fan above the mudroom we plan to remove.  We plan to generally see if we can tighten up the envelope a bit.  We have a fireplace, unfortunately (someday I may rip it down).
Our house is currently heated and cooled by a forced air system.  The supply ductwork is in the slab (cardboard sonatube forms).  The return duct is in the attic.  The condensate pan has trouble keeping up in the hot and humid times and water runs down from the furnace and collects in the supply duct plenum under the furnace.  It smells musty sometimes and we believe there’s a good colony of spiders and other bugs living in our supply duct.  Our furnace is original, the AC condenser is probably 15 years old.  I think our electrical panel is 100 amps.  None of our electrical circuits have grounded outlets.
That pretty much sums of the existing conditions worth noting.  Someday in the next few years we’ll put a PV array on the roof (likely DIY).  We’ll also probably do some upgrades to the electric.  Our water heater is currently gas, only 1 year old, and our stove is electric.  Dreaming of putting an ADU in backyard or expanding house for granny-apartment.  Wouldn’t mind having the garage better conditioned some day.
For my top priority, I’m on a mission to install a ductless mini split system to replace our current HVAC.  Prior to reading much on GBA I thought I would have a 5 head, 48,000 BTU system.  Now I know that is total overkill.  I also know I should run a manual-J or some other load calcs, but frankly I don’t know whether I’ll get around to it, it seems like there are tons of wags/assumptions involved.  We’ve got quite a few large windows on the South, and smaller windows on North, no windows East or West.  Shade from neighbor trees.  3′ overhangs.
I’m pretty convinced I want a mitsubishi hyper heat. I’m thinking a two head system should cover me.  I think one head in the hallway by the 3 bedrooms, leaving doors open during day.  The other head in the middle of the living / kitchen area.
Floor plan attached.
My wife likes the aesthetic of ceiling cassettes.  I understand the install is a little more work, and they’re a little less efficient, and they’re more expensive.  We would be fine with wall units.  I assume I could use my existing electrical wiring to the existing condenser.  I’m not sure if my two head system should run on 1 or 2 condensers.  I understand two condensers would be more efficient, but likely not as cost effective up-front.
I think I need something like 20,000 BTU.  I am wondering which head and which head location will push air through the bedrooms best (2 bedrooms already have ceiling fans).
I think I’ve got DIY capability for everything except vacuum work.  Are precharged linesets going to be sufficient?  If I can find a HVAC guy willing to come help me charge the lines and commission the system I’ll probably do that.  I have a BSME and have worked in general contracting for 10+ years and am pretty handy so I think I can handle it, plus I don’t trust the local HVAC contractors as much as myself based on conversations so far.
I’ll probably by the gear online.  Looks like I could get a 18,000 BTU 2 head 9+9 non-h2i for $2,600 + linesets and stuff.  Looks like a 18k BTU 3 head 6+6+6 h2i is $4,500 + misc.  Both of these are ACWholesaler prices.
Any suggestions on best place to buy this?  Best way to confirm my sizing calcs?  Hyper heat a good idea?  Mitsubishi is good bet right?  Best way to get air to flow in bedrooms?  Which wall unit features to I want (remote thermostat?, oscillating?  I’m not sure of all options)
I had kicked around the idea of a ducted system for the three bedrooms but I just don’t like ducts, and the install is quite a bit more intimidating.  That’s my brain dump.  Looking to purchase this stuff within next 30 days and hopefully install shortly thereafter before it gets to hot to go up in the attic.  After we get a new minisplit system running we’ll be able to turn our furnace closet into a pantry and seal up the ductwork and hopefully have better air quality, a better performing house, lower utility bills, etc.  Then onto the next project.
Thanks for all the articles and conversation I’ve been able to read so far, I’m looking forward to any feedback.

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  1. Expert Member
    Peter Yost | | #1

    Hi Matthew -

    I would strongly advise you get a BPI-certified home audit before you do anything else. The building you describe is ripe with opportunities for building enclosure work that will have a significant impact on the sizing, performance, and thermal comfort delivery of whatever HVAC system you ultimately select.


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