Changing to minisplit questions
So I’ve been considering converting to an all electric house since solar is looking a lot cheaper these days and due to a combination of not having AC and finding out this week that my flexible ducts in the attic have been chewed up by rats I’m looking at installing a multi-head mini split. I’ve got few questions now that I’ve gone through the work of doing I think a mostly accurate manual J at coolcalc.com
Before I dig into it I’m in San Jose CA, and here are the top level calcs I have:
Of that 1401 square footage I have ~1000 great room that is living, kitchen and dining (the 3 bedrooms are all very small, biggest is ~12×14).
My house is basically split in half with new construction 2×6 r-19 walls and then 100 year old construction no insulation walls that are very thick (inside -> out, lath and plaster, full dimension 2×4 cavity, 1-2″ clapboard siding, more modern wood siding, stucco siding). Underneath the old half of the house I also have ~7′ high basement that I just finished air-sealing and adding insulation to which I’m hoping will help with the envelope.
Onwards to how to select a mini split :). So looking at this report I only need ~14K BTU of heating from my mini split system? that seems awfully low for a house that had virtually no attention to air sealing (I’ve done a decent job in the attic and the remodel had all the top plate and bottom plate penetrations spray foamed) and half the walls insulated – attic has r-30 and crawl-space (about half of my floor square footage) r-19. If this is the case, laughably the HVAC contractor installed an 80K BTU 96% AFUE furnace (Goodman Model: GMSS960804CNAA) in the attic with insulated flexible duct, and since starting to learn more about all of this late last year I’ve had a feeling that I’ve been oversized since my furnace always seems to run in short bursts. Now, only needing a 20K mini split would be great news (and I guess is close to a rule of thumb I’ve seen of ~21K BTU for ~1400 sq ft?) since I was expecting something closer to 30K or 36K BTU, but I wanted to double check that my numbers seem reasonable?
Also, I was originally putting a head in the great room and in the two biggest bedrooms here are the calcs for all 3 rooms:
BedRoom1 (1.5 exterior walls, r-19):
BedRoom2 (1 exterior wall, no insulation):
However I’ve read a comment elsewhere from Dana that heads don’t make sense in smaller rooms as you end up short cycling and losing efficiency.
Not listed here is my tiny ~9×11 bedroom that has two exterior uninsulated walls but is heavily shaded by an oak tree that is an evergreen. I also did not list the basement I just insulated because it only gets chilly in the winter and pretty much doesn’t get over 80 in the summer. My main questions are:
1) Does a multi head configuration work?
2) Should I be concerned with just one head in the greatroom (50′ long x ~12′ wide)
3) Should i be concerned about that tiny room never being comfortable since it is uninsulated and leaky?
4) On the face, do my numbers seem somewhat accurate?
5) Anything else I’m overlooking?
Also as a side note, if it matters, I plan on doing the install myself and then hiring someone to evacuate, leak test and charge the system, hopefully this doesn’t seem crazy. I’ve mostly lost all faith in HVAC contractors after learning how things should mostly be done and seeing the install I paid for.
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