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Community and Q&A

Changing to minisplit questions

AlexD2022 | Posted in Mechanicals on

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

Before I dig into it I’m in San Jose CA, and here are the top level calcs I have:

Heated square footage:1,401
Cooled square footage:1,401
Heated volume (above grade CF):14,010
Cooled volume (above grade CF):14,010
Exposed wall area (SF):1,190
Heating BTUH:13,791
Cooling BTUH:18,184
Sensible cooling:16,023
Latent cooling:2,161

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:

Total Cooling BTUH:
Total Heating BTUH:

BedRoom1 (1.5 exterior walls, r-19):

Total Cooling BTUH:
Total Heating BTUH:

BedRoom2 (1 exterior wall, no insulation):

Total Cooling BTUH:
Total Heating BTUH:

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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  1. DevinCarlson | | #1

    Those loads aren't too out of line but they do seem a bit high to me. I wouldn't overstate the air leakage if you've air sealed the attic and the basement. My old 1200 sq ft cape cod with original mineral wool in its 2x4 walls has a measured heat loss of ~20k btu/h @ -12.3°F and heat gain of ~6k btu/h @ 80.9°F.

    The 2017 ASHRAE design conditions for San Jose specify a 99% Heating DB of 37.7°F and a 1% cooling DB of 87.8°F. I'd be surprised if your heating load reached 10k btu/h, which would make your current equipment 800%+ oversized. Unless they thought you needed exactly 4 tons of AC their equipment selection doesn't make a lot of sense, as all smaller furnaces in that line support up to 3 tons and all larger units up to 5. Good HVAC contractors are definitely hard to find.

    I wouldn't put a single head on a dedicated compressor into a room with <1k btu/h loads, let alone a multi-split. With a basement available, I'd recommend a single ducted unit. A ducted unit would also allow you to condition the tiny bedroom and the basement itself as well as better address the great room. A poorly insulated bedroom is going to be uncomfortable without any airflow, especially if the door is closed for an extended period of time.

    If you don't have a local designer, there are a few firms that work over the Internet and could draw up plans that you could work from or take to a local contractor to have installed.

  2. AlexD2022 | | #2

    Thanks Devin.
    I will admit I don't understand what goes into a ducted mini split, but I believe they still need a return plenum and supply ducts, right? I'm a little wary of that as head space is at a premium in my basement now that I've built up the floor and have 7' or less of headspace (yes my joists are varying sizes :D). I also feel like it would be slightly easier on my to just install 2-3 heads, but I'm happy to be educated.

    My two bigger bed rooms are right on the cusp of a 1K BTU load, so maybe it would work with a small head? Although I can't seem to find any heads less than 7K BTU, are there any heads that can go on a partition wall and condition the air in two rooms at once? Something like that could be useful for my two smallest rooms since they share a wall.

    Clearly I'm a little out of my depth when it comes to designing an HVAC system, but I am motivated to get my system out of the unconditioned attic while also getting off of natural gas. I just don't want to do this in a way that causes us to be scrambling for plugin heaters in the winter or to cause my planned ~4KW array to be insufficient to drive my electric bill to near zero kWh used.

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