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Community and Q&A

Gut renovation in NH – heating/insulation questions

Paul_Gareau | Posted in Mechanicals on

Hi everybody.

I’m amidst a complete gut renovation of a 1000 sq ft ranch home in central New Hampshire. Our plan is to primarily heat with wood using a wood cook stove in the kitchen. Our “backup” heat source and AC will be a 24k BTU Fujitsu mini split, with two 7k BTU heads in the bedrooms and a 12k head in the kitchen/living room. I’ll be using rockwool in the 2×4 cavities and wrapping with rockwool 2in Comfortboard on the exterior. The west-facing wall will get closed cell spray foam to make a future addition easier. We’ll do blown cellulose in the attic for at least R50. Floors over a full, unfinished basement will be insulated, but the method hasn’t been chosen yet. (Misc.. We’re milling our own wood for the exterior cladding, interior walls and floors and will run a 3.5kw grid-tied solar array).

A bulk load in CoolCalc gave me approximately 21k BTU for heat. The calculator roughly matched with approx 19k BTUs. 

Now, on to the questions…
1) Does the mini-split design and sizing seem reasonable? (Final design of 7/7/12k came from a contractor). Also note the small office on the south-facing wall of the house won’t be conditioned.
2) What can we do to promote air circulation from the wood stove? Our ceilings are low and adding transoms over doors with floor vents might not be realistic. Bedroom doors would be mostly open during the day and closed at night when heating is needed the most.
3) Feedback on the insulation plan would be welcomed.
4) We’ll be using a Kohler 14kw generator for backup power. The specs say that it has a maximum THD of less than 5%, which is generally low enough for sensitive electronics. The Fujitsus are supposedly very sensitive and their technical people have “no comment” when it comes to backup power. I saw another thread where someone had a similar plan and assuming it’s not uncommon to run a mini split from a generator, I’m curious about how it actually works in practice.

Thanks in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    The calculator is a joke. Use for a second opinion, since it's at least Manual-J based.

    To know whether a 2 ton multisplit is "reasonable" required the actual model numbers as well as your 99% outside design temperatures, and the zone by zone loads. Not all 2 ton Fujitsu multisplits have the same capacity at your outside design temperature. The AOU24RLXFZH is good for 25,500 BTU/hr @ +5F but only 13,500 BTU/hr @ -15F, whereas the AOU24RLXFZ (no -H) isn't good for even that much.

    What are the individual room loads of the bedrooms and office? A 7RLF or 9RLF mini-duct cassette with ducts in the basement to those three rooms may be more appropriate than a 7RLS per bedroom, with no direct heating of the office.

    A 9RLFCD single zone mini-split is good for 14,000 BTU/hr @ -5F, and can throttle down to 3100 BTU/hr @ +47F, much lower than you'd get with 24RLXFZH. A separate 9RLS3H single zone mini-split for the area with the wood stove could be turned completely off when burning wood, but would deliver 13,500 BTU/hr @ -5F, or 11,000 BTU/hr @ -15F. The overall capacity (and efficiency) would be greater than a 24RLXFZH, and the installed cost could be comparable or lower.

    I'm not clear how closed cell foam on the west wall makes a future addition easier(?).

    >"Floors over a full, unfinished basement will be insulated..."

    Is the insulation plan for the basement to insulate the basement walls (recommended), or is that intended to mean the underside of the first floor is insulated?

  2. Aedi | | #2

    First, a note: be sure your renovation includes a plan for air sealing.

    2) To my knowledge, there is no good solution for moving heat like that without involving some powerful fans.

    3) I am somewhat confused by your insulation plan. Mainly, I do not understand the plan to use spray foam to make the addition easier. Spray foam will reduce your access to your wall cavities, which you will probably need to use, and will make creating new openings slightly more annoying. I know spray foam will perform better than batts alone, but since this is a temporary situation you should just go with the batts. You might even be able to reuse them on the addition (or leave them for a sound proofing benefit). As a side note, any particular reason for preferring comfortboard over rigid foam? Foam is cheaper, especially if you have reclaimed foam available in your area. And your willingness to use spray foam means you are not anti-foam on principle.

  3. Paul_Gareau | | #3

    Thanks for the responses so far.

    Re: spray foam and the addition. To meet code requirements for insulation I need to either use closed cell spray foam, or rockwool in the cavities /and/ on the exterior. I don't want to insulate the exterior only to tear it off in a couple years, so spray foam is the option that's left. We'll pre-frame doorways and knock out the walls and insulation when we're ready to bulid.

    Dana, re: "AOU24RLXFZH is good for 25,500 BTU/hr @ +5F but only 13,500 BTU/hr @ -15F". Yes, it's the "H" model that we would be using. Where do you get the ratings at different temperatures? Our design temp is about 0F where we are.

    I don't remember which tool I used for individual room loads, but my notes say about 3k for the bedrooms and 2.5k for the office. The kitchen/living room was about 9k BTU (contractor says 12k is needed). My original idea was to use a ducted mini split for the bedrooms, but I was steered away from that idea by the contractors I spoke with, based on the lower efficiency of those units vs ductless. An issue related to the location of the return(s) came up too - where would that go?

    I like the idea of using a second condenser for the kitchen and living room. I'll ask for a quote for that option.

    The floor will be insulated, not the basement walls. We need to work on some drainage issues before anything is done to the basement/foundation walls.

    Aedi, I was hoping convection would do the job of air circulation, but not having room for the transoms is a problem...

    We'll be sealing as well as possible and using a vapor barrier on the rockwool walls. My understanding is a vapor barrier shouldn't be used with cellulose insulation. Haven't looked into what's needed for the floor yet...


    1. Expert Member
      Dana Dorsett | | #5

      >"My original idea was to use a ducted mini split for the bedrooms, but I was steered away from that idea by the contractors I spoke with, based on the lower efficiency of those units vs ductless. "

      Send those contractors back to school!

      The 9RLS3H single zone wall unit is indeed very efficient, with an HSPF of efficiency of 14.0:

      That's about 15% more heat per kwh than the 9RLFCD ducted unit's HSPF 12.2:

      But the HSPF efficiency of the AOU24RLXFZH with ductless-heads-only is 10.3 : (this link isn't working right now, but trust me :-) )

      That means the 9RLFCD ducted minisplit delivers about 18% more heat per kwh than the 24RLXFZH with a ductless heads in AHRI testing.

      18% more heat for the same power use is a significant step.

      The 9RLS3H delivers 36% more heat per kwh on a single zone unit than the very same head 9RLS head would deliver when used on the 24RLXFZH.

      36% more heat per kwh is a HUGE step.

      So sure, in general ductless is more efficient than ducted. But single-zone is also more efficient than multi-zone, enough so that a two single zone mini-split (one ductless and the other ducted) easily outruns the multi-zone solution on both efficiency and capacity.

      The capacity at different temperatures numbers come from a combination of the RLF technical & design manual, and the NEEP spreadsheet (not currently downloadable from the NEEP site.)

  4. Paul_Gareau | | #4


    I missed this question before "As a side note, any particular reason for preferring comfortboard over rigid foam? Foam is cheaper, especially if you have reclaimed foam available in your area. And your willingness to use spray foam means you are not anti-foam on principle."

    My goal is to only use spray foam where necessary. As far as rigid foam panels, we would need a lot of it to keep the dewpoint outside of the wall cavities. Then there's the whole issue of innie windows vs outie windows, drainage plains and a bunch of other stuff I can't be bothered with. :) The 2 inches of comfortboard shouldn't pose much of a problem.


    1. Paul_Gareau | | #6

      Quick correction as I refresh my memory on some of this... Foam probably wouldn't have to be too much thicker than the comfortboard - my mistake. I think what pushed me away from foam was that comfortboard is more permeable and can dry better to the outside.

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