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Abandoning my floor heat with addition for mini-splits, does it make sense?

DIYJester | Posted in Energy Efficiency and Durability on

If you’ve read any previous post you may have seen I bought a house with hydronic heated (electric resistance, all electric home) floors that were “fully” insulated. I have proven since buying that there is no insulation in the floor and the tubes or more than likely at the bottom of a 6″ slab.

This past year I added new windows and exterior insulation and I’m building an addition this spring. I am trying to decide how to heat and cool it. To me it almost seems like a no-brain decision to abandon the current system or just use it to try and temper the floor.

Cost to re-pour original slab: ~$7700
Cost to pour special thin-set mix in addition ~ $4300
Cost of PEX, zone valves, manifolds, etc. ~$3500
Cost to move current forced cooling system and add ducting ~$5000?

These costs don’t take into account that I would have to demo all of my original homes interior walls, cabinets, etc., saw cut the whole slab into pieces and bury them on my property with the Bobcat.

I keep trying to convince the boss that mini-splits would be much cheaper and much less labor intensive. My only concern is the house will be ~3200 sq. ft. on the first floor and an 1100 sq. ft basement and has quite a few rooms. Is it possible to do this well?

Total heating load I have calculated is about 40k Btu/hr total with the basement.
3 Beds (w/closets): ~500 sq ft master, and 2 200 sq. ft guest rooms
3 Baths: 200 sq. ft master, 60 and 40 sq. ft additional
Utility Room: ~165 sq. ft
Office: ~270 sq. ft
Kitchen/Entry/Great Room : ~1240 sq. ft

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  1. Dana1 | | #1

    The thermal mass of a 6" slab is nearly impossible to manage as the primary space heating radiation over the diurnal load swings even if the slab WERE insulated.

    It's possible to do this well with mini-splits in most US climate zones. It's also possible to temper the slab at high efficiency (covering a large fraction of the load) with a 2-ton Chiltrix air source reversible hydronic chiller rather than an electric boiler.

    What is your 99% outside design temperature?

    What are the individual room loads at the 99% outside design temperature, using a code-min 68F interior design temp?

    If it's easy to calculate, what are the individual room loads and whole house load at +47F?

  2. GBA Editor
    Martin Holladay | | #2

    I hope that you have had an opportunity to install vertical rigid foam insulation around the entire exterior perimeter of your home's slab.

  3. DIYJester | | #3

    Thanks for the reply. I did my heat loads using Siegenthaler's Modern Hydronics Book so it was geared to a slab floor heat system. I made a spreadsheet for each room and the house as a whole and made auto calculations for all blocks, so most of the changes are simple. I'm not sure how much difference it makes that I used this versus a Manual J method for these estimates. I would have the manual J done by the installer,and installed, as I would like a warranty.

    I am in zone 5, we are flooded in from all but one road right now. I had my degree days set for 4758 with a design low of 8 F. I'm not sure where I got those numbers anymore. I put the 47 F design in parenthesis.

    Master Suite: 9208 Btu/hr (3221)
    Guest Bedrooms (x2 ):~2400 Btu/hr (825)
    Office: 3900 Btu/hr (1375)
    Great Room: 15875 Btu/hr (5555)
    Utility: 2025 Btu/hr (710)
    Smaller Bath 1 600 Btu/hr (210)
    Smaller Bath 2: 552 Btu/hr (194)
    Basement: 3580 Btu/hr (1253)

    The way the floor plan is, the master suite, office and two bedrooms would be the west wing,with the master suite and office on the south and bedrooms on north. The great room is largely exposed south with large windows, and has the laundry and other two bathrooms on the North side of this area.

  4. DIYJester | | #4

    I have installed 4" of Roxul Comfortboard IS 4' down vertically on nearly the entire southern and eastern exposures of the existing home. The western exposure will be getting the addition and the northern exposure requires me to remove the brick (which would be part of the addition as its a HOA requirement to have a 1/2 brick front). A portion of the eastern exposure is directly connected to my garage slab and has no insulation at this point, nor had I planned on at this point.

    The same southern and eastern exposure that has gotten the Roxul has 4" of XPS with taped and foamed OSB seams, and taped and stagered XPS. I went with Roxul below grade due to possibility of termites.

  5. GBA Editor
    Martin Holladay | | #5

    Sounds good. I think that your plan to heat your house with minisplits is likely to work. Good luck.

  6. Dana1 | | #6

    Rooms with +47F heat loads less than 1500 BTU/hr won't be well-served by a single head. The lowest-capacity output head in the industry put out about that much at minimum modulation. If it's cycling and not modulating most of the time it's both a comfort and efficiency problem. It's worth trying to figure out which rooms/zones can be reasonably served by mini-duct cassettes, and where/ how those cassettes can be mounted.

    The lowest modulating single-head mini-split in the industry is the Mitsubishi FH09NA, that can throttle back to ~1600 BTU/hr @ +47F, but can also deliver 10,900 BTU/hr @ +8F, which may be the right choice for the master suite. The Fujitsu 9RLS3H can work there too since it can drop back to 3100BTU/hr @ +47F.

    The Great Room is also a good candidate for a single-head solution with either a 1.25 tonner like the FH15NA or Fujitsu 15RLS3H, either of which can dial back to under 5555 BTU/hr @ +47F, but still cover the ~16K load @ +8F with some margin.

    The rest look like mini-duct territory, with the possible exceptions of the office and basement zones, if served by a half-ton multi-split heads with a multi-split compressor that can modulate low enough. Most multi-split compressors can only modulate down to ~6000 BTU/hr or so, or roughly maximum output of a half-ton multi-split head, so don't go hog wild on that as a solution. It looks like if you add up the +47F loads subtracting out the Great Room and Master Suite it only comes to ~4500 BTU/hr (3300 BTU/hr if you exclude the basement), well above the min-modulation output of most multi-splits. Any in the Fujistu xxRLFCD series can drop back to 3100 BTU/hr @ +47F.

    A bathroom with a 550-600 BTU/hr load is essentially heated by the occupant when occupied, but some resistance heating like 200-400 watt panel radiator controlled via both occupancy sensor & thermostat limits discomfort issues when toddling in there at 7AM on the coldest morning of the year, assuming it can't easily be served on a mini-duct cassette zone. A conscious & walking adult human is good for at least 300 BTU/hr, sleeping humans at least 230 BTU/hr. Add to that another 75-100 BTU/hr for some high-efficiency lighting, it's not much load- you can probably ignore it completely if not easily served by a mini-duct, and deal with it later if there are comfort issues.

    So, take a hard look at the layout, figure out what works. If there's headroom in the basement it might be easy to serve all of the small load rooms with a single mini-duct cassette, which would be ideal for limiting on-off cycling.

  7. DIYJester | | #7

    Thank you Dana,
    Based on the layout I figured one cassette would be in the great room and cover the two small bathrooms and the laundry/utility room. All three of these rooms are on the north side of the great room and attached via normally open doors.

    I figured one cassette could cover the master suite (bath, room and closet).

    One cassette could possibly sit in the hallway between the doors of the remaining three first floor rooms (bed 1 and 2 and office).

    A last unit might be in the basement.

    Since I'm new to these units, I'm not sure how feasible having one in the hallway or greatroom cover other rooms.

  8. DIYJester | | #8

    I should also note that the current layout which is 2 beds and office and laundry is completely heated at this time by a 47.8k btu Vermont castings woodstove. We've had a mild winter so far, but we've enjoyed 75+ house for the last few weeks on just a few loads of wood a day. This is before re-insulating the attic, the north side of the house, the west side, and about 1/3 of the southern exposure.

  9. DIYJester | | #9

    Here is a basic floor plan with what I'd hope to be able to accomplish. The red x's would be a larger cassette install, preferably in the knee wall. The blue x's would be a ducted split and the rooms it would supply. The basement would have it's own split as well.

    Does this seem possible?

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