Abandoning my floor heat with addition for mini-splits, does it make sense?
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?
这些费用并没有考虑到我必须演示所有原始房屋的内墙，橱柜等，看到整个平板将整个平板切成碎片，然后用山猫将它们埋在我的财产上。
我一直试图说服老板，小型切片将更便宜，而且劳动力密集得多。我唯一关心的是房子在一楼的3200平方英尺和1100平方英尺的地下室，并设有许多房间。可以做得很好吗？
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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Replies
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?
Mike,
I hope that you have had an opportunity to install vertical rigid foam insulation around the entire exterior perimeter of your home's slab.
Dana,
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.
我在第5区，我们现在从一条路以外的所有道路上淹没了。我的学位日期定为4758，设计低至8F。我不确定我再获得这些数字了。我将47 F设计放在括号中。
Master Suite: 9208 Btu/hr (3221)
Guest Bedrooms (x2 ):~2400 Btu/hr (825)
办公室：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)
地下室：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.
Martin,
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.
Roxul获得的南部和东部曝光有4英寸的XP，带有胶带和泡沫的OSB接缝，并录制和停滞的XPS。由于白蚁的可能性，我与Roxul低于等级的Roxul一起去了。
Mike,
Sounds good. I think that your plan to heat your house with minisplits is likely to work. Good luck.
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.
Thank you Dana,
根据布局，我认为一个盒式盒子将在大房间里，覆盖两个小浴室和洗衣房。这三个房间都位于大房间的北侧，并通过正常开门。
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.
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.
这是一个基本的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?