Considering minisplits in a dome home
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Considering mini-splits in a dome home.
35 foot diameter and 22 feet tall with full livable basement, floor insulated and walls.
1370 sq. ft in basement with 9 ft. walls and 1370 sq ft on first floor, with half of a loft area of 450 sq. ft.
Cubic feet of basement is…
I have three extensions on dome: one four ft to the south 14 ft. wide, one on east 13 ft by 14 ft wide, and one to the west 14 ft wide 12 ft.
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Replies
William,
You forgot to ask a question.
If you decide to ask a question, it might make sense also to tell us your location or climate zone.
thanks sorry about that. Eastern ky zone 4 A . how many mini-splits and what size for the entire dome
William,
You want to design a heating and cooling system. The first step is to perform a heating load calculation and a cooling load calculation. If this concept is new to you, you can learn more by reading these articles:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
When calculating the loads for a dome, you will be using software (or a pencil-and-paper method) that assumes that a space has a level ceiling. You'll have to estimate the average ceiling height when entering data for the dome.
After you have finished calculating your heating and cooling loads, you have to decide how many ductless minisplit units or ducted minisplit units you need. Here is an article that will get you started on your system design: Rules of Thumb for Ductless Minisplits.
okay thanks I will get to work on it
okay martin I have done the manual J whole-house heat loss & gain worksheet after converting the dome into a tw0 story house of same volume 32 by 32 with 9 feet ceilings and came with an btuh loss of 22,730 and btuh gain of 12,804 does this mean that I need ac of at least 1 ton cap and heat of near two tons? still thinking of mini-sp[its. thanks for your time and input.
William,
Most people use the word "ton" for cooling only, not for heating. A ton of cooling is equal to 12,000 Btu/h, so (assuming that you performed your cooling load calculations accurately), you're right that you need a one-ton cooling system.
Assuming that you performed your heating load calculations accurately, you could heat your home with two ductless minisplit units, as long as each unit had a heat output of at least 11,000 or 12,000 Btu/h at your lowest expected outdoor temperature in winter. In your climate zone, outdoor temperatures don't get very cold, so it should be easy to find a minisplit meeting these requirements.
For more information, see How To Buy a Ductless Minisplit.
There are a few 1.5 tonners that put out 20,000BTU/hr or more at the relevant outdoor temperatures:
http://usa.mylinkdrive.com/uploads/documents/5083/document/MSZ-FH18NA_MUZ-FH18NA_Submittal.pdf
including a ducted version:
http://www.fujitsugeneral.com/PDF_06/Submittals/18RLFCD%20Submittal.pdf
Without flat exterior walls to mount it on, it may be easier/better to go with a ducted version, with the ducts routed at the basement ceiling (with maybe one extension up to the loft area.)
You don't mention the outside design temp, which makes a difference, but I'm guessing it's something like +15F(?). Assuming a heating/cooling balance point of 65F, that's a 50F temperature difference, which implies a heat load that grows 22,730 / 50F= 455 BTU/hr for every degree below 65F. This is important for figuring out the temp at which the unit begins cycling on/off rather than modulating. (You don't get the modulating efficiency UNLESS it's modulating.)
The minimum modulated output of the above Mitsubishi 1.5 tonner is 5150 BTU/hr @ +47. So the temp at which it begins cycling on/off is roughly 65 F - (5150/455) = 54F, which isn't bad(!)
The minimum modulated output of the ducted Fujitsu 1.5 tonner is 3100 BTU/hr @ 47F. So the temp at which IT begins cycling on/off is about 65F - (3100/455)= 58F, which is better.
The cooling cycling vs modulating minimum-modulation differences.
Even though the HSPF and SEER numbers of the Mitsubishi are marginally higher than the Fujitsu, the fact that the Fujitsu will be modulating nearly ALL of the time would likely give it a higher as-used efficiency if properly implemented. It also has a heating/cooling distribution advantage over wall-coil versions. If you're going to do it with a single mini-split rather than two, this would be a pretty good choice.
If you're doing it with a pair of wall-coil types, the Mitsubishi FH09NA would be the right way to go, since they can individually modulate down to 1600 / 1700 BTU/hr (heating/cooling), and you have the option of turning one off during when the loads are ultra-low to avoid on/off cycling:
http://www.centralmaineheatpumps.com/sites/default/files/ec_pro/centralmaineheatpumps/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf
Ok Martin & Dana thanks once again for time and help about the design temps they are from design condition s for Lexington ky closet big city to me.Winter inside temp 75 outside design temp 8 ht diff 67 . summer outside design temp 89 inside 75 cooling temp diff 14 . About the basement, the house sits on the side of a hill facing south,basement in hill with !st floor at ground level. basement has rigid insulation on outside of walls and slab floor and footers ,also have three 6'' earth tubes coming on west side of basement that come up to surface in nearby woods also have three windows in cupola at top of dome that open to pull air from earth tubes have windows in south side of basement for solar gain to heat floor have three small electric wall mounted heaters in basement hope to build a small masonary woodheater next year in basement with stone flue going through !st floor and pipe up to cupola . floor plan is open in basement and !st floor except for large bedroom in each loft is open except on west side small office. thinking of mounting mininsplit on !st floorbedroom wall to the open area and another in loft on office wall to open area.hoping to send some pics been building this for two years will be 65 in June
The ACCA calls out +10F as the 99% temperature bin for Lexington, not +8F
https://www.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
A second-opinion puts the 99% design temp at +12.7F, with the 99.6th percentile temperature bin being +6F:
https://www.captiveaire.com/catalogcontent/fans/sup_mpu/doc/winter_summer_design_temps_us.pdf
Code only requires a minimum indoor design temp of 68F. Assuming the ACCA's +10F design would reduce the delta-T to 58F. You can still run the place at 75F all you want and it'll rarely (if ever) fall behind (but not by much). But if you start with excessive design temp constraints you risk oversizing the equipment to where it can't operate efficiently.
Base on that, the 22,730 BTU/hr per-code heat load number is really (22,730 x 58/67 =) 19,677 BTU/hr at code-specified indoor & outdoor temps.
The nominal heating capacity of the equipment also falls a bit with higher indoor design temps and higher delta-Ts, which means you probably wouldn't be 100% covered with a single ducted mini-split if you absolutely required 75F indoor temps when it's 10F or lower outside, but you pretty much would be if 70F was considered a tolerable low temp for 1% of the hours when it's 10F or lower outside.
Run the heat load numbers for the walk-out basement and above grade floors separately, and report back. The loss characteristics of basements are different enough from fully above-grade space that they don't really don't balance well when operated as a single zone. Depending on how it all works out, there may be a 2-head multi-split solution that works reasonably using a combination of a mini-duct cassette a small wall coil for the basement zone and a bigger one (or a mini-duct cassette) for the above grade portion.
Ok Dana thanks for the help for the help. I went to the second opinion website that you suggested and they have the design temps for the city of Somerset ky which is 12 miles from where I am building . reworked the numbers and came up with 18457 on the heating load and the same on the cooling 12804. am a little confused on how to calculate the walk out basement as all factors for cooling are Os and the only factor for heating are .60 and that square feet of basement area or just perimeter btuh ,6oX32 X 4 sides =128 perimeter feet and also do I need to do a calculation of the window area for heat gain and cooling. 109 sq feet of glass, ,9 x 109 =98.1 and 25 x 109 = 2725 am I thinking right ?