Problems with configuring minisplit system as ONLY heat source
I am having trouble finding the best possible configuration for a mini-split system. I’m building a small house in NW Ohio, a story-and-a-half with full basement). Total calculated heat load is 16,000 Btu/hr.
I’d like to go with only a dual-zone minisplit without any supplemental heat source, if possible. Local HVAC contractor wants to put in a 36,000 Btu/hr Mitsubishi unit for almost $12K. Of course, that price is absurd. But I think he’s limited in his options because Mitsu does not make a multi-zone residential unit that can go down to sub-zero temps. He’s going with one of their smaller commercial units.
THE QUESTION: Does anyone know of a minisplit that can produce heat at 0′ (or even 5′), that can handle two inside units, and is sized in the 18-24,000 Btu/hr range?
And a secondary question: What is the ideal target for sizing HVAC capacity? How far above my Manual J load calcs is the ideal target? Or is that an old-school assumption; should I strive for an exact match?
My only two options, I fear, are to either go with two outside units, or to cave on the demand of no supplemental heat, and put in a radiant electric baseboard unit on each floor. But at least my minisplit system would go back into 4 digits!
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Replies
Andy,
My guess is that you can install two outdoor units for less than the price you were quoted. Some builders are installing two Mitsubishi minisplit units per house for $5,500 or $6,000 per house.
Don't add any fudge factor to the Manual J calculation. Any equipment that meets the Manual J load is sufficient.
Martin, thanks for your thoughts. You know, it's remarkably comforting to have feedback on the topic from someone who's not trying to sell me a piece of HVAC equipment.
Still holding out hope that someone will identify a manufacturer that meets my needs with a SINGLE outdoor unit at a reasonable price. I had to fight my wife hard enough for get landscape space for ONE of those.
If I go with two, can they be mounted side-by-side? or one above the other?
I've surfed the net for suppliers, but can't find the Mitsubishi dual zone unit that was specified in the $12K quote, the PUMY-P36NHMR. Seems like you can get all the smaller units easily, but is this a commercial product only handled by wholesalers?
A pair of 3/4 ton -FE09s would probably do ya, would put out over 20K (combined) at 0F, ( a combined 21,800 @ +5F, ~18,000BTU/hr @ -13F) and would come in at ~$6K (for the pair, installed) in my neighborhood. YMMV.
http://ecomfort.com/PDF_files/Mitsubishi/Mitsubishi_MSZ-FE09NA-MUZ-FE09NA_Submittal.pdf
A pair of -9RLS2 Fujitsus would get you there too, (with ratings down to -5F):
http://www.fujitsugeneral.com/PDF_06/Submittals/9RLS2%20Submittal.pdf
With mini-splits you get a modest efficiency boost out of moderate oversizing since they run at higher efficiency at part load than when going full-out, but that advantage starts turning the other way somewhere between 30-50% oversizing (for the output at design temp), since during the shoulder seasons it will cycle on/off rather than modulate, resulting in a larger fraction of standby loss. Standby losses ARE significant, and with two units you may be able to turn one completely off during the shoulder season to improve the duty cycle of the one left on, cutting the standby loss by more than half.
IIRC Sanyo has a pretty good~1.5-2 ton dual-head unit that's good down to 0F. I think this is it:
http://us.sanyo.com/HVAC-Multi-Split-Systems-Heat-Pumps-Outdoor-Units/Flexi-Multi-Inverter-Type-Heat-Pump
http://us.sanyo.com/dynamic/product/Downloads/Data%20Submittal-CMH1972_A-3192613.pdf
You can probably get a better HSPF out of some other dual-head though, HSPF of 8.8 for the Sanyo isn't exactly rockin' it on efficiency, but definitely in the running (it's hard to get high HSPF out of multi-splits, anything over 8.5 is pretty good.)
The Fujistu AOU18RLXFZ dual-head cuts it (with ratings at +5F anyway) with a 9.sumthin' HSPF and you'd be able to use a 7K head downstairs, 10K up or conversely:
http://www.fujitsugeneral.com/PDF_06/CapacityTables18-24-36RLXFZ.pdf
http://www.fujitsugeneral.com/PDF_06/HFI_Multi_Brochure.pdf
The 2-ton Mitsubishi MXZ-3B24NAis pretty similar, with flexible head options too:
http://ecomfort.com/PDF_files/Mitsubishi/mitsubishi_mxz3bsub.pdf
It's min-temp rating is also +5F, and may not be any cheaper than a pair of -FE09s rated down to -13F.
There are other options out there, and the LAST thing you'd want to do is put a 3-ton Mitsubishi on it that would be 3x oversized for your load at +5F, which wouldn't hit anywhere near it's HSPF numbers. I'd be a bit leery of using a contractor who recommended that "solution", but then again, good HVAC contractors for high-R houses are hard to find.
Don't let any body install the outdoor unit in the direct cornice-fall/roof-avalanche zone, or ground mounted where it can be hit with drifting snow. If you can bracket mount it protected under roof overhangs (under the rake is better than under eaves), well above the historical snowpack max depth it'll save you from having to dig it up or manually defrost multiple times per winter. (Many HVAC contractors tend to treat it as an air conditioner, which never has to work in 2' of snow.)
BTW: On a friend deep energy retrofit on a 3 story 3 unit rental he went with three separate -FEs, bracket mounted side by side 3-4' off the ground on the wind-sheltered side of the house. The overhangs aren't fully protective (I kept bugging him to put a small shed roof to protect them from cornice-fall), but it went through a few heavy-dump nor'easters this winter without any issues.