Is a ductless minisplit appropriate for a house with primary wood heat?
Hello,
I am contemplating installing a ductless mini-split heater in our new build house in Southeast Alaska. The climate is of course very wet, with fairly cool summers and fairly mild winters with the occasional cold snap.
We heat primarily with wood. Is a mini-split heater an appropriate choice when the wood stove is expected to do 80% of the work of heating? We want an auxiliary heater that will simply limit how cold the house will get while we are away for a long day or a weekend (to 50 degrees or so), but will not run at all when the wood stove is going.
Toyo and Monitor type through wall oil heaters are usually used for this purpose here. If you keep the wood stove going, they don’t turn on at all. Can a mini split function efficiently in the same role?
thanks,
JS
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Since mini-splits modulate over a wide range (some have much wider modulation ranges than others), they tend to "play well" with wood stoves, smoothly picking up the slack as the fire dies down, and modulating down to "off" as the stove brings the house up to temp.
Most don't have a minimum temperature setting lower than 60F though, but if it the unit is undersized for the heat load it could get to 50F (or colder).
Without knowing the size of your heat load it's hard to recommend a particular model, but for your purposes it sounds like it won't ever have to cover the full heat load at your 99% outside design temp, (http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ). Picking one with a low minimum modulation and a decent amount of output at your 99% temp is probably going to be good enough, as long as it's not a huge house. The 99% temp in Juneau is +7F, in Ketchikan it's +20F. Yours is probably somewhere between 0F and +20F, but with a ZIP code we'd be able to come up with a closer estimate.
A good possibility is the Mitsubishi MSZ-FH09NA, whicn can modulate down as low as 1600 BTU/hr @ +47F, but can still deliver 10,900 BTU/hr @ +5F, 12,200 BTUh/r @ +17F , all assuming an indoor temp of 70F. The output capacity is quite a bit more than those numbers when the indoor temp is as low as 50F. See:
http://usa.mylinkdrive.com/uploads/documents/4560/document/MSZ-FH09NA_MUZ-FH09NA_Submittal.pdf
If you run a heat load calculation on your place with an indoor temp of 50F at your 99% outside design temp there may be more appropriate models to consider. But the ~10:1 turn down ratio and low min-modulation of the FH09 makes it a good candidate unless it's plain too small, so small that wouldn't be able to keep the place from dropping below 50F over a cold weekend. With a description of the house, it's R-values, window types and square footage we could do a bit better than a WAG using rules of thumb for a likely range of heat load without doing the formal calculations, which may be good enough to pick mini-split for your purposes.
Justin,
Sure, you can use a ductless minisplit for the purpose you describe.
However, you may not be able to find a ductless minisplit thermostat that goes as low as 50 degrees F. You may need to keep your house a little warmer -- perhaps 62 degrees F. I'm not exactly sure how low you can set the thermostat of a Fujitsu or Mitsubishi minisplit in heating mode -- I would call up the manufacturer's tech help hot line and ask.
Additional advantage: at a lower indoor temperature, not only will capacity go up, but also COP, so when it's keeping the house at 50 or 60, it will be more efficient as well as needed to supply less heat than it would to keep it at 70.
A concern to consider: if you have a power failure while you are away, it won't keep heating whereas some kinds of combustion appliances could, or could do so with a smaller power backup system. But that's minor concern as your wood heat is your main power-failure backup plan.
Thanks for all the replies.
Dana, the house is in ZIP code 99826 (probably about the same climate as Juneau). It will be about 800 square feet to start with but will get to about 1,300 square feet in a couple years when a planned addition is built.
In brief, it will have R30 walls (R21 fiberglass, with 1 ½" of interior polyiso foam) and nearly R50 in the cathedral ceiling (R38 fiberglass and 1 ½" of interior polyiso foam). The walls and ceiling with have a ¾" airspace between the foam and the sheet rock.
The house is a basic cape with an ell, 16' x 24' in the front, with 16' x 18' in the ell off the back. Both sections will be two stories, with cathedral ceilings in the attic rooms upstairs (i.e. the walls are 10', overall. The upstairs "walls" will be 16" tall. The roof is a 12/12 pitch).
Windows account for about 20% of the exterior wall surface approximately 1,400 square feet. These are custom wood sliders with with low-E coating on one pane (the quoted U-factor of the panes is 0.34)
I'm not the only one interested in this discussion.... As of yet, no one has bought a mini split in this town. But, it seems like that they could become popular. Although 62 degrees seems like a very high minimum temperature for the house, perhaps overall energy use could be less than it would be if the house were kept at 50 degrees minimum using a Toyo oil heater?
By the way, we pay about $0.30 per kilowatt for electricity, and about $4.50 per gallon for heating oil.
Thank You,
Justin
On the Fujitsu 9RLS2H and I assume others in the series, you cannot set the temperature (heating) below 60F, but there is a Min. Heat Operation setting which will maintain the temperature at 50F.
In Gustavus your 99% outside design temp is something like 0F to +5F, depending on altitude. (Juneau's design temp is +7F at the airport), but your mean winter temp is about +30F:
https://weatherspark.com/#!dashboard;q=Gustavus%2C%20AK
A house that size & description would have a heat load at +5F with a 68F indoor temp of about 8000-10K for the 800' version, maybe 12,000-15,000 BTU/hr for the bigger version, depending on all of the particulars. At +5F outdoors with a 50F interior temp the heat load of the 1300' house would be under 11,000 BTU/hr.
The FH09 can deliver 10,900 BTU/hr @ +5F into a 68F room, the 9RLS2 or 9RLS3 would deliver even more (but if going Fujitsu you should really get the RLS2-H or RLS3-H versions to deal with the copious pan -ice from defrost you'd be getting during colder weather in your climate). At your load & use the 1600 BTU/hr @ +47 minimum modulation capability of the FH09 makes it a better fit than the 3100 BTU/hr @ +47F min-mod of any of the Fujitsus.
With a mean winter temp of +30F with the thing set to 60F indoors you'd be hitting the mid to high-3s for a seasonal average COP out of either the FH09 or 9RLS2H or 9RLS3H, but let's call it 3.3 just to be conservative. That means you get 3.3 x 3312BTU/kwh = 11,260BTU/kwh out of it. At 30 cents' kwh that's a $/million BTU cost of $0.30 x 1,000,000/11,260= $26.64 / MMBTU.
With a best-in-class oil burner running 87% efficiency you'd get 0.87 x 138,000 BTU/gallon= 120,000 BTU/gallon out of it. At $4.50/gallon that's $4.50 x 1,000,000/120,000= $37.50 /MMBTU.
Even though the total MMBTU needed at 50F is lower than at 60F, it's unlikely to make up for the 40% higher cost, even if 60F was the lowest you could operate the mini-split. In reality the mini-split efficiency average would be a bit better than that, and the oil-burner would do a bit worse. (I'm not sure what the Toyo's AFUE or steady-state efficiency rating is.)
Since nobody in town has a mini-split yet, getting one installed correctly and getting technical support for it may be a down side to going that route. It's not that the mini-splits don't work there (they clearly do), but if the nearest support is Seattle or Anchorage should the thing need a part or is acting up and needs to be diagnosed, you might not be able to use it for days/weeks/ months. Properly installed they're pretty reliable, but without a local knowledge base you may be on your own for figuring it out (a common theme for life in AK, eh? :-) )