Minisplits as heat in northern Vermont?
I purchased a 1976 ski house in Northern VT last summer. It is ~2800 sq. ft. with a finished mother-in-law apt. in the basement which I have not been using as it is freezing and damp.
The furnace is a retrofit 2010 Airco oil burning forced hot air furnace that has extremely poor duct work. Unfortunately the ductwork is buried in a sheetrock basement ceiling (of which I think I’m gonna have to tear out to get to it). My home is poorly insulated with ~ 6inches of pink in the attic.
I have had an energy audit recently and am planning on doing some serious air sealing and insulating to make the home more comfortable this winter. Unfortunately I also found out I also need a new roof, so I may incorporate that into the insulating as well.
My question to you is this: Do minisplits have any place in such a cold climate as a source of heat (secondary)? I used ~1200 gal. of oil last winter for heat and it wasn’t exactly warm in here. Other alternatives I am thinking of are Rinnai space heaters.
Thanks
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Replies
Mark,
Yes, you can use a ductless minisplit to heat a house in northern Vermont. For example, a Mitsubishi Hyper-heat ductless minisplit (model PUZHA36NHA) is rated for operation down to -13°F. The rated heating capacity is 30,000 Btuh. When the outdoor temperature is -13°F, it has an output of 18,000 Btuh. This model (and other P series models) exhibits 100% of rated heating capacity at 5°F and 87% at -4°F.
For more information, see:
Heating a Tight, Well-Insulated House
Just Two Minisplits Heat and Cool the Whole House
GBA Encyclopedia: Ductless Minisplit Heat Pumps
Martin and Dana-
Thank you very much. I think I've got my answer!
-Mark
There are two series of mini-splits well worth considering for a N-VT climate- the Mitsubishi "Hyper Heating" series, that Martin mentions, which are fully specified down to -25C/-13F. The popular MUZ/MSZ-18FE-NA 1.5 ton unit is good for over 20,000BTU/hr @ 0F, and can probably handle your average (but not peak) winter load all by itself, guesstimating based on your oil usage numbers.
Fujitsu has released a cold-weather version of it's very efficient AOU xxRLS2 series, designated xxRLS2H (the "H" is important here), which is fully rated down to -15F. The 1.25 ton version delivers a full 15,000BTU/hr @ -15F.
http://fergusonhvacchicago.com/wp-content/uploads/2013/03/9-12-15RLS2H-Sell-Sheet.pdf
The dirty-napkin heat load math on the 1200/year oil use works out something like this:
Source fuel BTUs: 138,000/gallon
Burned in an 85% burner delivers 0.85 x 138,000 = 117,300 BTU to the air.
With crappy ducts you're blowing at least 15% of that in extra infiltration and distributions losses, call it 100,000BTU/gallon actually delivered to the conditioned space.
A typical heating season in N-VT is 7500 heating degree-days. So with 1200 gallons of oil that's 1200/7500=0.16 gallons per HDD. Times 100KBTU/gallon you get 16,000 BTU/HDD. With 24 hours in a day that's 16,000/24= 667BTU/degree-hour.
Using 65F as the balance point (it's base-65 HDD after all), and a 99% design temp of -10F( see: http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ) that's 75 heating degrees, which means your peak heat load is about 667 x 75= about 50,000 BTU/hr.
But your average temp for January & February is more like 25F,(see: http://weatherspark.com/#!dashboard;a=USA/VT/Rutland ) only a 40F delta, for 40F x 667= 26,680 BTU/hr.
That means 1.5-2 tons of mini-split will cut your oil use by more than 3/4, since even at the outside design temp it can deliver half the heat, and at the wintertime average it can deliver the lion's share of the heat. The installed price of an -FE18 in my neighborhood is about $4.5K. A pair of 1 tons would run about $7K (maybe a bit less) and you'd have better heat distribution. At $3.50-4/gallon oil these things will pay for themselves in under 5 years on offset oil use at VT type electricity prices.
Any time you're installing a mini-split in snow country it's better to bracket-mount above the historical snowpack depth, protected by roof overhangs (rake better than eaves) or a small shed-roof to avoid clobbering it with roof avalanches or cornice/ice-dam falls. This is a common (and sometimes expensive) mistake- the LAST thing you want to be doing during or after a winter storm is digging up and de-frosting the mini-split.
Heating with mini-splits is usually more efficient with a "set and forget" strategy rather than deep setbacks, since they work far more efficiently at part load than when cranking the compressor & blowers at full speed as it would on a recovery ramp. This is very different from how you would operate an oversized fossil-burner like your furnace, which operates at the same efficiency no matter what, and has enough excess output to recovery quickly from deep setbacks.
As a total solution heat distribution can be an issue. For doored-off rooms remote from the mini-split head installing radiant cove heaters with both a thermostat and an occupancy sensor can take the edge off the extreme temp discomfort. Radiant cove heaters aren't any more expensive than electric baseboards to install, but since they heat the humans & objects first, not the air, the comfort as it's coming up to temp is MUCH higher, and the combination of occupancy sensor and thermostat control keeps power use to a miserly level.
See also: http://www.rmi.org/cms/Download.aspx?id=10410&file=2013-05_HeatPumps.pdf&title=Heat+Pumps%3a+An+alternative+to+oil+heat+for+the+Northeast
FWIW: A couple seasons back I saw a condo at Smugglers heated with a 2.5 ton Mitsubishi MXZ-3B30NA multi-split (up to 3 heads)- didn't get to chat-up the owner or renter, but it seemed about right for the size of the condo. (The unit I stayed in had a propane fired Rinnai wall furnace, which costs 2-3x as much to run.)