We're working on building a well insulated home in zone 6 (St. Paul, MN) with the goal of R35 walls, u 0.20ish windows, and R60 roof. Will a ductless mini-split system (Mitsubish 'HyperHeat') be sufficient as a stand-alone heat source in this environment with that type of construction or am I going to have a handful of days/weeks/months! where it just can't keep up with temps consistently below 0f? We're looking at doing PV as well so that also contributes to my desire to do electric. The likely alternative here is a standard forced air gas-fired furnace.

the Hyper Heats shut off at -18 +/- so if temps get lower than that you'll need a wood stove or other supplemental heat. Otherwise they'll work fine. I've built houses like that in north central New England with no problems and converted to Mitsubishi's in my old (but tightened) house, also with no problems.

Also consider using both - a HP for the highest efficiency in moderate weather and for cooling and nat gas (not necessarily fully ducted) for the most cost efficient heating when it's very cold.

Think about how cold closed door bedrooms will get.

I've lived in a two story home for about one year with insulation levels similar (wall and roof slightly higher) to your plan, in zone 6 (NH). One Mitsubishi FH12NA on first floor, open floor plan. Large PV system.

The short answer is yes, properly sized mini-split(s) can mostly do it. To keep the plan simple, you could put a small electric wall heater (Convectair or Stiebel Eltron) in each bathroom and bedroom, and maybe one in each main living area. These can kick in some heat on days (usually nights) when it gets below -18f. Or just have a couple/few cheap space heaters on hand to plug in those times. I would get these if building again, 120v plug in:
https://www.gpconservation.com/convectair-7358b05bm.html

While this standard electric heat is less efficient, you won't need them that much.

Some things to keep in mind:
-definitely go for at least those insulation numbers or better, great air sealing, great windows....don't skimp on this
-if going only one or two ductless, you'll want open floor plan and be ok with having bedroom doors open as much as possible, we keep bedroom doors open 24/7 (no kids)
-mount outside unit above usual snow level and area where it won't get pummeled by snow

Without calculating the heat load it's hard to come up with the "right" solution.   With mini-splits it's critical to get the sizing right, looking at both the max capacity at your 99% outside design temp as well as it's min-modulated output at +47F (part of the HSPF testing protocol.)  A mini-split that's undersized for even the average wintertime load may only hit a seasonal COP of about 2, since it would be running at it's maximum speed/lowest efficiency most of the time.  A mini-split that's ridiculously oversized can spend too much power just idling (usually only a problem for PassiveHouse type loads.)

But optimally sized for the load you can hit the high-2s or even low 3s, as long as you don't screw it up.

Some other ways to screw it up are to install the outdoor unit where it is in the drip-line of the roof edge spending all it's time defrosting or can be clobbered by roof avalanches. 

A subtler but still real issue in a space heating environment is installing the interior unit high on the wall.  Mini-split heads pull in air from the top, and send it out the front out the front.  If installed within inches of the ceiling it's incoming air temp is higher than the average room temp, leading to lower efficiency, since then has to cycly up to an even  higher output temperature to deliver the heat. Putting it close to the ceiling in a corner sometimes causes it to short-cycle, with bad effects on efficiency. In general it's better to put them no higher than head-height, and it's not insane to put them at chair-height, where the average temp might be close to or below the mean air temp in the room:



If I had to guess I'd hazard that MOST mini-split heads are installed too high on the wall for optimal heating efficiency. If installing it low would be too much of an intrusion into the room, a mini-ducted version would likely beat it on real-world efficiency, pulling in 65-67F air at the floor level rather than 70-74F air with a wall blob mounted at the minimum clearance below the ceiling.  HSPF testing presumes 65F incoming air, and that's simply not realistic if pulling air from a few inches under the ceiling.

The Fujitsu xxRLS3H series have a specified output at -15F, and won't stop even if it hits -30F.  The drawback with that series is that the min-mod output @ 47F is 3100BTU/hr, even for the 3/4 tonner, so for very small loads it might not be a good fit.  The Mitsubishi -FH09NA doesn't put out quite as much heat at low temp as the -9RLS3H, but it has a whopping 11:1 turn-down ratio, and can pull back to ~1500BTU/hr @ +47F, which means it spends a lot less time cycling on/off, but may be maxed out much of the time, depending on your actual heat load numbers.

Bottom line, you CAN make them work at low temps, and you can even make them work EFFICIENTLY, but you can't just throw any cold-climate mini-split at it and expect it to hit the efficiency numbers. You have to calculate the load at both the 99% design  temp and at at the shoulder season temps to find the best compromise.  Do the math, be aggressive on your assumptions to avoid oversizing, and be smart about where/how you place both the interior & exterior units.  A modest amount of oversizing is good for efficiency, but if it's cycling on/off whenever it's above freezing, you've shot yourself in the foot on both comfort & efficiency.

Great input, thanks everyone.

Dana1, regarding placement of the interior units, would your concerns be alleviated with locating them high up on the 'low' wall of a room with a sloped ceiling? That is, a space with one wall being 8' tall, and the opposite wall some 10-14 feet away being a few feet taller, with a sloped ceiling in between.

It's really all about the temperature of the inlet air at the head. The higher you are in the room, the higher the temp, which reduces the efficiency. The difference between a low-mount and a high mount can be as much as 2 HSPF points in some cases. (An as-used HSPF of 10 instead of 12.) Predicting the amount of room temperature stratification isn't something easy to do on a web forum- there are many factors to think about. But placing the head where you expect the room temps to be lowest is better for efficiency than some random location.

Ducted units can slurp up up the coolest air off the floor, but take a hit in performance due to the extra power needed to drive air through ducts- there are trade-offs.

Also consider locating the unit on an exterior wall, below a window.

In central VT, 1800 sq ft and less insulation on the walls but otherwise same (although we have a lot of windows!).  We put 1 Fujitsu 15RLS3H in the open living / dining / kitchen on the first floor. Also open doorways to the mudroom and a play room.

Electric bathroom heaters in each bathroom - I used the Panasonic WhisperWarm in ceiling.

I decided to put an 11k BTU Rinnai direct vent heater in the basement to help the Fujitsu along. I couldn't figure out a logical or easy place to put a 2nd Fujitsu.
I would have probably put the smallest Rinnai but they require the 11k btu unit in order to be able to vent vertically before going horizontally.

This is our first winter so we will see what happens.

This is our first winter so we will see what happens.

If would be interesting to measure the temp of closed door bedrooms during cold weather and see if the theory (based on insulation levels, etc) matches actual numbers.

I have a 1.5 ton hyper heat out did a great job heating. It still was making 108-114 degree heat when temperature were below 0 which was impressive since it usually did around 120 when it was warmet. They definitely work but you might need a way to push the air through the house if you have one. A floor fan helps out with that.
I just added a fugitsu .75 ton unit and can't wait to see how it performs. From what I read "how to buy a mini split article " these units are better or heating and hold up better then the hyper heats in cold temperatures.

In North Central VT, we installed a Fujitsu 15RLS3YH for our living room, kitchen, entrance foyer and hall, maybe 700 sq feet. With a cold, snowy November under our belt, we are very pleased with the performance. On a zero degree day with the human sensor Energy Saving mode, the zone goes from about 60 to our desired 66-67 in less than an hour and stays there even with single digits outside. The house is built to 70s and 80s standards. I plan the next install to be a multi-head unit to finish off the house. Our November electric bill is about $50 higher, but we have no idea how much heating oil we've saved as the primary heat is oil boiler to hydro baseboards. The entire house is 2500 square feet with two additions that were built with 6" walls vs 4" in the older part.

You realize the prior post is more than 3 years old, right?

LOL! You misspelled welcome to the forum. I got here from a friend who is debating the best way to heat/cool a retirement home he plans to build. He learned a lot from this thread, and knowing I had installed a Fuji xxRLS3YH a few months ago, sent me the link. I was impressed with the level of knowledge and joined the forum. It wasn't until after my post that I realized how old the thread is. Even if no one learns anything from my experience, maybe someone will learn something from a thread that hasn't been active recently. Good day and happy heating.

Posted By SkiSmuggs on 14 Dec 2018 01:47 AM
LOL! You misspelled welcome to the forum. I got here from a friend who is debating the best way to heat/cool a retirement home he plans to build. He learned a lot from this thread, and knowing I had installed a Fuji xxRLS3YH a few months ago, sent me the link. I was impressed with the level of knowledge and joined the forum. It wasn't until after my post that I realized how old the thread is. Even if no one learns anything from my experience, maybe someone will learn something from a thread that hasn't been active recently. Good day and happy heating.

I wasn't really intending to give you a hard time, just didn't want you waiting with bated breath for a response. :-)

A handful of March vacations ago I happened across a condo at Smuggs (a few down from the one I was staying in) heated with a 2 zone MXZ series Mitsubishi. They're long since ready for prime time in climate zone 6. The newer versions are even better, and I expect that trend to continue.

Any minute now I'm expecting Fujitsu to come up with fully cold-climate compatible version of their xxRLFCD "slim duct" mini-splits. They already have the right type of compressor, with decent capacity & efficiency at -5F. What it's lacking is a pan heater for managing defrost ice build up, and a tested-rated capacity at -15F (like their cold climate minisplits all have.) People are already using them in zone 7 parts of Minnesota despite those shortcomings.