Find out the model numbers that they are proposing. Study the spec. sheets and see if the outdoor unit will be capable of providing heat at -20F. Mine don't go that low for heating range (Fujitsu). The range is 5F-75F for one out door unit and 14F-75F for the other. This is the temperature range that the outdoor unit can provide heat in. The cooling range is listed separately and goes up to 115F.

But technology is changing all the time so maybe they work at lower temperatures now. We have in floor radiant heat for our normal heating needs and the ductless mini-splits for AC. We have used them for back-up heat when we need to replace a piece of equipment for our in floor radiant but for our area I don't think they would be suitable as a sole heat source because the outdoor temperature gets too low.

Hi, Mic111
I take your point about the low temps. How has your experience with these units been otherwise -- and are they quiet? We are looking at Mitsubishi.
Are you in a super insulated house?

Hi Mulchie,

I don't know if ours would be considered super insulated. Probably not. It was built in 1994 with in floor radiant heat and swamp cooler. We just installed the minisplit last year to get AC.

We love the cooling capability of the ductless minisplit. Last year through the brutal 100+ degree temp. it did a wonderful job. I don't think they are particularly quiet or noisy. You can look at the specs. to see the dB put out at the various speeds for the different units. Since our heating is in floor radiant in gypcrete we have no air circulation noise from that so are used to dead quiet from the heat. However we also run humidifiers during the winter and those are noisier. Because of local forest fires I also have air cleaners near all the exterior doors and those are quieter unless they ramp up to high speed.

I did find that the people supplying them don't seem to really understand how to size them or install them. That was the most difficult part. You need to do your own homework and not rely on what they say for a configuration. I think we could have gone a size smaller in our master bedroom and that would have been a little tiny bit quieter. We did have to cover the LEDs at night as they are very bright. Also placement over a bed is not advised as it will blow on you all night long. We put a ceiling cassette at the foot of our bed to air condition a large master bedroom. We then paid extra to close off the vent that blew over the bed.

Thanks. This is all very helpful. If we go this way we are going to do a lot of research. With a superinsulated house, the dynamics change and we shouldn't need to generate that much heat. I worry about overbuilding/overheating. This will be a delicate balance.
Your thoughts on noise, light and placement are valuable.
Cheers.

I don't know if the insulation matters. The outdoor unit is outdoors so it isn't in the insulated environment. Its ability to produce heat falls as the temperature falls below its operating range. In a prolonged cold spell you could find yourself with no heat as your house cools down. I suppose you could supplement with pellet stove and electric heaters but I don't think I would plan that in for a new house. However if the specs. go down to the temp. range for your area with a little comfort margin then it would probably be a good solution.

I think we'll be ok on low temps. Spec for this go to -20 and we are installing a zero clearance fireplace that throws 55k btus so we can definitely make up the difference if there is a severe and sustained cold snap. The house is also very passive solar, so it will only be the double thump of extreme cold and prolonged cloudy that will be hard to manage.
So many variables, though! And it seems not many folks know all the implications of super insulation. Pretty interesting direction, however.

I am a Mitsubishi mini-split user, not a pro, in a hot-humid climate (9b). I grew up in 4b and am most familiar with hydronic and forced air heat for your climate. I can be of some help, but I am limited. Is your wood heat automated or manually-controlled? I suppose that you have no natural gas.

15K sounds like a lot in new construction for three units if they are high ceiling units. I put in 7 for about that much in a retrofit, but I think that I got a bargain. If they are small, ducted units or ceiling cassettes with some of the more expensive controls, or even the high-ceiling units with more expensive controls, it might be more reasonable. You might live in a much more expensive market too. I'd get another quote.

My Mitsubishis don't have resistance back-up heat in them. I believe that others do. If you don't have it, you won't have heat when they are defrosting. Check out other good brands for their features as well. This is especially important for controls. Only three indoor units? In zone 5, I would think that you want heat in each room, even bathrooms. Are you skipping any rooms? Again, this changes if you are using small-ducted units that can serve more than one room. Check how the controls work very carefully. I think that things have changed in two years, but I think that there is no way to do true set-backs with the remotes supplied with my Mitsubishi heat pumps. I can only do on/off. I prefer to sleep in a cool room, but have the floor warm when my feet hit it in the morning. I think there is no way for me to do that with optional controls either. I also think that things have changed so that is no possible with their high-wall units. Note that I think true set-backs are possible with other manufacturers and were with ceiling cassettes and small ducted units when I put in my system. Note that I have been using a set-back thermostat for 35+ years in highly varied climates.

I find that in heating, as my air handlers slow down their blowers as they approach the set point, my high-wall units turn into ceiling heaters. This is predictable. It is also not much of a problem since I don't need that much heat. I compensate by increasing blower speed manually or using a paddle ceiling fan. My house, however, is a poorly-insulated, 1940, raised house which is much different from yours.

Good luck

Thank you for this thoughtful reply.
With super insulation the heat should be retained well while the exterior unit is defrosting (if I understood this right). We are still sorting out # of units, etc. and the 15k is a rough estimate. I am not sure about the final cost or the exact number of units. Our wood will be essentially a wood stove (very manual!)but, again, with good heat retention we really see this as taking the edge off on super cold days. I will look into the controls and other issues you raise. I'm new to this style of heat. There is no NG available at the site. Other options are geothermal or propane with a more conventional ducted system.
I did not understand what you meant by "set backs." An automated control with a timer? Sorry.
How noisy are your Mitsubishi units?
Thanks,

Hopefully you're planning on having resistance backup heat or some other additional source that isn't "manual", as you say? A fireplace/woodstove won't be of any help if the temps drop and you're out of town.

During defrost, the unit will be blowing cold air back into the house if it does not have auxiliary heat.

A 'setback' is when you want to automatically turn down the heat say at night and have it automatically come back up at a pre-determined time in the morning. Same goes for cooling, except you raise the temperature.

By my standards the Mitsu are quiet unless you manually put them into the highest fan speed. I don't think that they use that speed in any automatic mode. Note that In defrost mode, there is no blowing that I can detect*.

Investing in ground-source heat pumps in a very efficient house may not make a lot of sense. You'd have to do some calculations to compare it to a conventional heat pump with propane or oil back up vs just using electrical resistance heat. You MUST have something to keep the pipes from freezing if you are absent. It might needed to satisfy your building code. As a practical matter, you need it unless you are dairy farmers and someone needs to be there every day to milk the cows anyway.

* This is more a comment for the HVAC pros rather than the OP. Sometimes I wonder if my units would defrost faster if I have the units in turned on in rooms that I am not heating. If I just set them to a very low temp, they are activated in some way, but not heating. The defrost cycle might use them as cold sinks in addition to the ones in the room that I want heated speeding the process. For all I know, they might be used that way even if they are not activated. I'd really like to squeeze that information out of someone knowledgable at Mitsubishi. I have not tried because I think that my chances are poor.

Good information. I was unaware these units required defrosting so I will look into that further. As to the aux heat, I don't know the plan there yet. I'm not a dairy farmer, but we don't plan to be away much. That said, this is a key point.
We are still in the middle of some serious research. And it's clear we may need more.
Cheers,

I don't think either of my minisplits put cold air into the house when they are defrosting... I don't think that is correct... (I have an LG and a Mitsu).

Both are quiet.

The Mitsu officially heats when it is colder outside than the LG, but the LG is about 5 years older, so I suspect some technology influenced that too.

When it gets cold, how long does it STAY cold? -20 for overnight is different than 0 for 10 days. The 0 for 10 days will actually be harder on your house heat than the -20 over 1 night, imo.

Your pipes won't freeze until your house falls well below 32. It takes an unlived in house a long time to fall below 32 if well insulated. Just saying... You're more likely to break a water line from wind blowing the wrong direction into a house vent than your house not being warm enough.

It is my understanding that when either the LG or Mitsubishi mini-splits are in defrost mode, the condenser fan AND the interior fan are not running. Heat is taken from the air in the interior ducts or within the inside unit and "pumped" to the outside unit.

Saltidawg-- That sounds logical. And I can't imagine the defrost lasts too long. But I'm curious to find out.

Kirkhall-- Could you elaborate why sustained weather at 0 might be harder on the system? That is a more likely scenario (my zone is actually 4b) than a lot of time hovering around -20F.

We're not too worried about frozen pipes with heavy insulation, us mostly at home, a wood stove (zero clearance fireplace) for backup and lots of passive solar. I'll check with the builder, though, and see what the plan is for unfortunate extremes.

" Heat is taken from the air in the interior ducts or within the inside unit and "pumped" to the outside unit."
That is another way of saying that heat from inside the house is moved to the outside. How significant it is, is hard to say. I notice defrost mode when the house is warming up after the heat has been turned down or turned off. That would be first thing in the morning when the bedroom is cold and I would like to have it warm.

sustained cold temperatures outside are more likely to drop your interior temperature more significantly and is harder for your system to warm you back up.

If you are -10 from 3am until 6am, but up to 28 by 2pm, your system can get your house up to temperature by the end of the day. But, if it is 0 degrees for 10 days straight, your system will be in a constant "on" or defrost mode trying to keep your house at temperature. It is all about the temperature differential. -5 for 5 days is very bad. -5 for 2 hours, is more easily recoverable.

Does that make sense?

Maybe think of it as being underdressed in cold weather. You can tolerate really cold for a short period of time, and better, than fairly cold for a long time.

Where exactly are you located? How many floors is your house? What do you want to achieve with this heating system? Has a 'Manual J' calculation been done? Are you planning for an HRV or ERV?

This is what I would recommend, for a fireplace I would install a masonry heater or stove. I would install a liquid-to-water geothermal heat pump with radiant in-floor heating and water fed hi-wall units like the mini splits for A/C and for rapid change of room temperature during heating season.

You may need a system that is so small that the ground heat exchanger may not be the killer you think it is. There are no issues of defrost with geothermal; this mode does not exist. You may also be able to do radiant in-floor cooling and could fore go the hi-wall units altogether. Do you have the land for horizontal loops, a well or a pond?

Don't forget the tax credits installing geothermal!

SR

Here is a link that might be useful: Tulikivi Soap Stone Heaters

I'll add--geothermal is prohibitively expensive in the west...tax credits or not. So, worth getting an estimate, but be prepared for it to not be as low as many on the forum tout (esp if you are in the west).

Did you end up with the Mini-splits? If so, how has it worked out? Specifically, have you weathered any long cold spells?

I have a similar situation and am considering the Mitsu line of cold-climate heat pumps. However, I already have adequate heat, but it's resistance so fairly expensive to run, and I have no AC. I've been given a ballpark "guestimate" of $7k, based on an office visit (by me). Have put the project on hold because of the pandemic and probably won't do it this winter. I live in 4b.