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Air-to-air heat pumps are getting more popular as a primary heat source in colder climates. Here’s how to get the most from your system.

[Editor's Note: This guest post comes to us courtesy of Peter Talmage, P.E., an energy and design consultant and an instructor in the Renewable Energy & Energy Efficiency program at Greenfield Community College.]

I have heated my various homes with wood since 1975. It was always a love/hate relationship. The wood fuel was “free” off my land, but burning it was a very dirty business in many ways.

This Fujitsu 3/4-ton model 9RLS is in its third season as the primary heater for our 1,500 ft2 home in Northfield, Massachusetts. The interior unit is 18" off the floor, and certain creatures like that very much.
Photo Credit: Peter Talmage

Mini-splits in cold climates? Yes we can!

Three years ago, I installed a ¾-ton Fujitsu model air-source mini-split heat pump to heat my historic 1790 cape home here in Northfield, Massachusetts. It has been a great success.

During the winter of 2010–2011, the heater for my 1,500 ft2 home consumed 1,757 kWh from October 2010 to June 2011. For the warmer winter of 2011–2012, the usage was only 1,247 kWh from September 2011 to April 2012.

So far this winter, from October 2012, to March 23, 2013, the usage has been 1,501 kWh. I have a 5.4 kW PV array that supplies about 200% of my electrical consumption, including that of the heat pump, so the heating system is very “green.” I have since installed mini-splits in two other houses.

Below are my suggestions for successful house-heating with a mini-split—even in a cold, Northern New England climate like mine.

1. Reduce load first

Improve the thermal envelope of the structure to minimize the size you’ll need and to reduce overall energy use.

2. Size it right for typical low temperatures

Heat-pump output drops as the outdoor temperature drops. I recommend sizing the heat pump to meet heating load at, say, 10°F. During periods of lower temperatures, use simple electric resistance heating or another source to make up the difference.

The compressor in the Massachusetts house is located on the east side of the house and has a shed roof installed over it. The big pile of snow on the left had just slid off the roof cover.
Photo Credit: Peter Talmage

Also, remember that a heat pump doesn’t have the capacity to quickly bring a cold house up to temperature. I set the temperature to 60°F whenever the house is unoccupied temporarily or at night and down to 50°F for extended periods of no occupancy. (At the 50° setting, the interior units typically keep air circulating constantly to prevent overly cold spots from developing.)

3. Prepare for a little noise

The interior unit makes noise—not a lot, but a varying level of whoosh. Make sure you can live with it before you install one. Find an installation and listen. If you like a dead-silent house, a mini-split isn’t for you.

4. Let it snow—but not on your outdoor unit!

The outdoor compressor unit needs to be mounted at least two feet above the ground here in snow country. It also needs to be well protected by a roof or cover that does not restrict airflow but doeskeep snow off and away from the unit.

In normal operation, the evaporator will freeze moisture from the air, which takes some extra energy. This ice is melted off during the defrost cycle. The melt-water drains out under the unit and sometimes forms a small glacier. The energy balance of this evaporator freeze/thaw cycle isn’t all that bad because the ice releases heat as it changes phase.

What can drastically reduce the performance of a heat pump, though, is when the evaporator gets plugged with snow. There is no gain of latent heat here, only energy consumption to melt the snow out. If the evaporator is located so snow can easily be sucked into it, the compressor will spend a great deal of its time melting snow and not heating the house.

The compressor for this Kennebunkport, Maine, home is set up high on a stand on the south side of the house. It draws air from a three-season porch that has glass panels installed in the winter, pulling air up through the gaps in the floorboards. A protective roof will be installed as well. 
Photo Credit: Peter Talmage

My latest mini-split installation has the evaporator drawing air from an enclosed porch space. Air is pulled into the porch at low velocity through the spaces between the floorboards. Snow drops out of the air before it enters the porch, so it can’t plug up the evaporator. A second benefit is that the porch warms up in sunny weather, improving efficiency.

5. Get the low-down on indoor mounting

For heating, the interior unit should be mounted about 18 inches off the floor and should have a good, clear shot into the living space. Mounting the unit low has many benefits for heating:

  •  First, it operates more efficiently because it is pulling in cooler air to warm up.
  • Second, the warmed air is blown out across the floor and mixes with the cold air at floor level.
  • Third, the air isn’t blowing directly on occupants, which can cause discomfort in the winter unless the moving air is very warm.
  • Fourth, it is very easy to access the filters for cleaning.

6. Right-size the pipes too

The interior and exterior units need at least 15 feet of piping to ensure no noise transfer from the compressor to the inside unit. Greater lengths of tubing are allowed, depending on the manufacturer, but will lower efficiency.

7. In warmer climes, get maximum efficiency

In colder climates, heat pumps need to strike a balance between efficiency (measured as heating seasonal performance factor, or HSPF) and lower operating temperatures. The warmer your climate overall, the more weight you should put on the efficiency side of the equation.

In central New England and south, go for units that have higher HSPF rating over lower operating temperatures. Most of the time, the compressor will be seeing temperatures of 20°F or higher. Rarely will it be running at –10°F.  The latest Fujitsu 9RLS2 has an HSPF of 12.5 Btu/Wh.

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1 Mini-split pump effective for large, old house in Vermont? posted by Tanya Tabachnikoff on 05/09/2013 at 12:54 pm

I am curious about this new technology but have heard different views regarding its use for a large, not-yet-well-insulated 1860s home in Vermont like ours. We know we need to insulate, but also have two old oil boilers that need to be replaced. Currently we have a hot-water heating system with old radiators. Is a mini-split heat pump a realistic choice for us to make?


2 answer from Peter Talmage posted by Paula Melton on 05/09/2013 at 04:13 pm

Tanya, the guest author has shared this reply with me via email.


Hi Tanya,I agree totally with Tristan. The first move when considering what to do about an old inefficient heating system is to improve the old inefficient house it's in. Mini-split heat pumps are exactly that. Mini. The ultra efficient mini split models are incapable of dealing with high heating loads unless multiple units are installed. Think many $$$.You might consider down sizing the area of the house you heat. (Create a super insulated core.)   I would recommend that you find an efficiency contractor in your area who can do the air sealing and thermal work as well as the new heating system. In light of the pending disaster all the current CO2 is going to bring us I have a really hard time recommending a fossil fueled heating system, so a mini split heat pump with a PV array would be my choice. Yours,  Peter

3 Mini-splits in Nashville, TN posted by Jo Neumaier on 04/20/2014 at 01:20 am

Hello, I am moving to Nashville and plan to remodel a 1956 single story 1700 sq ft home.  Nashville gets 50 inches of rain a year. It has 56% sunshine.  It is humid and get as hot as 109 degrees F for a couple of weeks in the summer.  Last winter my home, which I rent out, weathered 1 degrees F for about a week.  I had to have a plumber fix a broken water pipe to the washing machine in the unheated garage and insulate the pipes.  This is unusually cold weather for Nashville as it was unusual for all of the south.  I have read on line that mini-splits are not as efficient in extreme weather.  Is 109 degrees considered extreme?  What are the best manufacturers to look at?  Where can I read more information on the differences between forced-ari heat pumps and mini-split heat pumps for Nashville?  Is there a blog for teh Nashville area?

Thanks Jo

4 efficiency of mini-splits posted by Tristan Roberts on 04/20/2014 at 07:24 am

Jo, part of what makes mini-splits particularly efficient is that they have motors that operate at partial loads, in contrast with conventional AC, in which the unit is typically on or off, with no in-between. A mini-split will tailor its output to the conditions. So if your weather were constantly 109F, the mini-split might not be much MORE efficient than something conventional. But the weather constantly varies, and so does the mini-split's output, and that's part of where it gets its efficiency.

5 going to punt posted by Tristan Roberts on 05/09/2013 at 02:10 pm

Hi Tanya, I'm going to punt on this question, but hopefully in a way that is helpful. There are a lot of advantages to mini-split systems, but providing a lot of heat to a big leaky house is probably not where they shine.

But before you go too far down the path of what heating system to choose, I would go as far down the path as you can of getting your house well-insulated and airtight. The more you can reduce your heating load, the smaller and less expensive a heating system you can buy and install—and then pay to operate.

When it comes time to choose, I would consult with a few professionals. It's a complex topic and I wouldn't expect a novice to make a choice from reading a few articles, even excellent ones like ours!  My gut is that if you have a boiler now it's going to make sense to keep a boiler, but get a newer, more efficient gas model. But also look at pellet boilers, and consider tying in solar thermal.

6 Sizing posted by Kevin Gardner on 06/09/2013 at 10:35 pm

Great article - thanks. I'm surprised by the small size - literature I've seen for such model would probably say it would condition a 300 sq.ft. Room. Any comments on sizing?  I've got a 2000 sq ft space that we have insulated and sealed - wood is getting old and wood stove is now way oversized. 

I'm thinking I could get by with something a bit larger that would suffice 90% of the heating season here in NH.


7 Fujitsu heat pumps posted by Joel Ackerman on 03/30/2014 at 11:28 am



I had installed two Fujitsu heat pumps, 15000 BTU models at the East and West end of my house.  The east end tends to get plugged up with snow several times a winter but a sunny day, a leaf blower or a hot air gun remedies this.  We have a house rated at 36000 BTU's per hour when built in 1954 but it was poorly insulated.  We now have 4 inches  of fiberglass in the walls, better vinyl windows, 10 inches of fiberglass in the ceilings and 6 inches of fiberglass in the attic roof.  We were using about 700 to 800 gallons of oil in a forced hot air system, keeping the heat at about 63 degrees.  With the two heat pumps set at 70 in the day, a small fan in a doorway to  move the heat out of a large bedroom, we run this all day and it usually keeps the heat at 62 degrees with the furnace coming on once in a while on cold days.  We have turned the heat up once or twice on real cold days but will still burn less than 125 gallons of oil.  My wife complained that our electric bill went up less than $4. a day and I told her that was about 1 gallon of oil.  We got a couple of rebates and a friend gave us a  good deal on installation so it will pay off the actual $5000 cost in less than three years.  Our house is about 52 feet long by 30 wide and gets lots of sun on the south.  We are right on the ocean in Downeast coastal Maine.  I hope that with minimal maintainace we can get 15 to 20 years out of them.  My wife complains of cold feet and you need fans in the doorways to move the air more than an extra room.  These units were mounted up near the ceiling.  Maybe in Japan the heat drops down to the floor but not here.  I am very happy with them, my wife is happy with the oil bill.  I hope this helps

8 Winter with a mini split in southern NH posted by Amy InNH on 11/26/2014 at 02:33 pm

Hi Kevin.

My two Fujitsu mini split heat pumps take care of 1600 sq ft, one for each floor. First floor is open, second floor doors must be open to reach other rooms. The house is well insulated and is blasted with sun, when it's out. Snowing today, 34 degrees out, felt a temp drop when the snow started piling up, feels like 68 in the house. Read this is due to the outside units stopping to thaw any snow/ice, and recommended the units be roofed, to prevent that. Used for heat for the past month, November, electric bill is up $50. Last year, oil bill down $300 from using some of the time, late fall/early spring. I turned the oil burner on when I felt it cold in the house, some time in Dec, but I could be better about changing to the heat pumps during the day, when temps are higher. Over 15 degrees or so.

Summer high of $30 electric for AC, mid-July to mid-August. If you choose to turn it off, it's about 10 min. to comfortable, they work very fast. Dry and 78 degrees in a shot. Can't speak to Mitsubishi's Mr. Slim, but I find the controls on the Fujitsu energy smart and intuitive.

I've heard even better performance in the past year's heat pump model.

Installation, about 3 hours to install both units, $3K per unit including installation. There were cheaper quotes around, but my faith in their installation wasn't as strong. Installation is 1.5 years old, no troubles, no complaints.

9 defrosting posted by bob coleman on 12/01/2014 at 01:56 pm

The units stop to defrost regardless of snowfall, the colder it gets the more they defrost.

A roof can help protect it from being buried by deep snow but won't do much to shield it.

Watch for the pan icing up outside.

10 Sizing posted by Peter Talmage on 06/11/2013 at 10:29 pm

Hi Kevin,

Sizing a heat pump is similar to that of any other heating system. Simply put, the supply must meet the demand. Ideally you'd like to know the amount of heat needed to keep your house warm per hour during the coldest weather. This is the design heat load and if the heating system can deliver the same amount of heat per hour you're all set.  You plan to use your wood stove as back up in very cold weather so the heat pump can be size quite a bit smaller. Here's a way to get a rough idea of of the size you need in this situation:

1. Figure the total number of units of fuel you used last year to heat your house.

2. Calculate the BTU content of the fuel.   (#units x BTU content per unit)

3. Determine the house annual heat load.  (BTU x efficiency of heater)

4. Figure the BTU per Degree Day.  (Annual heat load / average annual heating Degree Days)

5. For a typical January day in NH there are around 50 degree days, so in January your house would have roughly an hourly heat load of: (#4 x 50) / 24 hr per day. This is the rated heat delivery of the heat pump you need.

Peter Talmage

11 electric mini splits posted by Dan Isbell on 11/23/2013 at 02:19 pm

We have heated our two story log home for 32 years with a large Jotul cast iron wood stove.  As we are reaching retirement age, we'd like to be able to travel more, so we're looking for a way to heat the home without anyone being home.  

We trade kiloWatts with our rural electric cooperative and make more pv electricity than we use on an annual basis. One array on a tracker rack is 1,200 Watts. The newer array is 2205 Watts, also on a Zomeworks tracker.  Our hot water is a drainback system with 80 gallon tank and 10 gallon drainback tank.

In east central Iowa, we normally see at least a couple of weeks where temperatures dip well below zero. Will electric mini-split air source heat work for us to keep our home warm enough in temperatures below zero if no one is home for several days, or should we look into, say, electric thermal storage heaters?

We have a very open floor plan with 26'x46' downstairs including one bath, kitchen, and utility room with washing machine, well pressure tank, solar hot water heater, water softener plumbing that could potentially freeze.

Upstairs is a loft roughly 1/3 open to the downstairs lined by railings, open stair with cathedral ceiling and one bath.

12 mini-split in midwest posted by Tristan Roberts on 11/25/2013 at 10:50 am

Dan, the mini-split air-source heat pumps that are currently on the market produce heat in below-zero temperatures—they just do so less efficiently than at higher temperatures. They basically become more like electric-resistance heaters rather than heat pumps. If you have a mini-split system that is properly sized for the home (which you would do in consulation with your local installer), then I don't see any obstacles to using such a system through the winter, and simply being aware of lower efficiency at cold temps.

13 Is a mini split system right for our house? posted by Margaret Tehan on 11/25/2013 at 06:43 pm

Hello, I think maybe you might have already answered my question in your answers to others above but some friends/builders/HVAC techs are telling me that they don't recommend a mini split system for our Connecticut house. So... we have a Colonia Revival built in 1918. It has radiators. I positively hate them. They're in the way of where I want to put furniture. I know that the first order of business would be for us to make sure that our house is airtight. I want to get either cotton denim insulation or cellulose insulation in our walls and attic. Would it be a good idea to insulate the basement ceiling as well? We have double pane windows but I don't know what brand they are or if they're super efficient energy-wise because they were installed by the previous owners of our house. But, despite the fact that we don't have any insulation in this house, it's actually pretty warm even on cold days before we turn the heat up so I'm thinking that the windows are pretty much airtight and efficient. But some of our friends are saying that we won't be warm enough on really cold days in CT and that we'll need an alternative heat source on those days. I would *love* to install gas fireplaces in our three bedrooms and in our living room and den but... that's a really pricey solution, isn't it? I see that Tristan wrote to Dan above saying "If you have a mini-split system that is properly sized for the home (which you would do in consulation with your local installer), then I don't see any obstacles to using such a system through the winter, and simply being aware of lower efficiency at cold temps." I have to think that Tristan's answer to me would be the same as what he said to Dan... that a mini split system that is properly sized to our home would do the job... but on the very cold days, it would work harder and cost more to run. Am I correct? I hope so. I really despise those big ugly radiators.

14 energy audit posted by Tristan Roberts on 11/26/2013 at 08:54 am

Margaret, in your situation my strong recommendation is exactly what you stated, to start with your building envelope and in your case, given that you have a lot of questions I would start with an energy audit. I don't know the market in CT, but in other states you can find multiple insulation and weatherization contractors who will do an energy audit for a few hundred bucks. They will identify how your house is performing energy-wise now, and identify work you could do to improve it. Where we live, if you do the work with that contractor, the fee for the audit is put toward the work.

Whatever heating system you choose, you will benefit from first investing in a tight and well-insulated building envelope, both to reduce your energy costs as well as to reduce your investment in a large heating system.

Given how much you hate those radiators I hope you're aware of the mini-split units that are typically installed on a wall in the room? They might be better than radiators, but they might not be everyone's cup of tea. 

It's also possible that your radiators (probably technically called convectors) are old and oversized. Modern radiators come in many different styles and profiles and might fit better in your rooms. You might find that upgrading your boiler and radiators is a good fit for this home.

15 There is no reason to not posted by Kevin Gardner on 11/26/2013 at 08:58 am

There is no reason to not consider it. We are just going in to our first winter with a mini split - we installed one 18,000 btu/hr minisplit on our first floor, and it has been keeping up just fine in our ~2,000 square foot house. When it dropped into teens with strong winds recently, I fired up the wood stove - not because the minisplit wasn't keeping up, but to go along with my plan all along. So if it can work in NH, I'm sure it can in CT as long as it's (or they are) sized appropriately.

16 Hello Tristan and Kevin, posted by Margaret Tehan on 11/26/2013 at 01:14 pm

Hello Tristan and Kevin, thanks so much for your responses. As I was writing my question above and I was saying that the first order of business was insulation, I was saying to myself... no, the first order of business is to get an energy audit. So... great minds think alike. :) Tristan, we want to have A/C in addition to heat so swapping out our big radiators for smaller ones isn't an option. And our gas boiler is only 4 years old. I have no idea if this is possible, but I'm hoping that we can sell it if we install mini splits because we won't need it anymore. Re: the mini split units installed on the wall... from what I understand, there are also ceiling units now available. In order to have ceiling units in our first floor, we may have to drop our ceilings a bit. I have to find out more about that but I'm hoping that we can just drop the ceilings. The 2nd floor units will work because all that is above them is our attic. If we can't have ceiling units, then, yeah... that's a deal breaker because I'm not about to spend all that money to swap out ugly radiators for ugly wall units.

Kevin, I'm wondering if I can still add a gas fireplace in our living room and one in our 2nd floor bedroom and maybe that would be our alternative heat source for cold days. There's only me and my husband living here so if we don't heat every room to a warm 68 or 70 degrees on very cold days, it's not a big deal. I would *love* to use programmable Nest thermostats with these mini split units but I guess that technology is not available for mini splits yet. Maybe it will be by the time we can afford to do all these renovations. Have either of you ever heard of Mesanna Ray Magic? It's radiant heat and a/c in sheet rock that you install on ceilings or walls. That would be my ideal heat and a/c source but I'm not sure if it works well in humid areas like CT. If you know anything about that, I'd love to hear from you.

17 Mini split dilemma posted by Bryan J on 11/28/2013 at 06:52 pm

I have been reading the comments on the mini splits, let me tell you what I have and what I'm looking to do. Any comments will help me. I have a home built in 1932, it's a story and a half and until recently, had an unfinished half story. I took out a bedroom downstairs in order to put up a staircase to the second story and had a 15 foot dormer put in to get the ceiling height for the staircase. Our intention was to make the half story upstairs a master suite. I have friends and family in the heating and cooling business, and I was basically told that the design of my house and the layout I have, it would be impossible to get any kind of runs from my furnace up to the second story without sacrificing square footage on my first floor. A mini split was highly suggested for heating and cooling. Doing research online, and from what my friends in the heating business said, I'm looking into the Fujitsu mini split (12,000 btu model). I have about 450 sq ft area, fully insulated and that includes a closet and a small interior bathroom. So basically, half of the upstairs is closet and bathroom and the other half of it is sleeping quarters. I live in michigan, where the temps can get down to zero in the cold months and I know the new Fujitsu minis splits are rated for the heat pump down to -5 degrees. so my questions/dilemmas are these.... Should I even use a mini split, if so, hoW should the inside unit be mounted, down low for the heating or up high for the cooling?? And how energy efficient are they for the heating/cooling?? Please help!! Not too sure on what I should do!!

18 mini-split for heating in cold temps posted by Tristan Roberts on 11/29/2013 at 12:04 am

Bryan, if you want to provide heating and cooling, and if doing so electrically is attractive, then I don't see any issue with using a mini-split. As discussed in these comments, mini-splits get less efficient at lower temperatures, but properly sized they still work. Each model has spec sheets that gives you the performance curve at various temps.

19 Placement posted by Bryan J on 11/29/2013 at 12:29 am

Thanks Triston, but what do you think would be a suitable location for the inside unit, should I mount it up high, about 7 feet or mount it lower, say underneath a window or maybe on my knee wall?? And is there any other alternative for heating and cooling my upstairs beside the mini split?? I'm leaning towards the mini but was just curious to see what other options are out there. Thanks again. 

20 mounting mini-split posted by Tristan Roberts on 11/29/2013 at 10:07 am

Bryan, the article above discusses mounting considerations relative to heating. I think it's up to you to weigh whether those considerations are relevant to you or if cooling is more important.

If you are looking for one system that will both heat and cool your upstairs, a mini-split is probably the most sensible option. But there are always other options, particularly if you separate out heating and cooling, for example an efficient gas heater and a window-mounted AC unit.

21 Mounting external unit posted by Robert Henshaw on 08/17/2014 at 03:26 pm

I am concerned about noise and vibration transmitted from the exterior unit to the inside of the house. I am considering a Mitsubishi system, or possibly a Fujitsu system. Does the wall mounting bracket for the exterior unit have grommets or other noise isolators to reduce vibration conducted through the wall? If the external unit is hung on springs would that reduce conducted noise?

We have an attached greenhouse/green space. The winter air temperature never goes below about -5 degrees, and then only for a few hours. The summer air temperature generally does not exceed +110 degrees, and then only for a few hours. Would this be a good location for the exterior unit?


22 not a problem posted by bob coleman on 08/18/2014 at 08:30 am

These units are not big monster noise makers like the typical outdoor compressor. Of course vibration could be an issue due to some sort of home construction, but it shouldn't be. One cause is using a short lineset, but your HVAC tech will take care of that.

As far as using a green house - if I understand you correctly - mounting the outdoor unit in a greenhouse would not be practical. These units move A LOT of air CONSTANTLY; it can't be restricted by placing inside another structure, and the green house affect wouldn't do much good as it would suck through that air quickly.

23 Noise and green house installation posted by Robert Henshaw on 08/18/2014 at 11:38 am

Thanks for the comments

24 We're looking at basically posted by Ben Wiechman on 12/08/2013 at 10:37 pm

We're looking at basically the same situation located in central Minnesota.

I've been looking primarily at the Fujitsu and Mitsubishi split units. They each have an option that is supposed to work to -13/-15 or so degrees. From what I can see none of the very low temperature models from those two vendors support anything other than the wall mount internal unit or more than a single internal head.

From Fujitsu the ones that seem operate to the lower temperatures are the RLS2H models:


Mitsubishi has a couple as well with similar BTU output ratings, but slightly different output ranges:

MSZ-FE09/12/18NA models. These are listed as two part models - MSZ/MUZ are the indoor and outdoor units respectively.

From what I can see the Fujitsu models have a slightly wider range out BTU output than the two smaller Mitsubishi models, but the Mitsubishi MSZ-FE18NA is rated to a higher BTU output, but lists an output range starting at 8000 BTU on the low end, while the other five models list a low end output of roughly 3000 BTU.

Not sure if anyone has any real world experience with these models. It would be interesting to know.

There was a test comparison made between the Mitsubishi MSZ-FE12NA and a Fujitsu 12RLS2 (a model that typically is listed as having heat output down to roughly 0* F, where the 12RLS2H model typically lists -15* in Fujitsu's information online).

Are there other models from other manufacturers that we should be considering? 





25 I was looking at the same two posted by Kevin Gardner on 12/09/2013 at 07:54 am

I was looking at the same two models (I live in NH). I ended up going with what my selected contractor preferred (which happened to be Mitsubishi MSZ-FE18NA). But I did show then the NREL report, and challenged them on that. Their arguments were mostly related to quality of construction and reliability. My choice was also influenced by having a contractor that I trusted, that was local, and was going to stand by their installation/product and be able to maintain it effectively.

It's performed great so far this winter (and used the AC a bit this summer, which also was great). I have it hooked to a dedicated electricity monitor, so I'm tracking electricity used along with heating degree days and such. I won't get BTU output, but can guestimate that from their literature and the NREL report.



26 Operating Temperatures posted by Peter Talmage on 12/09/2013 at 10:37 am

For higher performance from a mini-split system the outside temperature regime is important to consider. Heat pumps designed for operation in very low temperatures typiclly suffer a lower COP in the higher temperature ranges than units with a higher minimum rated operating temperature. In western Massachusetts where the typical lowest winter temperatures dip to -10 for very short periods of time, the typical temperatures are in the teen's and 20's. I chose a Fujitsu 9RLS for my house in Northfield with this in mind. At that point, three years ago, the comparable Mitsubishi model was rated to -13 F while the Fujitsu was rated to only + 5 F. However, the Fujitsu had a significantly higher COP operating in the teen's and 20's. My thought was that if the Fujitsu couldn't deliver enough heat during the short super cold snaps I would use a small resistance heater. At very low outdoor temperatures the COP of a heat pump starts to get closer to 1 so using a resistance heater isn't too much different. I was quite suprised to see the Fujitsu operating one cold morning with the outdoor temperature at -9 F and still putting out useful heat. The newer Fujitsu models now have higher COP's and lower rated temperatures.  Their new "H" models go down to -15 F. If I was installing a mini-split in Minnesota and  wanted to minimize back up heating sources I would go with a low temperature rated model. Bitter cold weather periods in Minnesota are much longer than what we have in central New England. 

27 going through my first winter with a 9RLS2 posted by bob coleman on 12/22/2013 at 06:45 am

Located in central IL.I was curious to see how the unit performed outside before I constructed a shelter.It seems an ideal shelter would be adjustable, or at least removable outside winter season. It would shield it from the sun during the summer, while obeying the minimum space distances, and in the winter it would allow the maximum sunshine while providing a snow cover roof, to keep blowing snow and freezing rain off the rear coils, off the top (which thaws and drips down and freezes), and from falling into the face. A high wind baffle in front might be useful, and it would further keep blowing snow out; Mits sells some for some models.Those models don't have heating in the pan; have you had any ice clogs or do you monitor them?  I can see how after about 3-4 days of deep cold weather and little sun the bottom ports would freeze over and start to build up, but it would take quite some time for it to get anywhere near the fan. Performance might suffer a bit.I know the H model has a pan heater but I'm not impressed with how it's implemented; it's basically a 65 watt heater cord that runs in temps below 40degrees which for me would be all winter 24 hours a day. It drops the HPSF factor from 12.5 to 10ish.I'm debating adding my own heater with a lower thermostat and lesser power output. I'm also debating drilling a couple extra holes for the water to drip before it freezes - the potential negative is allowing more airflow to bypass the coil but it should be minimal.The back frame also acts as a nice place for snow and melted water to refreeze. The front grill will ice up nicely too in a heavy wet snow storm as the blades stop during constant de-icing which allows it to stick.The other day I had snow pool on the top, melt to water and slowly drip down the front, then refreeze like a long icicle that hangs off a gutter. It eventually made its way into the fan blade cowel and stopped it.I thinking of building a structure some like that of a large umbrella that is used on outdoor patio tables with the light weight fabric and aluminum pole construction. The base that raises the unit off the ground provides a good foundation anchor.The heating performance has been impressive - the A/C performance was nothing short of a miracle. I've had one -5 degree night, have a -10 forecasted.I do wish they would add a resistance heating option to the indoor unit as an assistance as sometimes you want heat now, or don't want to get hit with the blowing cool air. It would be a performance penalty, but I'm willing to pay an extra penny in electricity occasionally to have heat when I need it. It wouldn't be too expensive and could be user selectable or possibly an addon like the remote upgrades.

28 ice posted by Peter Talmage on 12/25/2013 at 10:03 am

Hi Bob,

Great comments on shelters and the ice problem. I find it very necessary to mount the outdor units with plenty of clearance underneath so melt water has a clear path to flow out. I've seen a glacier 3" thick under mine. I haven't yet had an ice build up great enough underneath or on the grills to compromise operation on my east side and south side mounted units. Though beneficial for AC operation, ice build up can be much worse on compressors mounted on the north side of buildings on on sides heavly shaded by evergreens. The sun definitely improves ice melt and it would be pretty easy to add shading screens for the summer. Painting the grills and pan black might aid melting even in low sun situatuions. Maybe the pan needs a layer of low thermal mass material to reduce the freeze up before draining problem. Using external heaters to melt ice seems to be a "patch" solution.

29 the pan posted by bob coleman on 12/26/2013 at 05:15 pm

Unfortunately I'm in a north side install, although I'm not sure how much this would help during the long bitter cold snaps that have little sun.I've had the pretty glacier too. I used 18" tall decorative concrete pavers stood on end set on a concrete pad, drilled and epoxied some bolts on top, put those rubber condensor bushing mounts on the threads to tamp vibration and then set the leg holes on. It was more work than I wanted keeping it all square but I like the look.My goal for to try to learn just how the ice built up through monitoring the first winter. It seems like less of a problem than I thought. You might pay a performance penalty. The way this model is it would take a tremendous amount of ice to even reach the blades.Also surprised to not see much of a steam show at all after de-icing.Like you mention, having a cold metal pan doesn't help. I don't know if another material would do much better as the environment is constant. Besides for shipping protection, ease of cost/assembly, the pan really doesn't need to extend back under the coils at all - it has to reach them and create a little bit of seal to prevent by-pass air flow.I think I'm gonna try adding two more drain holes in between the edges and center so it can escape before freezeing and preventing more backup.I'm gonna use some flat fridge magnets to cover the holes during the summer if it happens to make a difference in airflow.Still thinking about adding the pan heater, but the issue is it could also cause problems by being restrictive, the thick wire and covering could act like a small damn. Without access to one I can't tell if the H model uses a hard coil that sits partly elevated, or even if Fuj did any engineering but to apply a lot of heat. The specs just show its shape and using a piece of tape to secure it so at one point its flat with the pan. I also have to cut a hole to thread it up and into the pan underneath the fins which is not ideal while its on the machine due to the low clearance. The H model runs the wiring internally to a socket on the upper inside.Ideally if I could use a 240v based heater wire, that was triggered when the fan stopped and off when it resumed so enough heat was generated while condensate was dripping to prevent most freezing, I think it would be very effecient. Put a switch on it to only turn it on during winter. Found a six foot 240v heater wire which is about the same length Fuj uses. Leaving it on constantly is just to energy wasteful at 65watts. I have a low wattage 3 foot pipe wrap cable at only 8watts but i'm not sure if it's enough to be useful. Was trying to think of a good test experiment to try replicating the conditions before actually installing it. It won't be warm enough to warm the pan, but it may provide enough of a warm path to the outlet to prevent it from completely plugging. It also has a thermostat and gfci so it's a safe package.Was thinking put it in a wide cookie baking pan filled with water next to unit, let it freeze, then plug it in and see how much action there is.


30 melting ice posted by Peter Talmage on 12/26/2013 at 08:16 pm

Melting a pound of ice takes 144 Btu. One watt-hr of electricity equals about 3.4 Btu, so to melt a pound of ice takes 43 watt-hr if all the heat gets into the ice. Obviously the heat goes elsewhere too, so a 65 watt heater running continously doesn't sound all that excessive for high ice build up conditions. There are lots of variables involved in how much ice will be produced, so the amount of electricity needed will vary as well. Maybe a simple IR beam activated switch that turns on the heater when the ice builds up and shuts off when the ice melts below the beam.

31 Using the heat pump in cold weather posted by J S on 01/02/2014 at 11:17 pm

Are there any resources you can recommend for homeowners about how to use the heat pump in winter for maximum efficiency?  the user manual which came with my Mitsubishi heat pump is not very useful.

32 not an expert posted by bob coleman on 01/06/2014 at 01:15 pm

The effeciency comes from having the heat pump installed properly and sized accordingly to the needs home/area it services.A snow canopy is recomended in heavy snow climates along with mounting on blocks, or judiciously keeping it clear. Best to seek the advice of someone who installs/works on these types of units to make sure you don't construct one that restricts the airflow which will hurt effeciency.Also keeping the filter maintainedOther than that, it is just like any other HVAC equipment, use it less (ie turn down thermostat in winter) and it's more financially effecient.Some models have more features, like the Fujits model 9RLS2 and similar have a motion sensor to adjust when no one is in the room, and an economy program to alter the temps, along with 24/7 timer like a tradional furnace.

33 size of multi split heat pump posted by Duane Erickson on 01/04/2014 at 02:55 pm

I want to heat/cool my 600 sq' garage (workshop) and 750 sq' bonus room with 1 multi split heat pump.  I live in West End NC 27376 where the winter temp may get as low as 20 degrees F.  Both rooms are insulated and have windows.  What size do you suggest I use for a multi split heat pump and would each of the room units have to have aux electric heat coils?  Thanks.

34 with this cold snap posted by bob coleman on 01/06/2014 at 01:00 pm

Nice work on the numbers Peter Talmage. I don't think Fuji engineered it though, it looks like they used a comercial product because the cabling has the same specs you can buy privately - there is only one 240v short run cable around. Fuji just added a connection point inside the box.Running 60watts 24 hours a day for 4 months heating the outdoors is a tad ineffecient, especially for those of us trying to go super effecient. 60 watts is more than all my common lighting usage, or same as my PC.The ice is more of a buildup issue, rather than it making ice like an ice machine. I think a lower wattage strip could work. Just need to prevent freezing over of the drainage holes so the collection doesn't start to grow.I've got a 8 watt strip, but its shape is not ideal; just waiting for decent weather to install and try. It also has a temp sensor so it can stay plugged in continuously. -Got some good testing in of low temp use last night and today.Hit -19 outdoor temp and has stayed at -15 for many hours still (windchill -43) and it still runs and produces heat, but goes into defrost mode frequently, and the actual output has really dropped.I was expecting the unit to kick off possibly after enduring several hours at that temp but it continues plugging away. It's fairly impressive.

35 Are they really that efficent? posted by Garrett Kerns on 02/03/2014 at 10:44 pm

Hello,I have a 1760 sq ft home (plus finished 1600 sq ft basement) in Michigan that was built in 1998 with all radiant infloor heat (80% efficent boiler) which I love.  Just added more insulation to the attic (total of about 14"), have high performance Andersen windows all around (a lot of windows and 2 sliding doors) Unfortunately, I live about 2 miles from where the natural gas line ends so I have to use propane.  I live in the house alone and am only there on evenings and weekends.  With hydronic radiant heat you basically just set it and forget it and not adjust it for when you are home or away (or so I have been told).  Due to how much it has been costing me in propane in winter months (between $300-500 @ $1.90 / gallon) I have been keeping my home set to 60 deg which is cooler than I would like, but as I am only home from about 8pm to 6am, it doesn't make much sense to keep it any warmer. 

I orginally was looking at the mini split systems because I wanted to use them as AC for primary, but the more I read about them, are they really that efficent for heating?  I have been pondering a pellet stove or something similar to offset my heating costs, but this may work.  I have a large open floor plan and vaulted ceilings in most of the upstairs.

I guess my question is, it sounds too good to be true.  After reading the article and comments, people are using less that 2000 KW/hr to heat a 1500 sq ft home in for 8 months including winter?  That is only about $200 in electricity!

I am having my local installer come and quote my house this week and would also like to know if there is anything specific I should ask?

Also, if I install one, what would be the best way to regulate it to use both the in floor and the heat pump so the floors are warm and the air as well?

Thanks and keep up the great articles,


36 cost posted by bob coleman on 02/04/2014 at 10:13 am

First concern is likely cost, if the current system for one of the online charts that allow you to compare propane's potential heat output at your equipment's effeciency to that of a mini split heat pump using electricity at a COP of 2.5ish.Of the top of my head, $1.50ish propane is probably close to equivalent; outside factors like service charges come into play too.You would likely get more economical heat in the fringe seasons with a mini-split, with quicker adjustments as the COP reaches 4 on top models when it's warmer; you get immediate heat only when you need it rather than burning the propane constantly to keep the temp up. You can also often buy cheaper propane in the fringe seasons too.The homes people mention with very little use are ultra effecient, insulated, and air sealed. They also allow for effecient distribution of the air.A 'blower door test', and air sealing is likely your best bet to improve heating costs and comfort. Your electrical utility company can likely recomend a contractor and possibly offer rebates for the service and improvements. They'll document where the heat is escaping and offer solutions that give you the most bang for the buck to remedy the problem.

37 Cost posted by Garrett Kerns on 02/04/2014 at 12:01 pm

I knew that a lot of these houses had to be a lot tighter than mine.  I have gone through with IR camera and found and repaired all of the drafts that I could find, replaced bad windows (rest are all from 1998).  I know that I am not going to heat my house for only $200.  I am trying to offset the cost of propane somehow and this seemed like a good way to do it.  I was planning on the mini split because I need to add some sort of AC to my house and it sounded like a good way to kill two birds with one stone.

I talked to the contractor and he is suggesting the Fujitsu 1AOU12RLS2, 12,000 BTU, 25 Seer with Cooling capacity 3,100-13,600 btu, heating capacity 3,100-22,100 btu.  Currently he quoted over the phone $3167 installed, but is coming out on Thursday to confirm the bid with the layout.


38 Cost posted by Garrett Kerns on 02/04/2014 at 02:40 pm

I knew that a lot of these houses had to be a lot tighter than mine.  I have gone through with IR camera and found and repaired all of the drafts that I could find, replaced bad windows (rest are all from 1998).  I know that I am not going to heat my house for only $200.  I am trying to offset the cost of propane somehow and this seemed like a good way to do it.  I was planning on the mini split because I need to add some sort of AC to my house and it sounded like a good way to kill two birds with one stone.

I talked to the contractor and he is suggesting the Fujitsu 1AOU12RLS2, 12,000 BTU, 25 Seer with Cooling capacity 3,100-13,600 btu, heating capacity 3,100-22,100 btu.  Currently he quoted over the phone $3167 installed, but is coming out on Thursday to confirm the bid with the layout.


39 two new heat pump articles posted by Tristan Roberts on 02/04/2014 at 11:45 am

For this interested in how heat pumps perform in very cold weather, our editor Alex Wilson just posted a great report on how the cold weather has put his new mini-split to the test.

We also have a new post on how these darn things work.

40 Installation Cost posted by Peter Talmage on 02/04/2014 at 08:28 pm

Hi Garrett,

That price of $3167 is very reasonable. Fujitsu now offers a floor mount inside head, 12RLFF, with an HSPF of 11.6 and a SEER of 22.7. The 12RLS2 has slightly higher performance, but if the indoor head is mounted lower to get the heat and cooling closer to you make sure it's protected from being bumped. The infloor heating can be used to give a background heat level, but if it gets very cold in the winter quickly the heat pumps output will drop as the heat load of the house goes up and the infloor heat will take a while to start making up the difference. You might need to get used to moderate temperature swings, but nothing that a good sweater and warm slippers won't handle.

41 Regulate with Radiant Heat posted by Garrett Kerns on 02/18/2014 at 02:53 pm

Well the day is finally here.  My unit is going to be installed on Weds.  My question, is I currently have standard thermostats throughout my zones in my house.  How do I regulate the hydronic in floor with the heat pump to maintain both a warm floor with the heat pump?  Is that possible?  Do I not want to do that?  I am only installing one 12RLS2H on my main floor that is 1760 sq ft and I am going to run the in floor in the basement.

I was thinking I would just set my house to 55 deg for the in floor and then set the heat pump to 65 deg.  If the heat pump can't keep up the in floor would kick on, but I don't think that would be very efficient as it takes the floor so long to heat up.

Do I need to buy new thermostats with a floor sensor?  Do those exist?


42 Hydronic regulation posted by Mike Clarke on 07/09/2014 at 11:21 pm

I live 'down under' in New Zealand. I believe the secret to successful hydronic floor heating is the use of sensors that can accurately measure floor temperature. In other words floor type sensors are an absolute requirement. Air sense type sensors, usually wall mounted around shoulder height, just never give the right message. Honeywell can supply thermistor based, wall mount, floor sensors - try for TH131-AF or TH133-AF.
Your mission, should you accept it, is to somehow get these thermistors into your concrete floor without cutting through the existing pipework - good luck.
Mine were installed at contruction, inserted into small bore copper pipe, and chased into the concrete floor, then plastered over (I knew where the pipework lay, from the plans i had).
I'm a huge believer in hydronic floor heating, but floor slab insulation detail is critical.
It is just so much more comfortable than having heated air blasting around one's house, and the noise of those outside compressor unit whirring away on a quiet, cool night is downright.......chilling

43 sizing and installation questions posted by Michael Cyr on 03/09/2014 at 11:04 am

Hi Peter!   Wonder if you remember me.. I bought an old VW engine-powered welder from you sometime back in the early 1980s.

I now live in Central Maine in a 2500 sq ft saltbox I built.  Its pretty energy efficient and I heat with 1.5 to 2 tons of wood pellets/year. 

The house has no backup heat and I'm looking to make a mini-split my primary and the pellet stove the backup.  This article, and the discussions here, are just about the best I've come across, thanks!  I do have a few questions for anyone who can help.

1. Did I do this BTU calc correctly (based on Peter's formula above)?  2 tons of pellets, @ 70% efficiency appears to be 23m BTU.  Average annual total degree days for Waterville, ME is 7261 (5 year avg).  23m/7261 = 3168 BTU/degree day for my house.  Avg degree days per day in Waterville for January (our coldest month) is 44.  3168*44/24 = 5808btu/hr.  Does this sound right? 

2. Does it hurt to oversize a unit?  Will this improve low-temp performance?  We've been close to or below zero almost every night for the past month.

3. Are heat pumps more efficient at cooling than a windows A/C?

4. If the outside unit is mounted on an outside wall, will it transmit much vibration into the wall?







44 sizing and installing posted by Peter Talmage on 03/12/2014 at 09:38 am

Hi Michael,

Wow a blast from the past. I do remember that welder. I'm always amazed at some of the stuff I spent many, many hours building and then never used. Dreams of 60 foot steel hulled boats. Anyway, to your questions:

1. That's an OK calculation, but it gives you an average heating load. In the dead of night the exterior temp will be much lower than during the day, so the heat load will be greater than the average. A house temp set back will help reduce the difference somewhat.Also, your pellet stove probably heating one area of the house much better than other sections so to replicate its performance the heat pump inside unit would need to be in the same general area. If you are going to use the pellet stove as the back up heater when things get really cold then these calculations aren't so critical. I'd probably add 50% to your number.

2. As long as you select a heat pump with an inverter driven compressor, the ones that can vary compressor and fan speed according to demand, the efficiecny stays pretty constant. The benefit from a large heat pump is the higher output at lower temperatures. Note that the the typical family of 9K, 12K and 15K or 18K heat pumps are basically all the same unit with higher compressor and fan outputs. The efficiencies drop a little because the evaporators and condensers have not been increased in size.

3. Minisplits are much more efficienct than window units. The best window units have an EER (Energy Efficiency Ratio) of 11 Btu/Wh. The best mini-splits have an equivalent EER of about 24.  Mini-splits are actually rated by SEER (Seasonal Energy Efficiency Ratio) which is about 115% of EER.

4. Heat pump compressors are fairly quiet, but depending on load a can sometimes growl a little. Mounting on a wall bracket for the most part will be OK for a well built, well insulated wall. However, I recommend a ground mount on a tall secure stand with good shielding from snow. Vibration from the compressor can be transmitted through the refrigerant lines so keep the lines no shorter than 15 feet.

This winter has been a good test for my 4 year old 3/4 ton Fujtsu mini-split. I sold my old woodstove so the only other heat I have is electric resistance heaters (if I care to use them, NOT), passive solar gain and the excess energy from my solar hot water system. What I usually do in the dead of winter is turn the temp down to 62 at night when we turn in. The house will slowly coast down overnight and if its around 0 or above in the A.M. the house will be right around 60-62. That's downstairs. Upstairs its a few degrees cooler. The heat pump is puttering along delivering warm air at a moderate pace. If it has gone down below 0, like -10, the heat pump is still working, but only putting out luke warm air and the house will be down around maybe 58. I set the temp back up to 68 and as the outdoor temp rises the heat production climbs and everything warms up. Having the compressor on the east side of the house where it sees the sun early helps. I certainly dress up with a good sweater, (thanks pres Carter), but even on cloudy days that little 3/4 ton unit brings the house back up to 68. Yes my house temp cycles and yes I making use of the thermal mass characteristics to work with the heat pumps abilities. This works very well and on sunny days when the passive gain and excess solar thermal energy take over I shut the heat pump off.


45 Thanks! posted by Michael Cyr on 03/20/2014 at 11:31 am

Peter.. Thanks for much for the great followup info!  I'm working with Sam Zuckerman up here in Maine.  His company is Maine Solar Solutions.  Really like working with him.  He did my gridtie PV array too.

Thanks again and if you ever find yourself up around Harpswell, would love to take you for a sail and talk alt energy. 


46 MSZ-FE12NA system -- is this normal? posted by mark g on 03/20/2014 at 09:56 am


I live in southern NH and have the above Mitsu system installed. Its setup upstairs with about 20' of piping going to outside compressor.The situation i'm having is that the inside unit cycles on/off constantly. It comes on and heats up area, then the vanes go back to a 'parked or level' position and the unit outside makes a whooshing noise which i'm assuming is the defrost cycle. When it goes to heat the area up again, the vanes move to the set position (lower) and fill area with heat. It does this several times throughout the night and is usually associated with what i can describe as a clicking or cracking noise of either the inside unit or the piping. It sounds like plastic is expanding/cracking almost. That noise varies in decible level to barely audible to rather annoyingly loud. Seems to happen constantly when temps are in 30's and hardly at all during lower temps. It's definitely caused loss of sleep. Has anyone else experienced this clicking/cracking noise from the inside unit?The unit itself on the wall can be physically slid up on the wall. It is easily moved up by applying upward pressure on bottom. I was not around during install, but imagine the installation of the unit slides down onto a track or similar. Is the inside unit supposed to have this movement?


I've contacted Mitsubishi and they said to contact installer. Contacted installer and he said the unit can go into defrost cycle whenever it feels like it. Mine seems to go in/out of defrost several times throughout the night. The clicking/cracking is really my issue. I hate the fact that it keeps me from sleeping.


Thank you all,


47 crackling posted by bob coleman on 03/20/2014 at 02:22 pm

Unfortunately you are experiencing one of the symptoms I would have liked to known about before having bought the unit.The indoor unit is made of flimsy plastic and will expand and contract with heat gain and loss, similar to the pinging of old heat radiators. Because the defrost cycle has a cooling effect, it will be more dramatic around those times. The metal tinging sound is the metal expanding/contracting.If the unit is located in a sleeping quarters there isn't much you can do besides move it.I grew up with metal electric heaters and recognized the sounds right away. It felt like I was inside a submarine during the winter with a constant sonar metal pinging sound. The plastic just pops.If you absolutely can't move it, you could experiment with tinkering with the plastic panels, but it is likely to be a maddening effort with little or confusing results and would definitely kill your waranty.Occasionally I imagine due to manufacturing tolerances a more severe problem may develop; replacing the indoor unit would fix that but it's an expensive gamble, and it might get worse.

48 crackling posted by mark g on 03/21/2014 at 08:36 am


Thank you for chiming in.You describe exactly what i was thinking.The installer came out and padded areas all around to try and stop the cracking/popping.It DID help, but still is there. Unfortunately this is right in the bedroom and moving it is not an option.I'm going to experiment with window/door expanding sealer and report back with results.I've already had very good noise reduction by spraying this foam sealer into the cable / hose covering leading upstairs. The foam virutually elimnated the transfer noise through the pipes coming into the bedroom via the indoor unit.


49 More crackling posted by Peter Talmage on 03/20/2014 at 08:13 pm

I wonder if this is an issue with just the Mr slim units. None of the Fujitsu units I am familiar with make anything but a slight guggling when they go into a defrost cycle. Heating and cooling of baseboard hydronic elements always produced ticking noises in my parents house, but a little fiddling with the enclosures usually stopped them. It might be possible to locate what parts are producing the noise in the indoor unit and fiddle with them as Bob suggests.



50 more cracking posted by mark g on 03/21/2014 at 08:44 am

Peter, i'm definitely fiddling with this thing.I will eventually get it right even if it means re-plumbing the entire system.

There is no joy or comfort in having a unit that pops/squeaks/cracks while your trying to sleep.That is certain!

I am hoping engineers at Mitsu (or Fijitsu) eventually resolve this for future customers.



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