Passive house heating and cooling systems using ductless mini-splits from Fujitsu: 9-12-15 RLS2h, RLS2, RLS heat pumps

Passive House eating and cooling systems with Fujitsu ductless mini-splits (heat-pumps):
Fujitsu 9-12-15 RLS2h, RLS2, RLS

Latest Fujitsu 9-RLS2h, 12-RLS2h, 15-RLS2h Ductless Mini Splits

While we fully expected that we would not require active air-conditioning in our passive house, we did anticipate the need for active heating.

The notion of passively heating a home from the thermal energy of the sun alone, certainly sounds appealing enough, but trust me, there are going to plenty enough days during the colder months when long-periods of overcast are going to prevent these sorts of natural interior heat-gains.  And given the sheer size of our home and the layout of the rooms--even with a very open floor plan--active heating was going to be required.

We considered a number of options: a conventional heat-pump forced air system, geo-thermal heat-pump forced air-system, hydronic radiant heating, and combustion-based systems.

While hydronic radiant floor heating is regarded as being a superior way of heating, we would still have to concern ourselves with a means of air-conditioning (if we ever needed it or wanted to use it) using another system.  This, in my view, added unnecessary complexity to the active heating and cooling systems--since we would have to install and maintain two systems.

Geothermal initially looked appealing, given their advertised high COPs (coefficient of performance) and the ability to optionally provide domestic hot-water (for "free").  However, these units are expensive to install, lose a lot of their touted efficiency to their ducting and air-handling systems as well as their fluid circulation systems.  Furthermore, these systems' heating and cooling capacities would far exceed the heating and cooling loads that our passive house would present.

Since our desire was to use electric everywhere (to easily offset with a photo-voltaic system) to achieve an either net-zero or net-negative (generating more onsite electricity than consumed) building, we decided on the use of ductless mini-splits to do the job.  A great thing about ducted mini-splits is that they are dirt cheap and super energy efficient.  Effectively equaling (if not exceeding) the efficiencies of their more elaborate subterranean counterparts. In the case of Fujitsu, their minis (heat-pumps) are rated at a whopping 27.2 SEER and have an EER of 15, at optimal outside conditions.

Ductless mini splits can also be purchased as ducted units, however adding additional ducting work was not desirable and ultimate efficiencies are reduced relative to their non-ducted counterparts.

The approach we used was to install two ductless mini-splits centrally located in our two story great-room and to use the Zehnder passive house ventilation system to effectively distribute the conditioned air throughout the home.

At the time of our selection, Fujitsu was just about to release their revised Fujitsu 9RLS2, Fujitsu 12RLS2, Fujitsu 15RLS2 ductless mini-splits.  The minis were designed to operate at temperatures of -5 degrees fahrenheit and perform at their rated heating capacity to 20F.  This was an improvement over their previous RLS models.  The timing was perfect because we were able to install, we understand, the very first RLS2 models introduced to North America!

The question then became, what specific model would we require?  There are three sizes available from Fujitsu, the 9000/btu/hr 9RLS2, the 12000/btu/hr 12RLS2, and the 15000btu/hr 15RLS2.  Interestingly enough, when it comes to heating, all three units provide nearly the same level of maximum capacity of 22,000/btu/hr.  It's only with their cooling mode, that their relative capacities become more differentiated.  Again, since our active air-conditioning requirements were expected to be absolutely minimal, we opted for the most efficient unit of the bunch, the 9RLS2.

During a recent conversation with Fujitsu, I was informed that Fujitsu will soon be introducing an update to the RLS2 models--hyper-inverter model versions: the Fujitsu 9RLS2h, Fujitsu 12RLS2h, Fujitsu 15RLS2h.

The interior air-handlers remained unchanged, but the exterior units will now be able to operate to -15F, opening up markets in more northern climate zones.  Performance efficiencies appear to be nearly identical, until the outside temperatures reach the extreme cold of the spectrum.  I also understand that Fujitsu has improved the efficiency of their defrost cycle with these updated units.

After twelve months of usage, we are very pleased with the performance of these minis.  In a future article, I will address some location placement tweaks we made to our ductless mini splits that are proving to be extremely effective.

It is really amazing to see that even during the extreme cold temperatures that we have been experiencing lately (lows in the single-digits and highs in the low 10s), that merely two space heaters can effectively heat an entire home of 6000SF using less than two tons of heating capacity!

During the summer months of last year when outside temperatures hit an excess of 105F, we were able to maintain an average in-house temperature of 67F using only the dehumidification (dry) mode of ONE unit! Amazing, indeed.


  1. Hi Bob
    I'll be very interested to read your future post about the location tweaks of your mini-splits.

  2. Hi Bob

    Getting ready to do a deep energy retrofit using the same Fujitsu 15RLS2 units.

    Love to know those tweaks you mention.

    Any chance you will be posting them soon.

    Great site that you have done here with lots of great info. Thank you so much.


  3. Sure Brian. I'll make that my very next article. I'll start working on it now.

  4. Thanks Bob for posting the ductless tweaks so quickly.

    My situation is a bit different where it is a deep energy retrofit on an existing house. We essentially are heavily insulating and trying to get off oil. On paper we have it nailed down but it will require 5 ductless units throughout the house. I am very impressed you can get buy with only 2 of the 9RLS2 for your size house.

    I am encouraged that I am on the right path as I had specked out many of the same products for my house before I found your blog.

    Do you have any idea what the 2 ductless units are costing to run each month?

    Also in a previous post you mentioned energy monitoring software. I am in the process of deciding what I am going to use. Please let me know if you found one that works.

    Thanks so much.


  5. Your more than welcome.

    I figured I'd invest the time to do so as both you and Mimi were interested. With respect to your questions. I believe I need more information before I can attempt a response.

    1) Where is the home located?
    2) Are you operating mechanical ventilation of some sort?
    3) Can you provide an overview of the layout of the home you are retrofitting?
    4) What is the cost per Kwh all in where the home is located?

    You probably want to consider the RLS2h series which is replacing the RLS2 series. I can then give you a better estimate of expected operational costs.

    I will be publishing some articles detailing what the operations have been like this past cold season.

    The cost of operation

  6. Bob

    I live in MA. I considered the RLS2h models but it only gets down to 0 degrees here for less than 24 hours a year. The h versions costs a bit more to run and where I need several of them I decided the RLS2 would work best.

    House is 3,700 sq ft that has 3 large rooms(great room, exercise room, master bedroom) that are 25 x 30 with 12 ft ceilings. Each of them will have their own 15RLS2 units. Another unit in the kitchen/Dining room 25 x 30 with 8 ft ceilings. Then 3 small bedrooms 10 x 12 and an office that will share 2 9RLS2. 5 units total and hopefully only 2 being used regularly. One of the 15RLS2 and both 9's are in areas that we don't frequent unless we have company so I am hoping to use these minimally.

    We are working on a plan for mechanical ventilation but we were told it is more for fresh air than to spread heated air?

    Cost is .15 KWH but once we install we should be getting a lower rate of .13 KWH.

    We have a Stiebel Eltron Accelera going in and our old oil furnace and boiler going out.

    We will also use Stiebel-CNS for back up heat if needed in master bedroom.


  7. I wasn't aware that the RLS2hs cost more to operate than the standard RLS2 models. From the charts that I reviewed, they appeared nearly identical, I believe.

    Of course I can not estimate how much heat your home will require, but I can suggest that the nominal operating costs of an RLS2 model is about 750watts or .8 Kwh/hr of operation. I hope this helps.

    In PH designs, ERV/HRV systems distribute the [conditioned] air around the building. They only make sense, of course, for tight building enclosures.

    1. Thanks Bob

      Wondering if you can break down that .8 Kwh a little more.

      I am concerned because I will have several of these units and I don't want them combined to be running 5 Kwh per hour.

      If the units are set for a very low demand do they use a lot less power?

      My house is old and has some major leakages that we are working on but we will not be in the PH range but should be in the 50-70% more efficiency range and most importantly saving $8-10K a year on oil.

      On the AHRI site the H models have an estimated cost that is about $50 more each year for heating, but cooling is the same.

      Thanks again for all the info

  8. I would like to see that report. I am not aware of increased operating costs, merely a small premium in purchase price. If you could provide a link, I would appreciate that.

    Since these are variable in their operation than can operate with as little (3000btu) to their maximum output. When set temperatures are achieved, the units go into "monitor" mode and only consume about 60 watts during this period.

    The rated efficiencies do drop to below two when one gets into the low 20s or high teens. However I would expect COPs to average at about 3.

    The amount of heat you need to supply in your house to keep it comfortable is fixed at whatever the house is performing at. So, in my view, it simply comes down to a COP rating of the unit(s) which puts that heat into your conditioned space.

    The units can be set with economy mode which automatically drops temps 4F below set temp when no motion in the room is detected.

    Of course the units can also be programmed with set times of operation.

    While not as efficient, have you considered ducted versions (multi-zones) with one outside unit?

    1. Here is the link, if it works.

      You might just need to go to the site and in the top right corner click on certificate directory. Go under Fujitsu and the units are right next to each other RLS2 vs RLS2H. Under estimated cost to run you will see the differences.

      I have run more calculations and feel that this is still my best investment. I have significant issues with the existing duct work so the ductless is more cost effective.


  9. You can also see Fujitsu posted HSPF for the new models and they are much lower, especially for the 9 model which goes from 10.5 to 8.5IIRC.

    That base element heater sucks some juice, and at lower temperatures is not smart like the heat pump and is always on at COP 1 in exterior weather. If you live in an area with frequent drops below freezing, it will really eat up the power.

    As you've noted you like simple solutions, the problem the heater pan element solves is an aesthetic one mainly. The outdoor unit doesn't need a 'base', especially in a shielded area. As long as you have decent clearance below, condensate can drip down as the defrost mode runs, and with no pan, will drop out of the way. With a pan, it collect and refreezes.

  10. Good post! Thanks for sharing this information I appreciate it. God bless!

    ductless heating and cooling Boston, MA


Post a Comment