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Ductless minisplit Location

Mathieu_Montreal | Posted in General Questions on

Good morning,

I am in the process of getting quotes for a new ductless mini-split heat pump for my house in Montreal area.  The overall dimension of my home is about 30’x30′.  The front of house is the wall where the stairs are.

I have quite a few quotes from different vendors (Carrier, Lennox, Directair, Daikin).  I am still trying to compare them for their efficiency and reliability.  Do you have any opinion on that?  I would like the unit to be efficient to heat the house in winter, as winter are cold and long here in Montreal.  The unit would cool the house about 20-30 days a year.

I am doubting the efficiency of the mini-split that would be located on the second floor.  I have received 2 propositions for the location.  The first one would be a air handler located where the red rectangle is.  I am pretty sure that this would be useless to cool down the rooms and most of the air would be pushed down the staircase.

The second location (yellow square) would be a cassette.  Do you have an opinion on the efficiency of this installation?  Would this be enough to cool down the rooms?  Would only the small hallway be cool and the unit be constantly cycling?  Do you have an opinion about the reliability of such devices?

Thank you for your help.

Mathieu Leduc

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  1. this_page_left_blank | | #1

    We'll need to know the exact model numbers to compare efficiency numbers.

    I'm doubtful of your ability to both heat and cool the whole house with a single ductless. I had a single unit on the first floor, and while it effectively heated the entire house it was woefully inadequate for cooling the second floor. I suspect you'll see the opposite for a second floor placement. I now have one ductless on each floor. The bedrooms upstairs are a couple of degrees warmer (at worst; when it's not really hot outside the difference is less) than the hallway when the doors are open.

  2. minmaxdesign | | #2

    I have done this in two very, very efficient homes. One was single story and one was two story. You will definitely need one unit upstairs and one down if you want cooling on both floors. You might be able to get away with one unit downstairs for heating only but there are quite a few considerations.

    In the two-story home with a wall unit in the hall, I found it was difficult to cool all three bedrooms adequately. The bedroom area is just too closed off and the master bedroom was far enough away. However, all of that was with the bedroom doors open. Once the doors are closed, it's a whole different story- it severely hinders any type of cooling. There was a big difference between the hall and the bedrooms with the door closed. To me and my family is was manageable, but definitely could have been better. I don't have quantitative data or anything, but can just say we could notice the difference and didn't need a thermometer.

    In the single story we are in now, we have one unit in the hall heating/cooling three bedrooms and two baths. Similar issues, doors open no problems, door closed and noticeable but tolerable temp difference.

    Ultimately it comes down to few key things:
    -Can you tolerate relatively large temperature swings?
    -Are you willing to sleep with your bedroom doors open?
    -How well insulated and air sealed the home is and what the heat load analysis shows for each room.
    -Might want to consider a ducted minisplit system.

    Kohta Ueno of Building Science Corp has a report on minisplit placement. Additionally Marc Rosenbaum talked about his experience with minsiplit placement.

  3. Mathieu_Montreal | | #3

    Thank you for your reply. And thank you for the reference to the reports. I plan reading them.

    I plan to have 2 ductless. Since the first floor is more open, I am less worried about the ability to cool/heat the whole area. What I am worried about is the bedroom on the second floor. We would not mind sleeping with the doors open when it is very hot. However, I would like to know if the additional cost of a cassette (yellow square) would be worth it and allow a better cooling of the bedrooms.

    Thank you again

  4. Expert Member
    Akos | | #4

    If you want proper cooling for the bedrooms, the only way is with a slim ducted unit with ducts running to each bedroom. I've mounted these before either in the hallway ceiling or the ceiling of the bathroom.

    Generally the problem is that most HVAC guys won't want to install this or deal with the bulkhead work in the house. If you can get those done either by DIY or subbing out and just have the hvac folks install the unit. It is a relatively straight install and should be close to the same cost as a wall mount.

    If you don't want to go that route, the ceiling cassette is a better option as you can direct the airflow out of it towards the bedroom doors. With the doors open, it should work reasonably well. The only challenge with these is they are big, so means cutting ceiling joists, framing in and installing a proper air sealed/insulated cubby over the unit in the attic.

  5. Mathieu_Montreal | | #5

    I forgot to mention. Here are the 3 models that were proposed to me
    Daikin Aurora 2MXL18QMVJU
    Carrier 38MGRQ18B-3
    Lennox MLA012S4S-1P

    1. Expert Member
      Akos | | #8

      Usually the most energy efficient and flexible setup is two single zone units. Multi splits tend to have much lower modulation range, correctly sized they can work but it is hard to get there when you need much more heating capacity.

      You also want to make sure the unit is a low temperature hyper heat type with a pan heater.

      The Diakin or two of the Lennox units would be your best bet. If you go with the multi, make sure the Daikin is correctly sized for you actual cooling and heating loads.

      If you have previous heating bills for the place you can run through the steps here to check:

      A great technical resource for compering systems is:!/product_list/

  6. STEPHEN SHEEHY | | #6

    One suggestion is to use double pocket doors for the rooms. A five or six foot opening may help spread the cooling/ heating. In our former house, we had two 3' pocket doors in the bedroom upstairs. We had no heat upstairs, but leaving the doors open kept us warm enough.

  7. Mathieu_Montreal | | #7

    Thank you for your answer.

    Akos : where would the slim duct unit be mounted? also, where do you run your ducts? Do you run them in the ceiling corner of the room and then add gypsum to cover them?

    Thank you again

  8. Expert Member
    Akos | | #9

    You normally mount it under the ceiling and cover with a new false ceiling underneath (see attached). If the unit is centrally located, most of the ducts can run inside this false ceiling to registers in the bedroom walls. If you need longer runs, you can run them inside than build a small bulkhead around it. Generally running bulkheads on the inside above doors is not noticeable at all.

    Generally you loose about 12" to 14" of ceiling height, seems like a lot but not very noticeable once built. There are also some units that can be mounted vertically, these you can put inside a closet if you don't want to loose the ceiling height.

  9. Mathieu_Montreal | | #10

    Would it be safe to run the ducts in the attic between under the current insulation? The duct would sit between the insulation and the gypsum. I think my bathroom exhaust fan is installed this way.

    1. Expert Member
      Akos | | #11

      Hvac ducts and bathroom exhaust ducts are not the same. Bathroom exhaust air is air that is already leaving your house, losses there don't add to your building's load. With HVAC you are running conditioned air that will enter back into your house, any thermal loss through the insulation and from air leaks is now lost to the outdoors.

      Generally ducts in the attic are a bad idea. Can be done but you can have issue with condensation and can bump up both your cooling and heating load significantly.

      About the only way I would do it is if you can run the ducts just above the ceiling and cover the whole thing in spray foam.

      Ducts inside conditioned space are always better.

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