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800 cfm from a ducted minisplit?

reimhagen | Posted in Green Products and Materials on

I paid someone to perform a manual J calculation for me for my house renovation project, in which I’ll be converting my 3500 s.f. house to heated/cooled exclusively using minisplits.

He convinced me to use a ducted system for the large common area (kitchen + dining + living room).  I’d say it’s roughly 1000sf.  He came up with this design.  Is this too much for a ducted minisplit to handle?  He has told me he doesn’t do minisplit calcs often, usually they’re central systems.

I’ve been looking at LG products, and they don’t seem to have a ducted minisplit that can do 800 cfm.  What confuses me a bit is when he showed me this design, he says that he calculated it using a low static value (<0.3).  I don’t really understand the relationship between CFM and static pressure, but LG’s low static units don’t put out 800 cfm according to the specs.

Thanks in advance.

P.S. – I was originally intent on having wall-mount units instead of a ducted system here, but was convinced otherwise by the guy performing the calcs.  Let me know if you guys think wall units are still a better idea for this area.

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  1. joshdurston | | #1
  2. Jon_R | | #2

    > I paid someone to perform a manual J calculation

    But what you need (eventually) is Manual, J, S, T and D.

  3. reimhagen | | #3

    I saw that LG unit but didn't seriously consider it because, as you said, it's way overkill.

    I guess my question really is after reading about the downside of having the ductwork in the attic (which is what this plan is saying I should do) -- will wall units be sufficient?

  4. STEPHEN SHEEHY | | #4

    My open kitchen, living, dining area is about the same size as yours. A single ductless minisplit, Fujitsu 12RLS3H, heats it well. House is very tight and well insulated. Zone 6 (Maine).

  5. joshdurston | | #5

    If that floor really is completely open. Then a single well placed wall mount would likely serve it very well. Perhaps located on the living room wall (right hand side of the image), discharging towards the dining room. I would recommend getting a wall mount controller/thermostat for better sensing, and placing it appropriately.

    A wall mount unit will be more efficient than the ducted unit, and losing the ductwork will save substantial costs. If you are going to oversize, then going with a 1:1 unit will be better since it will have a low min capacity versus a multi split. With Multi splits the heads typically don't modulate their capacity so getting the sizing right and not oversizing significantly is even more important.

  6. BillDietze | | #6

    I second the wall mount unit in the open area. I have a single Fujitsu 15RLLFH in my basement and another on the main floor above. The walkout basement is ~1,400 sq.ft. and the main floor is 2,000 sq. ft. Design temp is -6F and the main floor unit heats the living, dining, kitchen, entries and hallways just fine. There's an open stairway so the basement unit helps heat the main floor. If the doors to the main floor bedrooms (at end of hallway) are left open, at night they run about 3 - 6F cooler than the main area, depending on the outside temp. I have cove heaters in the bedrooms to help out when it's 5F or colder outside.

  7. reimhagen | | #7

    The area is mainly open with the exception of a wall separating the entry foyer from the kitchen. It's a wall of cabinets that will also act as pantry space. One concern the guy brought up about the wall units is most of the air throw is down low, so he said it would be easily blocked by couches and tables and various other pieces of furniture. In your guys' experience, is that a valid concern? Is there some sort of mini-split feng shui I should keep in mind when layout out furniture with wall mount units?

    I've attached a rudimentary diagram of how I'm considering designing the system. Removed the ducted system, and put in 2 head units to condition the entire space. Do you guys think it's overkill? I assume in this case I'd just divide up the zone's BTUs evenly between the two units.

    1. Expert Member
      Akos | | #8


      You are on the right track with the ducted unit, you will end up with a much more comfortable house.

      Typically the units are not sized by CFM but by BTU. With a typical split that 800 cfm works around 25000 btu head.

      Depending on where you are locted and how big your windows are, it feels like 25k BTU is oversized.

      Also the LG units don't have very good low temperature heat performance (go from 27000 btu at 47F down to 13000 btu at 4F), depending on where you are located, it might not be the best choice for heating.

      If the 25000btu is correct, LG has a mid pressure head (LMHN240HV) that will do 0.4" HG and around 800 cfm at max that will work. If you are going with a multisplit, make sure you check the combination tables as the minimum size outdoor unit for that head is a 36000 btu.

      If you are going with multi splits, it is important to get the loads correct as the turndown on them is not great (they'll be expensive to run if they cycle).

      1. reimhagen | | #13

        Yeah, the load for the room is calculated via manual J as roughly 13K heat and 17K cool. The LG unit that can do 800CFM is way oversized for the space. I guess going with wall units is still the better option for an efficiency standpoint, while ducted is better from a comfort standpoint. Given the other problems related to the ducted units (mainly the presence of ductwork in my attic), I think the wall-mount units is likely the better path to take.

    2. STEPHEN SHEEHY | | #9

      Reimhagen: On our Fujitsu minisplit, you can adjust the vanes both vertically and horizontally, by using the remote. So you can direct the airflow as appropriate. Unless your calculated heat load requires two units, I think that space could be heated ( and cooled) with a single wall mounted minsplit.

      1. reimhagen | | #12

        Thanks for the info. I'll consider just one unit. Do you have a recommended placement for it? Pretty much where #1 is on my drawing? (right hand side)

        1. STEPHEN SHEEHY | | #14

          I'd put the inside unit at #1, unless there's an installation issue concern. There are maximum and minimum distances between inside and outside units.

        2. Expert Member
          Akos | | #15


          If you are going with a wall/floor mount I would put on your hand drawing at the end of "dining room" label (near the jog). You want it central and your kitchen cabinets would block flow otherwise.

          A single wall mount would work for cooling but will be uncomfortable for heat unless your house is air tight with good windows. If you are going with the single wall mount, I would install electric baseboards or electric floor heat in the entry and kitchen area. If you don't do it now, you will end up doing it later for comfort. Going this route will be cheaper in parts and only slightly more expensive to run then the ducted unit.

  8. BillDietze | | #10

    Yes the floor mounted units should have a clear air path in front of them. No special mini-split fang-shui. If you don’t have a place for the floor unit with zero obstructions, then you shouldn’t use one. If there is only one, it should also be centrally located with a nod to where the windows are if you have a lot of them and a low design temperature.

    1. reimhagen | | #11

      I was referring only to the wall-mount units, not floor-mount units. Is what you said valid about wall-mount units too?

  9. joshdurston | | #16

    I would recommend a wall mount over a floor in this case, just to get better coverage since it can move air above obstructions. But a carefully place floor mount can work well. I like the idea of the floor mounts for heating pulling the cold air off the floor (both for efficiency and comfort).

    Part of the reason for putting it on the living room wall, is that you tend to be stationary, and relaxing there, which makes it a comfort priority. The rest of the floor will be pretty close to the same temp. Some areas like the front entrance are transient and don't require the same precision for comfort. The dining room and kitchen will be fine too.

    If you committed to LG, their top performing unit is a wall mount. The Art Cool Premier LA150HYV2 is a pretty good match for loads, and has some extra head room between the rated capacity and max.

    1. reimhagen | | #17

      Thanks Josh. For reference, I never mentioned floor units, my dilemma has always been between wall units and ducted in-ceiling units.

      Not that committed to LG either, based on what Akos said above I looked at the effiency ratings at 17F and agreed with his evaluation that LG's cold-temp performance is not that great. What is your recommended brand(s)? I wasn't able to find the Engineering Manuals for Daikin or Fujitsu units, but I saw Mitsubishi's units with the hyper-heat technology vastly outperforms the LG at low temperatures. I'm in the Seattle, WA area though, so our temperatures only rarely dip below 20.

  10. joshdurston | | #18

    All those brands have some quality offerings. I don't want to sound like a cheerleader for a certain brand but here is my opinion. I'm generalizing here, and there are exceptions for sure.

    IMHO Mitsubishi and Fujitsu are the front runners, but each with its own strengths. Fujitsu is the king of ultra low temp heating (below -15F), maybe not that important to you.

    Mitsubishi often has lowest min modulation, and good part load efficiencies. And great low temp down to at least -17F.

    You really have to look at the specific model being considered for your application, there are duds scattered all over everyone's product line. You have be an informed purchaser.

    That being said the LG unit I recommended seems to match your loads better than any of the Mitsubishi FH models. The Fujitsu 15RLS3YH would be good too. The comparable Mitsubishi MUZ-FH15NA comes up a bit short in cooling capacity compared to your 17kbtu design number (EDIT: It looks like the FH15 has enough cooling it's rated capacity is 15kbtu, but it's max is 18kbtu). Perhaps with windowshades, or overhangs you could cut enough BTU off to make it work too.

    You could go with two smaller heads, in that case I would consider the mits FH models for nice low minimums. I would only go to two heads if you were still going 1:1 rather than multi split.

    I think the LG Art Cool Premier line is pretty decent too, it's notably better than most of the other LG offerings. The HSPF and COPs are pretty comparable with the best. I'm actually considering one of them for an area of my house right now.

    It's sometimes hard to translate specs into real world since the operating sequences and controls are bit different for all of the them.

    So LG, Mits, or Fujitsu (and maybe Daikin) all have a decent wall mount units that may work for you. The contractors you have available may be what ultimately makes the decision for you.

    I'm in Canada where heating dominates my utility bills, if you are somewhere that is more cooling oriented there are more and cheaper options than the ones I mentioned. What's your outdoor design temperature?

    1. reimhagen | | #19

      Great, thanks for the info. Brand availability isn't that much of a concern as I was planning on purchasing the equipment myself online, do most of the install myself, and then have an HVAC guy come out to do the final refrigerant charge / commissioning of the unit. I know that doing this will void the manufacturer's warranty of several of the brands, but imo the money saved is worth it.

      Could you explain what 1:1 vs multi-split means? Due to the shape of the room I still slightly prefer having two smaller heads instead of one big head. Is that 1 indoor to 1 outdoor unit vs multi-zone outdoor units?

      He has my winter (heating) design outdoor temp at 28F and indoor at 70F; summer (cooling) design temp is 82F out and 70F in.

      1. joshdurston | | #20

        The 1:1 units have a dedicated outdoor unit for each head. The multi splits have 2-8 indoor heads tied to one outdoor unit.

        The 1:1 unit has more flexibility in modulating its capacity and often can run at a wide range of capacities, often giving an 8:1 turn down. (example cooling between 12000btu/hr and 1600btu/hr but without ever shutting off). They also tend to be cheaper even though you have more outside units. And also more efficient. The HSPF (measure of heating efficiency ) often approach or exceeds 13 on a top tier (brands you mentioned) cold climate 1:1 wall mounted mini split.

        Multi splits tend to be a bit less efficient than the top 1:1 units, they package nicer since you only have one outside unit. But can sometimes have complicating additional details like branch boxes to distribute refrigerant and control wiring as part of the install. The outside unit can modulate over a range of capacities as different combinations of inside units run. But the individual units tend to always run at their rated capacity. So a one ton unit will provide about 12,000btu/hr of cooling until the space is satisfied, then it will shut off and wait for the temp to rise again, rather than just modulating down and continuing to run. This makes them more sensitive to sizing, and even if you get the size right they will still cycle on/off rather modulate most of the year.

        A multi split system can still be a great system, and may become a necessity once you exceed 3 or 4 heads, since it may get a little awkward and ugly to tons of outdoor units all over the outside of your house.

  11. joshdurston | | #21

    If you are doing two heads a pair of Mitsubishi FH09 or FH06s might be a good setup. The FH09 and FH06 have the same minimum outputs, so there doesn't appear to be an oversizing penalty if you go with the FH09.

    1. reimhagen | | #22

      Those units look nice. Is your suggestion of those units still predicated on the assumption that I'm going 1:1? The whole house is going to be mini splits, so I was hoping to have a total of 3 multi-zone outdoor units. The house in total will have 7 heads excluding this common area that we've been discussing. I'm assuming the downside for multi-zone is just from the perspective of energy wastefulness; will the cycling affect the lifespan of the equipment? How much more energy are we talking about? 10%? 20%?

      With the current plan, the two head units in the common area will go to two different outdoor units (both are still multi-zone outdoor units with multiple heads connected). Not sure if that affects things at all.

      1. joshdurston | | #23

        If you're doing 7 heads multi split is the way to go. But I would be surprised if you need three outdoor units. The small high HSPF, high SEER 1:1 are probably in the ballpark of 10 percent more efficient. It's hard to say since the multi may often be running low and efficiently with just one or two heads calling. A well designed multi split is still a great solution. Just be careful not to oversize your heads.

        Adjacent zone can share capacity a bit. So if you're a couple thousand BTU short in one area as long as you have enough overall you should be fine. Also, you can sometimes take advantage of some diversity especially for solar loads. If rooms are getting peak sun at different times of the day, the loads are not necessarily additive.

        You could also do a hybrid of 1 or 2 singles and a multi. I think you can go as high as about 8 heads on one outdoor unit.

  12. reimhagen | | #24

    So if I understand you correctly, I should try to make my predicted most-used units to be 1:1. Previously I was going to share the #1 head in my drawing with two heads that share the same wall in the basement below. I expect the kitchen/living room unit to be run way more than the units downstairs in the basement. So, to gain some energy efficiency, I should strive to make the kitchen unit a 1:1, and have the basement units on their own separate unit. That way the kitchen unit can modulate down to keep the set temperature. The equipment seems affordable enough for me to put some more units outside; it'll just be somewhat more of a visual blight. If you think the energy savings would be significant, it'll be something I'd seriously consider.

    You said to be careful about not oversizing the units -- in two of the bedrooms of my home, the load was calculated as only 1200btu heat / 2800 btu cool, and 3345 btu heat / 3128 btu cool. I was going to have both of these units hooked up to a multi split. Since the lowest capacity indoor unit 9k cool / 11k heat, I was just going to put one of those heads in each of the two rooms, sharing an outdoor unit. How would you tackle a scenario like that? It seems I'll be oversizing either way, and from the sound of it, that'll be exacerbated by how the multi split operates.

    Thanks for your time + advice, by the way. It's helping me a great deal.

    1. Expert Member
      Akos | | #27

      You generally don't want an onversized head for a bedroom. On a multisplit you can reduce the capacity of the heads by oversizing the indoor units (ie 45k of indoor units on a 30k outdoor unit) but this does not reduce the output enough for a 9k head to handle a bedroom.

      Bedrooms / bathrooms/office is best handled by a ducted mini split (I have a 9k head in my bedroom and had to do a lot of DIY air flow mods to make it comfortable).

      If you want cheap and efficient, your best bet is two ducted units with dedicated outdoor units. One for upstairs and one for the main floor / basement. The extra bit of cost of ductwork and bulkheads (you don't want ducts in attics) is way less then going with a 5 or 8 head multisplit. It will also be way more comfortable than stuffing 9k heads into bedrooms.

      I regret my decision to go multi split, if I could go back in time, I would go with a single ducted unit and save a bundle.

      1. Expert Member
        Dana Dorsett | | #28

        What Akos said- a 9K head on a multi-split for a bedroom will only be comfortable with the windows open. It is groteque oversizing- neither efficient nor comfortable.

        A 9K or 12K mini ducted casssette with output split between three ~3KBTU load rooms can work with a multi-split.

  13. joshdurston | | #25

    I'd like to hear what Dana says about optimizing towards 1:1 versus multi splits.
    I'd see it as an optimization, as in you do it where you can but don't lose sleep over it.

    For low load bedrooms a shared low static ducted cassette is often the way to go (something like a Fujistsu 9RLFCD min heat/cool is about 3000btu/hr.). You could combine bedrooms, or slave one off the master bedroom and bring you head count down. With a lower head count the higher performing 1:1's make more sense.
    With low static heads you need to change how you think about ductwork though. You really want to place the unit close to or in the zone, against a wall or ceiling and soffit in probably less than 15' of ductwork, sometime it may only be 4 or 5' of ducting, just enough to get a diffuser into the room from the hallway.

    If you decide to go multisplit with wall mounted heads... Mitsubishi makes a 6000 btu head, and Fujitsu a 7000, both of these would be a step in the right direction for the bedrooms.

    If you are trying to mostly self install, the 1:1's can be a little more straightforward. Some of the multi splits require more complex branch box setup, others are simply home run line sets from each head to the outdoor unit (fairly simple).

  14. Expert Member
    Dana Dorsett | | #26

    13K heat/17K cool is a perfect fit for the 1.5 ton Fujitsu 18RLFCD ducted mini-split. It's not a bad fit for the 1.5 ton Mitsubishi FH18NA ductless either:

    Bedrooms with a ~3K cooling load and 1.2K-3K are NOT a good fit for a half-ton head apiece on a multi-split. There isn't enough load to run efficiently, and it will be prone to overheating/overcooling. Even a 3/4 ton modulating ducted mini-split (NOT on a multi-split compressor) split between two rooms isn't optimal if it's only two rooms, but it's better than a half-ton head each.

    1. reimhagen | | #29

      Thanks for the input. I'm a little confused over the bedrooms then. If I put each bedroom on its own 1:1 wall-mount head using MSZ-GL06NA (6,000 BTU cool / 7,200 BTU heat), would that be able to modulate down to heat the room comfortably? (i.e. two outdoor units, two indoor units). If so, I would prefer that over having to soffit in some ductwork with a shared ducted unit.

      1. joshdurston | | #30

        I'm not sure about the GL06 (it looks to be a bit of unicorn documentation wise, I suspect it's a newer model and the all the docs haven't been updated to include it yet). I don't think the GL heads units turn down as far the FH. I think the min modulation on the FH might be half of the GL. So on the small bedrooms the FH06 is the least worst option. The min modulation is about 1600btu/hr on the MSZ-FH06NA & MUZ-FH06NA (1:1). For the small size units it's worth the couple hundred dollars to go up to the FH series.
        So it would work ok for the high load bedroom, and probably ok in the smaller Definitely better than most grossly oversized conventional HVAC, and much better than multi split heads that can't run lower than their rated capacity.

        I've got the same problem, a bunch of 2000-3000btu heat load bedrooms in a finished house. I'd much rather hang some heads on outside wall, than soffit ductwork between bedrooms.

        You may want to consider installing the remote wall mounted room temp sensor (replaces the internal one) for better temperature feedback, on all your wall mounts. The built in room temp sensor (in the air inlet) tends to get skewed because the high wall position and potential short cycling of supply air back into the return.
        They are cheap (on ebay you can buy 4 for $27), compared to the other wall mount control options.

      2. Expert Member
        Dana Dorsett | | #31

        The GL06NA is a multi-split only type head, and not fully modulating. The FH06 and FH09 can modulate down to ~1600 BTU/hr heating, 1700 BTU/hr cooling, assuming they have their own appropriate single zone compressors.

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