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Ducted vs. Cassette Heat Pump Minisplit

ryan9751 | Posted in Mechanicals on

In the early stages of a gut remodel and addition on a 2 family home with identical units and trying to get a general idea of placement and types of units to recommend to the GC.

Before the remodel the units were heated with a 30K LG system, LMU300HHV and 3 wall mounted indoor units. I had these systems refrigerant recovered and stored for possible re-use.

The new construction increases the size quite a bit, but I have had a manual J done and because of the new spray foam, windows , etc, the requirements stay the same at under 30K per unit.

If I try to re-use the LG units, I would have space to add one more head, as there is a max of 4 heads, but I am not sure how I can get one head to cover the remaining areas which are titled “new living room, new dining room and kitchen” in the attached plans.

Would a cassette unit in the ceiling possibly cover these areas or should I go with a ducted unit? Rooms are approx 13.5×13.5 and 15×26

I’m also considering trying to hire a HVAC designer to do this, but it seems most consultants are used to working with traditional ducted design as opposed to mini splits.

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Replies

  1. Expert Member
    Akos | | #1

    In most cases, the best setup is a ducted unit for the bedrooms. Since the LG needs at least two heads on it, I would leave the head in the living room and dining room and add a ducted unit for the rest. Looking at the layout, you can put a slim ducted unit into the ceiling of the laundry room with a return in the hallway and simple runs to the bathrooms and bedrooms.

    I would even go as far as try to take out the dining room wall mount and use the ducted unit for that as well. By getting rid of the wall mount you eliminate a fair bit of extra maintaince, on less coil and blower wheel to clean and one less drain that will inevitably clog.

    The last thing you want is to add a fourth head to this setup. That is a recipe for a lot of maintaince, comfort issues and low efficiency.

    1. ryan9751 | | #2

      Well that wasn’t quite the answer I was hoping for it is probably the most logical.

      Because one of the bedrooms won’t be used frequently and the occupants of the two other bedrooms prefer vastly different temperatures I wanted the individual control of the wall mounted units.

      Curious as to why you say this would provide less comfort though - I have checked the engineering units for the indoor units and they should modulate to less than the required heat/ cool load. (They are 7K)

      Understand that an additional unit is potentially additional maintenance but wall mounted units in each room seem to be the norm in the rest of the world.

      I think I would definitely go with the ducted for the living room / kitchen as the filter change will be easier and larger coverage, plus the existing wall mounted are 7k and would be on the small size.

      How would i go about determining if there would be comfort / efficiency issues . Hiring someone for HVAC system design?

  2. arnoldk | | #3

    Hi Ryan,

    We ran in to the same issue with our house built that ongoing where we wanted to use two cassette unit, one on the main floor and the second for the upstairs (slab on grade).
    We decided to go with a full duct system for some of the reason Akos pointed out. We were also told by two individuals (engineer and HVAC) that cooling with a cassette unit for the second floor wouldn't be able to adequately push air into all three bedrooms and of course if bedroom door are close, this exacerbates the issue.

    A fully duct system is more expensive but we wanted to make sure we would have even temperature throughout the house especially with two young kids who will become teenager in 5 years and those bedroom door will be closed most of the time.

    Arnold

  3. ryan9751 | | #4

    I think my situation is a bit different - I actually want to put individual heads in each bedroom, but have been advised by several people that this is not a good idea and will lead to comfort issues.

    I know Akos pointed out airflow issues in the past , but if the minimum BTU of the indoor head is close to the latent BTU for the room from the manual J I would think at least temperature / efficiency would be OK.

    I cant seem to find modulating info for the LG indoor units which is another problem, only the data on the outdoor units is published in the engineering manuals. (Looking at LMN079HVT )

    1. paul_wiedefeld | | #5

      The indoor modulation is overridden by the outdoor modulation for multisplits, so it looks like the LMU300HHV cools down to 8400 btu/h.

  4. Expert Member
    Akos | | #6

    Ryan,

    You are looking at a way too common oversized multi split with a wall mount per room setup. There are a couple of issues with a wall mount per room. If you look at the combination data for the your unit:

    https://lghvac.com/resource-service?filename=EM_MultiF_MAX_LGRED_ODU.pdf

    Say with 7+7+7+18, the minimum cooling and heating capacity is ~17000 BTU.

    Unless the house has no insulation and single pane windows, this well above the heating load of your place which means the indoor units and outdoor units will always be cycling and never modulating. In an oversized systems the zone calls rarely overlap, which means that in a bedroom the wall mount will generally run at rated capacity then off. This means you get blast of hot or cold and then nothing.

    All multi splits also bypass some refrigerant through all the zones, this results in a lot of crosstalk between zones. Lot of times a zone that is "off" is still producing heat which can overheat rooms.

    I have an older non hyper heat LG multi split with wall mount per room setup and I can tell you that if I could go back in time, I would have never installed it. A number of times I have seriously contemplated ripping them out and swapping in a ducted unit.

    I have seen first hand the issues above.

    Cooling is terrible, short blasts of super chill followed by long recovery until the temperature rises again. Almost no humidity removal as any condensate on the coil gets re-evaporated between calls.

    Heating is better but not ideal. If the living room unit is running, the bedroom will overheat. If I keep the living room unit off, the bedroom unit will cycle which causes random thermal expansion noises in the middle of the night. The worst thing though is defrost cycles. These happen randomly and usually in the middle of the night. The noise from the refrigerant flow reversing will wake you up.

    On paper it sounds like the wall mount per room would be a great setup, gives you individual temperature controls and easy to install as it needs no ducting. The reality is that this doesn't work here.

    The reason this can be made to work overseas is the buildings rarely have any insulation, most windows are single pane and air sealing is unheard of. This bumps up the cooling and heating loads enough that there is no oversizing.

    Have somebody competent, not an HVAC tech but and HVAC engineer, run a Man J for your place and figure out what your room loads are. From there you can pick a properly sized ducted unit to feed them.

    For local zoning, over provision the rooms slightly so the occupants can close the vents if they want less heating or cooling. You can even add an in-line damper driven by a local thermostat to make this automatic. The key is to adjust the damper closed position to allow some flow this way the controls don't need to be integrated with the ducted unit.

    You can also look at something like the Honeywell zoning system and connect it the optional thermostat interface module on the LG unit. Since the controls are simple two stage, you won't get full modulation but can be made to work.

  5. ryan9751 | | #7

    I did have a manual J done before the addition and the whole unit came in around 20K btu for heating, with the bedrooms requiring between 1800-2000 btu.

    The spec posted on the LMU300HHV heats down to 10K btu and cools down to 8K per Pauls post above. Keeping that in mind If the heat load is similar throughout aside from the scenario you pointed out where you have a head off would the system not evenly distribute the heating / cooling?

    I am looking to find a HVAC engineer - any recommendations are welcome. Energy Vanguard quoted 4K which I thought was a bit excessive.

    1. Expert Member
      Akos | | #8

      I posted the wrong link earlier. Here is the combination configuration link:

      https://files.lghvac.com/resources//CDM_MultiF_MAX_LGRED_ODU.pdf

      Look at table 13 on top of P.16. The min for four heads is 17100BTU. Maybe under certain configurations the outdoor unit can go down to 8000 but not when driving four zones. This is similar to other manufacturers, although it seems a bit worse (Mitsubishi is about 1000BTU/zone).

      You didn't say which climate zone you are in, so it is hard to say if 20k design heat load is correct, doesn't look way out there though. If you have the detailed report, post it here to see if anything obvious is wrong. The minimum installed indoor capacity on most multi splits is 70% of outdoor unit so you can go with something like a 7k wall mount + 18k ducted to get a pretty good match to your design load.

      The 1800-2000 btu for a bedroom is about right. This means even if you install a wall mount on its own compressor which has much better modulation range, the min modulation on it would be about the heat load of the bedroom, so any day outside of your design coldest day the unit will be cycling. On a multi split it will be much worse.

      1. ryan9751 | | #9

        Akos - thanks for pointing out the table 13 with the minimum BTU with 4 connected heads.
        I did not realize that the turndown would be affected by the number of heads.

        I'm in Massachusetts - agreed the 20K heat load doesn't seem bad, but this was just coming from a rough manual J that a energy rebate rater did.

        I'm going to have a proper manual J done and re-evaluate. It looks like I won't have a use for the existing 7K heads and will end up with either 2 ducted 18K units or a ducted 18K for the bedrooms and a cassette type covering the living room/dining/kitchen.

        1. Expert Member
          Akos | | #10

          With multi splits, more is always worse, match the ducted unit to the heat load. If the ducted is feeding three beds and two bath, you are probably looking at a design load under 9000BTU, so an 18k unit is way oversized. Same for the living space. Get the proper man J and select the indoor units from there.

          The ceiling cassette can work for the living space. The 4 way ones are pretty large, make sure you can fit it between the ceiling framing. They also need an air tight box above them in the attic. This is something you can make out of rigid insulation taped together and sealed to the ceiling bellow with canned foam. Make the box big enough to have clearance around to allow for servicing.

          The nice part of the LG four way unit is they support branch ducts. If you look at the picture, it has square knockouts on the sides where you can install jumper ducts to feed a larger area. About 50% of the rated flow can be sent through these ducts. This way you can get better coverage of the living and dining room.

          Condensate pumps inevitably fail, if possible always gravity drain. Might be a bit more work but worth it for the reduced maintenance.

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