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Exceeding multi zone mini split maximum connectable capacity

ryan9751 | Posted in Mechanicals on

Wondering if anyone has experience with multi zone mini splits where they exceed the published maximum capacity of the outdoor unit. 

I am looking to use an outdoor unit from LG – LMU300HHV which has a rating of 30K BTU, but capacity can be exceeded up to 40K btu. 

I had my contractor order and install 3x 7k wall units LMN079HVT and 1x 18k vertical air handler LVN181HV4 a total combined capacity of 39K BTU. 

Of course I missed the section of the engineering manual where it states that High static / VAHU units must be calculated as 1.3x their rated capacity, IE the 18k unit counts as 23.5K, putting the units over the max allowed connection – 44.5K. 

I reached out to LG engineering because if the system would still function I would keep it as is, even if they void the warranty. I’m doubt they will respond to me, and the HVAC tech just said the rep checked it in the software and it does not accept the combination.

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  1. walta100 | | #1

    Without an accurate manual J calculated by someone not bidding your install, every part indoors and out will almost certainly be oversized.

    It seems unlikely the installer will redo the install and if my guess is correct and everything is oversized it may run just fine.

    Making a successful warranty for more than $500 in parts is less likely than winning the lotto and the installing dealer eats almost all the labor making the warranty almost a nonissue.


  2. ryan9751 | | #2

    Actually the contractor wanted to oversize the system but I insisted on matching the system closer to the manual J. Some of the indoor units are slightly over sized but not by a whole lot. Manual J total load was around 29K BTU and the LMU300HHV is a max of 34K btu, with a max of 29K heat at 5 degrees. It's actually undersized for New England, but have been improving some insulation.

    The installer would re-do, there just isn't any equipment available that perfectly matches, unless someone here or an LG engineer can confirm that connecting 44.5K to an outdoor unit max rated for 40K will still function. (albeit not at more actual heating /cooling capacity, just more connected capacity)

  3. walta100 | | #3

    I think you are dreaming if you are hoping any manufacturer’s representative is going to put in writing anything different than the product manual says as it would be career suicide.

    Generally, in refrigeration when you over size the evaporator coils what would be your indoor coils when cooling you get less latent heat removal and more sensible heat removal. What that means is the system will not dehumidify the air as much as expected forcing you to set the thermostat to a lower set point so you feel cold and clammy. In heating mode, you could be a little more efficient.

    Push for the installer to connect one of the 7k heads to a second compressor or make it right in anouther way.


  4. Expert Member
    Akos | | #4

    I'm win Walta here. Either move one of the 7k units to a dedicated outdoor unit or reconfigure. I don't think there is a single zone 7k outdoor unit, so that means swapping it out for a 9k, the one-to-one LG units have excellent turndown so this is not an issue:!/product/25836/7/25000///0

    Because of the low load, it doesn't need to be hyper heat outdoor unit. Since this zone would have much better modulation than the rest, I would connect it to the main suite.

    A simpler option is to remove one of the wall mounts and connect the room to the ducted unit.

    The final option is to move the ducted unit onto its own outdoor unit and leave the wall mounts on the multi split. This is not the best setup as the modulation on the 300HHV is pretty limited so it will be cycling pretty much all the time:!/product/29567/7/25000///0

    Going down to the 2 ton 3 zone won't help much either.

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