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Q&A Spotlight

Problems with Multizone Minisplit System

After a gut renovation and multiplit installation, electricity bills are sky high and so is the indoor humidity

After renovating the inside of his Boston area house, Levi Tofias had a ductless minisplit system installed. It has suffered a number of problems ever since. Photo courtesy Levi Tofias.

Levi Tofias is one year out from a gut renovation of his Boston area home, and he’s finding that his new minisplit HVAC system is anything but a match made in heaven.

The house, located in climate zone 5A, has 2500 sq. ft. of conditioned space, including 600 sq. ft. in the basement. It’s 2×4 walls have been insulated with dense-packed celluose with an air-sealing membrane on the inside. The new windows are triple-glazed, and seven Lunos units provide whole-house ventilation.

Now . . .  to the root of Tofias’s unhappiness—the Mitsubishi multizone heat-pump system. As he explains in this recent Q&A post, it consists of two outdoor compressors, rated at 30,000 and 42,000 Btu/hour, and a total of seven indoor units. (Later, in an email, Tofias corrected the capacity of the outdoor units to 24,000 and 36,000 Btu/hour.)

“My electric bills have been between $600-$700,” he writes. “We also had humidity issues this summer in cool mode. Basically, turning on the units would both cool and add humidity to the room, which meant adding between 5%-10% relative humidity.” This problem apparently affected units running on the larger of the compressors. (Tofias has attached both a floor plan of the house and the Manual J calculations for heating and cooling loads.)

“Lastly,” he adds, “interior vibrations and exterior noise of the large compressor have become unbearable. It sounds like it is permanently in defrost mode. [We are] in an urban area, and the unit is 5 ft. from the property line. Neighbors want to kill me.”

The compressor is on a wood frame mounted to the side of the house, but the installer is coming back to move it. In the meantime, Tofias shut off the big compressor so he can get some sleep.

What now? That’s the puzzle for this Q&A Spotlight.

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  1. jameshowison | | #1

    Any thoughts on what recourse the homeowner has here? Where in the process did things go wrong and what safeguards could be put into place to help? For example, how do code official review the Manual J? How do these design issues get missed? Where in the process is the checkpoint to avoid the situation where a system design for peak heating isn't completely inappropriate for cooling? Should the design software have highlighted this issue? If I understand correctly Mitsu's design software doesn't model minimum capacities on multi-splits, only maximum capacities, so there is a lot of guessing.

    Finally, given that the design may have been done for 17° but without hyper-heat, where in the process could/should the homeowner be told of this difference in CoP between regular and hyper-heat? Think of the documentation on loans and interest rates, seems like something like that is needed?

    1. larkomundo | | #6

      It would seem to me that the HVAC licensing authorities should provide some assistance with such gross incompetence. Assuming the installer is properly licensed, there is more than sufficient evidence for discipline. Although each state is different, the licensing scheme exists to purportedly prevent stuff like this. It didn't work very well in this instance. But it's worth investigating the complaint process to determine if there may be a remedy to address the damage.

  2. jason_v | | #2

    Code officials don't review the manual J. Manual J's take a ton of assumptions into the calculations that may or may not be accurate. I commissioned 5 Manual J's from engineering firms and they all came back widely different. The home owner is always stuck between HVAC companies that will automatically oversize everything and inaccurate manual J's that might leave you wanting. Its a gamble either way.

  3. aaron55 | | #3

    This just makes me feel sick. I feel so bad for the homeowner

    1. Expert Member
      MALCOLM TAYLOR | | #5

      Me too. What a mess.

  4. wanmi | | #4

    I recently had a single Fujitsu Halcyon installed in a new 600 SF apt. It has a single compressor and 2 heads/cartridges (living room and bedroom).

    It operates efficiently but there is one bother: you must run the 2 heads on the same setting if run simultaneously. For example, I cannot run the bedroom on heat the the living room on just fan or vice versa.

    Still glad we chose this heating and cooling method, just annoying.

  5. larkomundo | | #7

    This is a classic case of gross incompetence by the contractor. A cursory examination of the multi-zone combinations would clearly have shown a violation of ACCA Manual S. Makes me wonder if the contractor even knows about Manual S.

    The sad part is Mitsubishi will likely be blamed for the failure.

  6. AndyKosick | | #8

    When I read "What now?" the phrase "Gross Incompetence" was the second thing that came to mind, the first was "Sue 'em!"

    I do not say that lightly and am seriously interested in GBAs opinion here. At what point does this constitute fraud? This case stands out to me in that Manual J documentation was provided and that is a clear implication that the ACCA design process was being followed. I can say with certainty the the Manual J above is incorrect and can guess with near certainty that the rest of the ACCA process was not followed. I am not a lawyer (and would really prefer not to involve one) but shouldn't the owner have some kind of recourse at this point? That system cost a lot of money. The installation may be impeccable but the design is completely wrong.

    I know this is a rocky road and I'm not sure it's the right one to go down, but after seeing so little progress in the very troubled residential HVAC community over the years, the question should probably be asked. We're going to need all the good HVAC techs we can find the the coming years, but we also need good design of heat pump systems. Maybe a little sting to weed out those not willing is needed? Of course, there aren't nearly enough that are willing as it it is.


    1. larkomundo | | #9

      Andy, the word for the day is incompetence. In the lawyer (truth twister) scheme, recall the quote from Chief Justice Warren Burger, “75% to 90% of American trial lawyers are incompetent, dishonest or both”.

      I’ve done expert witness work over 40 years and can say I’ve only met one competent attorney. Most are also lazy and not very smart. But they have no problem charging 50 to 100 times more than what they’re actually worth.

      Litigation is obscenely expensive with no guarantee of an equitable outcome. At the end of the day, a plaintiff’s case is decided by 12 people who didn’t have enough on the ball to get out of jury duty. Or the judge reflecting the riddle - what do you call a lawyer with an IQ of 75? “Your honor”

      The law doesn’t have a remedy for every wrong. And in most cases, the only winners are the lawyers.

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