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problems with Mitsubishi split system

I have a Mitsubishi eight zone system (MXZ-8B48NA, I believe) in

my Cape Cod (about 1300 sq feet between the first and second floor and another

800 square feet in the basement).  Seven

of the eight zones are connected and in use. 

The system was installed in April 2012, replacing several room air

conditioners.   The summer of 2012 and

the first half of the summer of 2013 were wonderful.  I was never more comfortable.  I kept the units on the first and second

floor set at 68 degrees and the unit in the basement set at 66 degrees.  The air in the house was crisp and cool.  The basement was cool and dry.  Condensate poured from all the drains on the

individual units.  At night, I closed the

master bedroom door a few hours before going to bed and turned the temperature

down to 59 degrees.  The unit kicked up

to high speed with both green lights on . 

By the time I went to bed, the temperature was down to 59 degrees and

the unit had gone back to low speed operation with just one green light

on.  In the morning, I turned the

temperature back up to 68 degrees and opened the door.  The compressor ran in single stage mode most

of the time, unless it was extremely hot. 

Life was good!

Sometime about mid-summer last year, I came home and

realized that the temperature in the house was well above the set point.  I called for service.  The technician turned off the electricity to

the system, let it sit for several minutes and turned the electricity back

on.  Once again, the air was cool and

shortly the temperature in the house was down to the set point.  The technician checked the compressor and

said it was fine.  I noticed in the days

that followed, the air in the house wasn’t as crisp as it had been although the

temperature was at the set point.  The

air in the basement felt damp. 

Condensate production stopped in most of the units and was greatly

decreased in the others. 

I called for service again and was told that the air coming

out of the units was cold (about 59 degrees, I think) and the house was at set

point so there was nothing wrong with the system.  I asked about the humidity and the lack of

condensate.  I was told that split

systems don’t dehumidify as well as a Unico, for example.  That didn’t explain why the system WAS

dehumidifying and doing a great job previously.

I ran the system in heat pump mode for a week or two in the

fall before I turned on my oil fired hot water baseboard, and again the spring,

after I turned off the hot water baseboard. 

The system seemed to work fine.

This spring, I turned the air conditioning on and had the

same experience as the end of last year… 

the house remained humid along with the basement; little condensate was

produced; and additionally, when I dropped the temperature down to 59 degrees

in the master bedroom, the unit stayed in high speed mode (both green lights)

all night.  I bought a digital humidity

meter and found the house running close to sixty percent humidity. 

I had my PM done in June. 

The tech observed again that the temperature was at set point and that

split systems aren’t that good at dehumidifying.  He cleaned the filters and the coils and put

a gauge on the compressor.  He added some

refrigerant.  The situation didn’t change

so he came back again.  I had found an

oily spot on the branch box in the basement. 

He said it was a refrigerant leak, tightened the fitting , and added

more refrigerant. 

The situation isn’t any better.  The humidity is running at 68-70

percent.  The bedroom will only drop to

64 degrees at night even when it’s set at 59 degrees.  The basement is really damp.  Wet laundry hung up takes forever to dry, even

with a big fan running along with the basement unit set at 66 degrees.  The compressor seems always to be in two stage

mode.  I’d like to compare my electric

usage to last year, but we only went to smart meters in August of last year

from a system of estimated bills two months in a row followed by a bill based

on an actual meter reading.  So, I don’t

have anything genuine to compare with right now.

The tech is coming back on Tuesday.  I have faith in him.  He’s a great guy and is knowledgeable about

AC, refrigeration, oil heat, gas heat, etc. 

He’s been with the company a long time. 

I just have the feeling something’s going on that he’s missing.  Is it possible that the system is running

cold enough to produce cool air but not cold enough to condense the

humidity?  If so, could that be caused by

something besides low refrigerant levels? 

I’m looking for suggestions and insights.  If I’m not satisfied after this visit, I’m

going to have to call the owner of the company, who, incidentally, sold me the

system.  I feel like I’ve been really

patient, allowing time to see if additions of refrigerant helped the problem,

etc.  Tell me what you think.


  • Harvey RamerHarvey Ramer Posts: 1,082Member ✭✭✭
    Low on freon.

    Most probably low on refrigerant. That oily spot he found would be a leak. The connection he tightened would likely be a flare connection. When a flare leaks, it is usually best to undo the connection, cut off the old flare and then re flare the pipe.

    If this were me, I would first check every connection for a leak. Then I would recover the freon and see how much was left in the system. Then repair the leaks and weigh in the freon charge that was calculated by system design.

  • SpenceSpence Posts: 316Member

    If the system is cooling yet not dehumidifying, I would make absolutely certain that the indoor coils are very, very clean. These are not easy to keep clean due to their position in the head and the rather gimpy filters the factory provides.

    Humidity comes from two sources: your lifestyle and your environment. Did something change in your daily life? For example, new house plants or another moisture source. Did something change in your house? For example, an opening to the outdoors of some type.
  • IronmanIronman Posts: 2,270Member ✭✭✭
    Bad Info

    Your tech has given you some bad info about the ability of the mini splits to dehumidify. They do an excellent job of dehumidifying - much better than ducted split systems. They are probably as good, maybe better, than high velocity (Unico, Spacepak) because of their ability to modulate.

    I agree with Harvey and Spence that dirty evaporators and/or low charge are your two most likely issues.

    Regarding charging your system: the ONLY proper method is to weigh in the TOTAL system charge. Harvey's spot on when he said the refrigerant charge should have been removed, the flare redone, the system leak tested, evacuated, the TOTAL system charge properly calculated and weighed in. That is the only correct method for charging a multi zone branch box system.

    This ain't your grandpa's air conditioner. Things that are acceptable on a single stage ducted system may be totally wrong on this one.

    One other note: setting the cooling to 59* inside is way below the system's design. It should not be set below 68* even though the control will allow it to go lower.
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • Bill_in_DelawareBill_in_Delaware Posts: 3Member

    Thanks to everyone for your replies.  The tech came out yesterday to look at things.  He'll be back tomorrow to take out all the refrigerant, leak-test the system, and recharge with the appropriate amount of refrigerant.  Let's hope that takes care of things.  Thanks again.

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