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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
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.