Mitsubishi dry mode
xlogger
| Posted in General Questions on
We just installed a Mitsubishi mini split on our trailer at the coast this past winter. It has two heads on it. I was down this past weekend and was testing out the dry mode on it. I cut it on and the humidity really didn’t drop any but the temp really did and it never cut off when in this mode. Just wondering what I might of done wrong. Also since there are two heads on one outdoor unit can you run one on AC and the other on dry if I figure out what I did wrong?
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Dry mode works by reducing the airflow and keeping the inside coil really cold. The idea is rather than cool a lot of air a little, you cool a little air a lot, which condenses out more moisture, since you air dropping the air significantly below the ambient dew point.
The problem is without the presence of sensible gains you will still overcool the space.
If the temp is dropping, but the humidity is staying the same (%rh), you actually are dehumidifying since your dew point is dropping, and there are fewer pounds of water in the air. The thing to remember about relative humidity is... that it's relative (to temperature). So it's possible to dry the air and increase the rh simultaneously.
At some point the absence of sensible gains will require you to use a dehumidifier, which adds the sensible heat back into the space to dehumidify without dropping the temperature.
If it's cool outside (say less than 50f), you may be able to "dehumidify" by simply by ventilating more (assuming inside it's kept around 70f). Crack a window or run the bathroom fans. This works even if the rh is high outside, because it's relative to a low temperature, thus a low dew point. The dewpoint can't exceed the temperature, or you simply get condensing since the air is saturated.
It could be that your head(s) is significantly oversized for the conditions that occur in dry mode.
I would suggest only running one head in dry mode, and leaving the other head off completely. Don't run in AC and dry mode at the same time unless you need the sensible cooling effect as well.
Mitsubishi has a strong building performance team now; I am checking with them on this. I am pretty sure that Josh nailed it, but will let you know what else I might find.
Peter
It's not clear why they even have a manual dry mode. The system should monitor humidity and temperature and adjust the sensible/latent ratio as needed.
Agree, I think Fujitsu has this on some units. At minimum dry mode shouldn't overcool by more than a degree or two. Lots of reports have overcooling to 60F which is ridiculous.
I was hoping this would work and get rid of my DH that we have been using. The temp was down to 62 when I cut it off and was getting ready to leave for the week. Not sure how low it would go or if it would cut off if we left it on. I probably really over did my trailer. We really did put two mini splits in each with two heads. Last year our first year there when we got down for the weekend it just took too long to cool it down and after the hurricane took out our ducts I decided to go with mini splits.
>"I probably really over did my trailer. We really did put two mini splits in each with two heads."
Throwing 40,000 BTU/hr of compressor at a sub-18,000 BTU/hr peak load it means all heads will be cycling (UNLESS you put it in DRY mode). The modulation range is not infinite, and unless the compressors are running and the coils are cold it won't be removing any moisture.
Try turning one of them completely off (throw the breaker), and run just one head on the other, so that it has enough load to work with when humidity is the primary load. (Turning off the other head with the remote doesn't turn it completely off- there is still refrigerant running through it whenever the other head is calling for coolth/heat.)