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  1. #1
    Join Date
    May 2006
    Posts
    312

    Ductless minisplit = Increased humidity

    Subject line is really a question, as I've received a number of inquiries about the constant circulation of the indoor unit (it runs perpetually, even when setpoint temperature is reached) while in Cooling Mode.

    So far, it seems to be that if indoor IR handset is 76 F, humidity climbs almost to 60% even while unit maintains indoor temp at 74.7 - 75.0 (it always keeps it at least one degree below setpoint), outdoor temp is 80 F at 70% RH

    This applies to Mitsubishi units I've installed, anywhere from 3/4 ton to larger units.

    There does not appear to be any way to make the indoor fan to shut off when setpoint is reached.

  2. #2
    Join Date
    Jul 2003
    Location
    Los Angeles, California
    Posts
    184
    From my experience with mini-splits, they don't shut down when they reach set point because the indoor fan, compressor and even the condenser motors ramp up and down based on load. The Fujitsu units that I usually install have a temperature control board which controls the VFDs and electronic metering valve using a PID loop. Depending on the inputs the PID loop will determine the outputs, such as fan speed, compressor speed and EXV position.

  3. #3
    Join Date
    May 2006
    Posts
    312
    just verified with two other Fujitsu installations, that this condition appears universal with minisplits.

    there must be some moisture taken back, copiously, from the wet evap coil with perpetual fan, into the indoor space.

    since the units do not have any RH% detection mechanism (and I don't see that in any installation/technical literature), the end result is high indoor humidity. initially, the units drop the indoor RH, while dropping indoor temperature, once setpoint is reached, RH starts climbing back up and temperature is maintained.

    even the "DRY" mode in the IR handset is not able to overcome the excessive indoor humidity.

  4. #4
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    WE had this problem on a guest room over a garage. I did the heatload and found the unit to be oversized. We swapped out the unit which solved the problem.

  5. #5
    Join Date
    May 2006
    Posts
    312
    this happens universally on all my recent installations, even using just 9000 BTU full inverter models, all using the lowest fan speed setting (also tested on Auto)

    the compressor barely comes on outside, while the fan blows continuously indoors.

    used my latest fluke toolset to log indoor RH over 24h period - how did you measure RH% ? I created a one-hour interval report on a spreadsheet and reported results back to factory rep.

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    Yes the minisplits run the indoor fan steady and this elevates humidity.

    I just went through a building with a high humidity complaint. There were many problems BUT, the absolute worst area was a small office, not connected to the main system.

    It had a grossly oversized ceiling cassette unit. The compressor would cycle on, RH would begin dropping down from 80 to 75% RH as water started to condense, then in 3 minutes, space temperature was satisfied and the compressor was off.

    Any water that condensed on the coil just re-evaporated back to the air. Nothing comes out of the condensate drain.

    Three minutes later the compressor is back on, and cycle repeats. The moisture just travels from air to fan coil and then back to the air.

  7. #7
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by nina View Post
    this happens universally on all my recent installations, even using just 9000 BTU full inverter models, all using the lowest fan speed setting (also tested on Auto)

    the compressor barely comes on outside, while the fan blows continuously indoors.

    used my latest fluke toolset to log indoor RH over 24h period - how did you measure RH% ? I created a one-hour interval report on a spreadsheet and reported results back to factory rep.
    Is the air coming out of the inverters always cold?

  8. #8
    Join Date
    Mar 2006
    Location
    Delaware
    Posts
    448
    In my experience, even here in humid Delaware at the beach in the summer on not so warm days, the key seems to be getting the size right. My preference is Fujitsu, and in 'DRY' mode, they do a reasonable job of dehumidifying, provided they aren't overkill sized. However, super dry doesn't happen if the cooling load isn't there. In my experience, it's not just minisplits, but ALL air conditioners that run into this. Add a bit of heat, (so the compressor kicks on) and things dry nicely.

    Quote Originally Posted by nina View Post
    Subject line is really a question, as I've received a number of inquiries about the constant circulation of the indoor unit (it runs perpetually, even when setpoint temperature is reached) while in Cooling Mode.

    So far, it seems to be that if indoor IR handset is 76 F, humidity climbs almost to 60% even while unit maintains indoor temp at 74.7 - 75.0 (it always keeps it at least one degree below setpoint), outdoor temp is 80 F at 70% RH

    This applies to Mitsubishi units I've installed, anywhere from 3/4 ton to larger units.

    There does not appear to be any way to make the indoor fan to shut off when setpoint is reached.

  9. #9
    Join Date
    May 2006
    Posts
    312
    > Is the air coming out of the inverters always cold?

    no as the inverter controlled compressor comes on at very low revolutions based on feedback from the indoor control board, satisfies setpoint and resumes continuous fan blow over evap coil.

  10. #10
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    Quote Originally Posted by kuryakin View Post
    In my experience, even here in humid Delaware at the beach in the summer on not so warm days, the key seems to be getting the size right. My preference is Fujitsu, and in 'DRY' mode, they do a reasonable job of dehumidifying, provided they aren't overkill sized. However, super dry doesn't happen if the cooling load isn't there. In my experience, it's not just minisplits, but ALL air conditioners that run into this. Add a bit of heat, (so the compressor kicks on) and things dry nicely.
    So I can use the sun to make my air conditioner a dehumidifier?

    Ah the Achilles tendon of the cool to dehumidify feature.

  11. #11
    Join Date
    May 2006
    Posts
    312
    it really would help if *only* the indoor unit would turn off fan when cycle is complete, ie. *not* running in "fan always on" mode

  12. #12
    Join Date
    Mar 2006
    Location
    Delaware
    Posts
    448
    Indeed. Unfortunately, minisplits with hot gas reheat are rare. Such as:
    http://www.vital-air.com/ProductLine.pdf

    Quote Originally Posted by adrianf View Post
    So I can use the sun to make my air conditioner a dehumidifier?

    Ah the Achilles tendon of the cool to dehumidify feature.

  13. #13
    Join Date
    Apr 2002
    Posts
    11,808
    Quote Originally Posted by nina View Post
    > Is the air coming out of the inverters always cold?

    no as the inverter controlled compressor comes on at very low revolutions based on feedback from the indoor control board, satisfies setpoint and resumes continuous fan blow over evap coil.
    I would think even witrh the inverters 'slowed down' that unless the fan coil air flow was ramped down, it would end up behaving like an undercharged system, most of the coil would be superheating the vapour, probably whatever mositure was removed would be re-eavaporated. Coil surface temperature would average above the rooms dewpoint.

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