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  1. #1

    Confused Mini-Split Humidity Issues

    I recently had Mitsubishi Minisplits installed throughout my 1950's cape cod in eastern PA. While the 2 units on the first floor control the humidity and keep it at a comfortable level, the 2nd floor is a different story. The 3 head units upstairs are hooked to a multi-zone Mitsu OD unit and all 3, particularly the 2 small bedrooms (~100sqft, 6000BTU units) have high RH (sometimes 70%+ @ 72F) after they hit their temp setting. The installer is confident they are sized properly (he showed me the manualJ calcs which were based on his pretty thorough measurements of the house) and also, given the fact the Mitsu's are variable speed, they can ramp up and down as needed. They've replaced the main board on the outdoor unit (all under warranty) and installed wireless thermostats, but still high RH. I recently had the crawl space above air sealed and additional insulation blown, so I'm pretty confident we're not getting high humidity air in.

    Even with keeping the fan speed on low (to slow amt of air over the coils) the problem still remains. However the RH isnt always high - if we lower the room temps and the units call for cooling, the RH drops down to the 50-55% range. The DRYING mode on teh units isn't helpful because it basically drops the room temp down as it periodically cools.

    Is this just a inherit flaw of the Mitsu minisplits? It's been pretty frustrating that we can't get a constant, reasonable RH level. As I said - the same system is installed on my first floor is working great with comfortable RH. The installer is somewhat stumped - saying b/c we have hardwood floors they suck up and give off alot of humidity - as well as the extreme heat we've had. But I feel like that's an excuse - there should be a better way to fix this problem?

    Help!

  2. #2
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,186
    I of course do not know your models,,, but I would take a stab that 6000 Btus is too large for the small rooms. These are the smallest units made and perhaps they do not modulate down low enough/short cooling times thus less dehumidification. The 24,000 unit is about 12,000 minimum. Here is a spec for a unit.
    http://usa.mylinkdrive.com/uploads/d..._Submittal.pdf
    ...

  3. #3
    Thanks Kangaroo. I'm running an MXZ-3B24NA OD (24,000) hooked up to 2 x MSZ-GE06NA (2 small ~100sqft bedrooms) and 1 x MSZ-GE12NA for the. They assured me the small 6000 units could ramp down as needed, but from researching I'm concerned too about what you noted - oversized for such a small space. The RH drops to a tolerable level when the room doors are open, but when they are closed for an extended time (kids' nap time or overnight) the RH climbs up - this morning in the 70%+ range. Any additional thoughts?

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,592
    Quote Originally Posted by pamarz13 View Post
    Thanks Kangaroo. I'm running an MXZ-3B24NA OD (24,000) hooked up to 2 x MSZ-GE06NA (2 small ~100sqft bedrooms) and 1 x MSZ-GE12NA for the. They assured me the small 6000 units could ramp down as needed, but from researching I'm concerned too about what you noted - oversized for such a small space. The RH drops to a tolerable level when the room doors are open, but when they are closed for an extended time (kids' nap time or overnight) the RH climbs up - this morning in the 70%+ range. Any additional thoughts?
    Nothing mentioned, the fan should in the "auto" mode. This means that the fan is off when the a/c is not cooling for cooling. What is the cooling cycle length like? Mins. on mins off? Cold supply air temp/%RH at the end of the cooling cycle? Room Temp?
    Regards TB
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #5
    Hi TB. Even with the fans set to "Auto" mode (or on Low for that matter) the high humidity problem still remains. My understanding (and experience from monitoring the units) is that indoor unit fans run continuously circulating air - never stopping. I dont think i've ever seen then stop. I'd have to time the actual cycling time, but generally what happens is as follows (night time example): Temp is set down to 70F (from previously set point of 72F, which is satisfied) and then as the rooms cool, the RH drops to an acceptable level (50-55%). After the room temp is satisfied (say ~1 hr for that 2F change), the cooling stops and maintains the 70F, but RH immediately begins to rise (up into 60's%, low 70's%). For instance, over night, the rooms will maintain the 70F temp, but by the morning, the RH will be close to 70%, if not higher. One additional observation: When the room doors are open, the humidity levels are decent (low 50%'s). As stated earlier, the high RH is mostly noticeable when rooms are isolated and doors closed.

  6. #6
    Join Date
    Aug 2001
    Location
    Pavilion, NY
    Posts
    2,186
    While the unit does modulate, it has a minimum modulation which I believe is 12600 Btus. If 1 or both of the 6k units are calling only it is in all likelihood sized to large for the room but that really is not a fault of the contractor as he sized it for the smallest unit made. The indoor blower I do believe runs continuous as it is sampling air for setpoint (the thermostat is IN the indoor unit and the remote only tells it what to run at and then is out of the equation. I have never had that complaint from a customer and I really don't know what your solution is
    ...

  7. #7
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,592
    Quote Originally Posted by pamarz13 View Post
    Hi TB. Even with the fans set to "Auto" mode (or on Low for that matter) the high humidity problem still remains. My understanding (and experience from monitoring the units) is that indoor unit fans run continuously circulating air - never stopping. I dont think i've ever seen then stop. I'd have to time the actual cycling time, but generally what happens is as follows (night time example): Temp is set down to 70F (from previously set point of 72F, which is satisfied) and then as the rooms cool, the RH drops to an acceptable level (50-55%). After the room temp is satisfied (say ~1 hr for that 2F change), the cooling stops and maintains the 70F, but RH immediately begins to rise (up into 60's%, low 70's%). For instance, over night, the rooms will maintain the 70F temp, but by the morning, the RH will be close to 70%, if not higher. One additional observation: When the room doors are open, the humidity levels are decent (low 50%'s). As stated earlier, the high RH is mostly noticeable when rooms are isolated and doors closed.
    The continuous fan is a problem. Any moisture collected on the cooling coil from a short cooling run is re-evaporated back into the space when the compressor is off but the fan continues. Most of these units have a fan only when compressor running mode.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  8. #8
    TB - What manufacturers make units that don't have continuously running fans? As noted, mine are Mitsubishi's. Actually in my case, the temp is monitored by the wireless thermostat (honeywell made) the installer recently put in - not via a measurement of the air being pulled into the continuous running fan. Do you think if the fan would stop when the compressor is off, we'd be good and the humidity would be normal - meaning, would this solve the issue? Or for proper de-humidification, does the air need longer time over cold coils (i.e - if the unit were sized smaller as someone suggested above?). Appreciate the help!

  9. #9
    Join Date
    Aug 2009
    Location
    Maine
    Posts
    3,577
    Don't have a solution, but maybe a quick fix. Is it possible to put a vent or grill in the door? Just a thought and I am sure some one with more knowledge will chime in.

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,592
    Quote Originally Posted by pamarz13 View Post
    TB - What manufacturers make units that don't have continuously running fans? As noted, mine are Mitsubishi's. Actually in my case, the temp is monitored by the wireless thermostat (honeywell made) the installer recently put in - not via a measurement of the air being pulled into the continuous running fan. Do you think if the fan would stop when the compressor is off, we'd be good and the humidity would be normal - meaning, would this solve the issue? Or for proper de-humidification, does the air need longer time over cold coils (i.e - if the unit were sized smaller as someone suggested above?). Appreciate the help!
    The compressor cools the coil. Moisture builds up on the coil until it drips to the pan and down the drain. It takes 10-20 mins. to buildup enough moisture. When the compressor stops, the moisture on the coil/pan evaporates back into the home. With the fan on, the evaporation happens in 20-40 mins. With the fan off, it takes +1 hour.
    No fan with the compressor off removes more moisture.
    Also less air flow lowers the a/c coil temp which will remove more moisture. Slow the fan or increase the filter restriction for more dehumidification.
    I am not familiar with your brand of a/c.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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