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NFT: A/C-Dehumidifier Question

GruningsOnTheHill : 7/10/2019 12:22 pm
Hi--

I'm on an island off the Gulf Coast of Florida, and when we built our house a few years back, I closed in a 300sq ft section off the back of the garage via drywall, and I added a mini-split A/C unit to convert it into a useable room. Thus, one of the 4 walls is drywall that backs onto the garage, and the other three are painted cinderblock with no insulation.

The room stays cool enough with the mini-split, but the air is musty and unpleasant. I assume the problem is due to lack of insulation that allows the cinderblocks to sweat through.

My question: Will a dehumidifier likely take care of this problem, or will I need to insulate the cinderblock with foam board and then install drywall over it?

Thank you.

Provided you have a large enough AC  
jcn56 : 7/10/2019 12:28 pm : link
and you're running it low enough - if the dehumidifier was going to solve the problem, the AC should as well.

I would go the polystyrene board route, sealed up with Tyvek tape at the seams.
Thanks JCN  
GruningsOnTheHill : 7/10/2019 12:34 pm : link
So you're saying I should try lowering the temperature? I keep it @77 degrees.

.  
oldhemi : 7/10/2019 12:42 pm : link
I would suspect the floor might be a problem moreso than the walls.
I also suspect that the a/c does not get run enough - maybe when you are only in there?
Re:  
GruningsOnTheHill : 7/10/2019 12:54 pm : link
Hi Old Hemi--

The mini-split A/C is running 24hrs. As for the floor, I hadn't thought of that. I laid the 12" vinyl squares right on top of the concrete slab.

Will a dehumidifier help rid the musty air?

The three windows are framed in, and though I can do the insulation/drywall myself, redoing the window frames to account for the extra thickness of the insulation and drywall materials would be beyond my realistic capabilities. I don't want to sink a ton of $$ into this if at all possible.
without insulating or even jcn suggestion  
pjcas18 : 7/10/2019 12:55 pm : link
my guess is during the warmer, humid, months your dehumidifier would run 24 hours a day if it's set at a reasonable humidity level like 40 or 45%.

You will need a dehumidifier with a pump that pipes water outside, so I'd think about where you could drill a hole if you need to. if you have to empty a basket filled with water from the dehumidifier you are going to be doing it every hour while it's on.

Even with insulation the dehumidifier may be on a lot.

Some AC's have both AC and dehumidifier, not sure if a mini-split as a dehumidifier mode, if so, it would solve some potential issues for you.
RE: without insulating or even jcn suggestion  
Jim in Fairfax : 7/10/2019 1:09 pm : link
In comment 14495954 pjcas18 said:
Quote:
my guess is during the warmer, humid, months your dehumidifier would run 24 hours a day if it's set at a reasonable humidity level like 40 or 45%.

You will need a dehumidifier with a pump that pipes water outside, so I'd think about where you could drill a hole if you need to. if you have to empty a basket filled with water from the dehumidifier you are going to be doing it every hour while it's on.

Even with insulation the dehumidifier may be on a lot.

Some AC's have both AC and dehumidifier, not sure if a mini-split as a dehumidifier mode, if so, it would solve some potential issues for you.

If a floor drain or sump well is available, a hose could be run there from the dehumidifier.
RE: RE: without insulating or even jcn suggestion  
pjcas18 : 7/10/2019 1:12 pm : link
In comment 14495979 Jim in Fairfax said:
Quote:
In comment 14495954 pjcas18 said:


Quote:


my guess is during the warmer, humid, months your dehumidifier would run 24 hours a day if it's set at a reasonable humidity level like 40 or 45%.

You will need a dehumidifier with a pump that pipes water outside, so I'd think about where you could drill a hole if you need to. if you have to empty a basket filled with water from the dehumidifier you are going to be doing it every hour while it's on.

Even with insulation the dehumidifier may be on a lot.

Some AC's have both AC and dehumidifier, not sure if a mini-split as a dehumidifier mode, if so, it would solve some potential issues for you.


If a floor drain or sump well is available, a hose could be run there from the dehumidifier.


Absolutely, that's what I do, but I figured this was Florida, maybe no basement and maybe no sump pump or drain - but that would be the easy solution if there was one.
We use a dehumidifer in our basement which is partially below grade in  
tangled up in blue : 7/10/2019 1:18 pm : link
a mountain house and it got rid of the musty, damp odor. Also, once I cleaned the mildew from the ceiling and walls, it has not regrown
RE: Thanks JCN  
jcn56 : 7/10/2019 1:52 pm : link
In comment 14495939 GruningsOnTheHill said:
Quote:
So you're saying I should try lowering the temperature? I keep it @77 degrees.


Yeah, that's nowhere near low enough for the compressor to be running often enough to pull the moisture out of the air.

Don't get me wrong - there are other environmental factors to consider here, both the floor and the walls being insulated will go a long way(in particular with something like EPS that offers both insulation and water vapor protection). Your immediate fix should be to run the mini-split at a lower temperature, which will extend the duty cycle of the compressor and get more of the moisture out of the air.

Some units will even have a 'dry' setting that's meant to remove moisture when there's not as much cooling to be done (effectively by lowering the fan speed).

Make sure the condensate line of the mini split is free and clear and you see water coming out the other end. These things are proned to backing up, so if you're not getting a steady drip in Florida another source of the humidity could be the mini-split itself.
Thanks  
GruningsOnTheHill : 7/10/2019 4:43 pm : link
The mini-split does have a "dry" setting on it, but in experimenting, I haven't noticed any difference between the A/C and the dry setting. How low would you say I should set the temperature?

I saw the GE 30-pint dehumidifier on special July 4th sale for $99/free shipping, and I made the impulse buy after seeing it gets great reviews. Will I be able to use it in conjunction with the mini-split for cleaner air, or have I wasted $100?
RE: Thanks  
jcn56 : 7/10/2019 4:50 pm : link
In comment 14496154 GruningsOnTheHill said:
Quote:
The mini-split does have a "dry" setting on it, but in experimenting, I haven't noticed any difference between the A/C and the dry setting. How low would you say I should set the temperature?

I saw the GE 30-pint dehumidifier on special July 4th sale for $99/free shipping, and I made the impulse buy after seeing it gets great reviews. Will I be able to use it in conjunction with the mini-split for cleaner air, or have I wasted $100?


They're fundamentally the same device - the dehumidifier is more useful when you don't want to cool down the interior space.

The dry setting on the mini split effectively turns it into a dehumidifier - you're cooling the air more to reduce humidity than to lower the temperature of the room.

I'd lower the mini split's temperature significantly - say to 70-72. I'd also get a humidistat, something like this:

https://www.amazon.com/ThermoPro-TP50-Digital-Thermometer-Temperature/dp/B01H1R0K68/ref=sr_1_3?keywords=humidistat&qid=1562791733&s=gateway&sr=8-3

This way you can track the humidity throughout the course of the day and relative to the temperature.

Keep the humidifier if you want - it might come in handy when it's cooler and you don't need to cool the space. It's less convenient, though - if it's an inverter mini-split, it'll be more efficient than the humidifier, and the existence of an external condensate line means you won't have to empty a bucket (unless you run a hose from the humidifier to the drain).
Thanks again, JCN  
GruningsOnTheHill : 7/10/2019 5:24 pm : link
The 70-72 will be pretty cold, but I would rather it be on the chilly side than have this dank air, where my clothes smell like the room in question for the rest of the day.

You brought up something I've never gotten an answer to: dehumidifying in the "winter."

I say "winter" because down here 50 degrees is considered frigid. What do you recommend I do in the November-February months, when it's too cool to run the A/C? I keep the windows open those months, but my electronics are essentially outdoors and 2 blocks from the Gulf of Mexico at that point.

Would you advise running the dehumidifier during the colder months? Even with the mini-split set to dry, it won't kick on when the temps get into the 50s...
RE: Thanks again, JCN  
jcn56 : 7/10/2019 5:36 pm : link
In comment 14496185 GruningsOnTheHill said:
Quote:
The 70-72 will be pretty cold, but I would rather it be on the chilly side than have this dank air, where my clothes smell like the room in question for the rest of the day.

You brought up something I've never gotten an answer to: dehumidifying in the "winter."

I say "winter" because down here 50 degrees is considered frigid. What do you recommend I do in the November-February months, when it's too cool to run the A/C? I keep the windows open those months, but my electronics are essentially outdoors and 2 blocks from the Gulf of Mexico at that point.

Would you advise running the dehumidifier during the colder months? Even with the mini-split set to dry, it won't kick on when the temps get into the 50s...


You're testing - so if you drop down to 'cold' temps - you'll see what the humidity does as the temp goes down that far. If you see a marked improvement, you might find a happy medium somewhere else. If it's too cold for you below 77, then it's time to set the drying mode on the mini split and set the temp around 74 (the temp will hover higher than that).

The winter months would be something you'd want to use the dehumidifier for, since you can take moisture out of the air with minimal cooling. You might not be able to get much, but then again it shouldn't be quite as humid during that time of year to begin with.
Dehumidify mode  
GruningsOnTheHill : 7/10/2019 10:41 pm : link

Got home from work later than I'd expected, and I did switch the remote to the dehumidify mode, which is the dew drop icon. As I understand it, the temperature cannot be selected in dehumidify mode. Does that mean the room will be like 63 degrees in the morning? Can these be left for long periods of time in the dehumidify mode, or are they meant to be switched back & forth between dehumidify and cool?

I did order the $10 humidistat as recommended above. On the humidistat, what is the optimum humidity I'm shooting for?

If I'm understanding, a properly-functioning mini split should be sufficient to curb the musty air issue in my situation if operated correctly? I should then plan on popping a hole in the wall for a drain hose at some point, and run the GE 30 pint dehumidifier in the colder months?

Thanks again for taking the time to consider my remedial questions.
RE: Dehumidify mode  
Jim in Fairfax : 7/10/2019 11:12 pm : link
In comment 14496390 GruningsOnTheHill said:
Quote:

Got home from work later than I'd expected, and I did switch the remote to the dehumidify mode, which is the dew drop icon. As I understand it, the temperature cannot be selected in dehumidify mode. Does that mean the room will be like 63 degrees in the morning? Can these be left for long periods of time in the dehumidify mode, or are they meant to be switched back & forth between dehumidify and cool?

I did order the $10 humidistat as recommended above. On the humidistat, what is the optimum humidity I'm shooting for?

If I'm understanding, a properly-functioning mini split should be sufficient to curb the musty air issue in my situation if operated correctly? I should then plan on popping a hole in the wall for a drain hose at some point, and run the GE 30 pint dehumidifier in the colder months?

Thanks again for taking the time to consider my remedial questions.

The only difference between cool and dehumidify modes is the fan speed. Dehumidify mode runs the fan on low, which allows more moisture to be drawn from the air as it passes over the coils. It will still provide cooling, but much less than cool mode since the air is circulating more slowly.
Dehumidify  
GruningsOnTheHill : 7/11/2019 8:25 am : link
I believe that with my Mitsubishi mini split, the temperature drops/cannot be controlled when switched into the dehumidifier mode...or at least that's what the instructions seem to indicate. When put into the dehumidifier mode (as indicated on the remote by a dew drop), the temperature setting on the little screen in fact goes away. I never understood why one wouldn't just leave it in dehumidify, as it would serve two functions. Then again, just lowering the air temperature would seem to be dehumidifying by definition, no?

At any rate, I checked the room this morning and it is cooler (although I have no thermometer yet; ordered the gauge yesterday as suggested above) but more importantly, the musty smell is only very faint now.

I will report back once the humidistat arrives and I will advise of the readings. What is the optimum humidity level I should be shooting for, and in my situation (Florida/near water/uninsulated cinder block room) should I leave the mini split in dehumidifier mode constantly throughout the 8 "summer" months?
You're looking for around 50%  
jcn56 : 7/11/2019 10:04 am : link
The reported 'ideal' is 40-50%. That's something that I think is subjective. Some folks feel comfortable at the higher level, some like it dry.

You start to get into trouble when the RH is over 60% on a regular basis, which invites mold, mildew and all kinds of pests.

Didn't realize the Mitsu's don't have a temp setting on dry. Fujitsu does, although as mentioned it's mostly a low fan speed setting, so changing the temp only marginally works.
Thanks JCN  
GruningsOnTheHill : 7/11/2019 11:26 am : link
Your info has been great, and I’m guessing you are in the trade.

The company that services our main A/C system never mentioned that I needed to lower the temperature on the mini split—or better yet, switch it into dehumidify mode. Each time I mentioned the issue of musty air/locker room odor, his answer was simply, “these mini splits don’t do a great job in this climate.”

Would you guess I will be best leaving it exclusively in the dehumidify mode, or will I be switching back & forth between dehumidify and cool?
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