Use of an air conditioner in my shop

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Forum topic by jasoncarpentry posted 07-18-2015 04:15 PM 2094 views 0 times favorited 24 replies Add to Favorites Watch
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149 posts in 3463 days

07-18-2015 04:15 PM

Topic tags/keywords: humidity air conditioning

I have a serious humidity problem in my 400 sq. ft. shop. Right now it’s about 70-75% RH, and the whole place smells like mold and mildew, which can’t be good for my health. I’ve gone thru a series of dehumidifiers from Home Depot, usually in the $200 range. These work OK, but they die within 2 years, and I’m tired of wasting money on them.

So I pulled an old window-mounted AC unit out of storage. This was used by my daughter for about a year, and worked nicely for her. FWIW, it’s a Samsung AW0528, 5000 BTU/hr. model. Before I go to the trouble of installing it so that it exhausts to the outside, I decided to run it just sitting on a workbench; i.e., the exhaust air just goes right back into the shop. For the first few days it blew cool air out of one side and warm air out of the exhaust. In other words, it operated just as an AC unit should. This may sound like a waste of time, but since it created lots of condensate, it WAS dehumidifying the room, even though it wasn’t doing any cooling.

Now the condensate flow has stopped and the humidity is at 75% (where it stayed while I was running the unit). I’m trying to figure out why. I know why it wasn’t doing any cooling, but why wasn’t it lowering the humidity in the room? After all, it was producing condensate. Any thoughts? Is the unit just too small for the room?

-- Jim in Tennessee

24 replies so far

View crank49's profile


4032 posts in 3780 days

#1 posted 07-18-2015 05:44 PM

I have a 384 sq.ft. shop in Tennessee. My AC is 11,000 BTU and is just barely enough to keep my shop comfortable, but that’s how I sized it.

View WoodNSawdust's profile


1417 posts in 1985 days

#2 posted 07-18-2015 06:00 PM

I am working with a HVAC guy to add permanent heat & air to my shop. He told me that the rule was 1 on per 400 square feet. 1 ton of air = 12,000 BTU. So yes I think you will need a bigger unit or multiple units.

Consider biting the bullet and buying a heat pump I went with the Mitsubishi Mr. Slim ductless unit. claims that it will work efficiently from 5 degrees to 95. I will know better once we get installed. They (eComfort) did not send one of the parts.

In any case, I DO NOT recommend eComfort (aka Power Equipment Direct)! I just made the mistake of dealing with them.

-- "I love it when a plan comes together" John "Hannibal" Smith

View Luthierman's profile


222 posts in 1896 days

#3 posted 07-18-2015 07:11 PM

I have the same issue. Strangely I can set the temp where I want it and it stays, however, the humidity never goes below 70% either. Heck, it sometimes goes UP when I turn on the a/c. Completely counterintuitive. It makes my brain hurt when I try to figure out why that is happening.

-- Jesse, West Lafayette, Indiana

View RogerM's profile


804 posts in 3208 days

#4 posted 07-18-2015 08:20 PM

I think WoodNSawdust pretty much nailed your problem. I am not sure what your walls are but humidity can and does penetrate some walls.

-- Roger M, Aiken, SC

View jasoncarpentry's profile


149 posts in 3463 days

#5 posted 07-18-2015 10:36 PM

Thanks for the replies so far. crank49: What brand AC do you have? And note Luthierman’s response. Like him, are you also not seeing a reduction in humidity?

Luthierman: Is your unit producing any condensate? How many BTU’s do you have?

-- Jim in Tennessee

View Pezking7p's profile


3358 posts in 2460 days

#6 posted 07-18-2015 11:22 PM

Check to make sure the heat exchanger didn’t freeze over. This will happen on window units after they run non stop. Once they freeze over the efficiency goes to crap and they stop making cold air and dehumidifying.

If it’s not frozen, then it’s probably just too much for the little feller to keep up with. 5000 BTU is pretty small.

-- -Dan

View Luthierman's profile


222 posts in 1896 days

#7 posted 07-19-2015 12:03 AM

I have a 10,000 btu in room a/c. It makes plenty of condensate, as I continually have to make sure the drain line is clear. I don’t have very much fall so it can get clogged from time to time. My shop is about 200 sq ft. I have plenty of cooling power. It is extremely humid here. My house is hovering around 70% too. This has been an unusual year weather wise. I just find it curious that the R.H. can go up when I first turn the unit on for the day.

-- Jesse, West Lafayette, Indiana

View WhyMe's profile


1300 posts in 2370 days

#8 posted 07-19-2015 03:26 PM

I have a 10,000 BTU window A/C in my fully insulated 400 sqft shop and it does fine with cooling and keeping the humidity down. I’m in central Virginia and the humidity recently has been terrible. I’ve never measured what the actual humidity level is but it feels comfortable. One thing I know is if the A/C unit is too large it will cycle on and off a lot and will cool fine but won’t run long enough to remove the humidity. So bigger isn’t better with A/C.

View longgone's profile


5688 posts in 4117 days

#9 posted 07-19-2015 03:57 PM

Good insulation is important for efficiently running an AC unit. My shop is 24×40 and has 10 ft ceilings. I insulated it with foam insulation and use a 24,000 btu window unit that keeps it at a low humidity level that is always below 40% and the temperature is always comfortable. In a shop you always have to be checking the filter and changing them frequently if you do a lot of dust generating woodwork.

i am not familiar with the heat pump models or how they would be any better…but they would also need constant filter monitoring because sawdust will always be the dowwnfaall of any ac unit if not kept clean

View Crank50's profile


173 posts in 2385 days

#10 posted 07-19-2015 06:53 PM

Thanks for the replies so far. crank49: What brand AC do you have? And note Luthierman s response. Like him, are you also not seeing a reduction in humidity?

Luthierman: Is your unit producing any condensate? How many BTU s do you have?

- jasoncarpentry

My little window AC unit is a Frigidaire and was just under $300.
It lowers the humidity just fine and flows condensate all the time.
Main thing I like about it is the “econo” setting where it turns off to a sleep mode but starts the fan every few minutes to sample the room air temperature and shuts back off without running the compressor if it’s not needed. This feature is great.
By the way, this is a window unit, but I didn’t have a window where I wanted to install it. I just framed up an opening the size of a half closed window and set the unit in the opening. Installed the side panels and foam rubber tube top and bottom like it was a window and this works just fine.

View jasoncarpentry's profile


149 posts in 3463 days

#11 posted 07-19-2015 08:06 PM

OK, crank49 and crank50: Are you guys just messin’ with me, namewise? I see you both live in Pulaski, TN. I’m in the greater Knoxville area. I’m assuming that the humidity in both parts of TN is about the same; if anything, Pulaski should have a slightly higher RH.

crank49’s shop has 384 SF & the AC is 11,000 BTU/hr. crank50: How big is your shop, and what’s the rating on your AC? And exactly which Frigidaire model do you have?

-- Jim in Tennessee

View crank49's profile


4032 posts in 3780 days

#12 posted 07-20-2015 01:54 AM

Well, I am Crank 49 at work and Crank 50 at home ;^)
Hate having to log in every time I use a different computer.
So both shops are 384 sq ft (16ft by 24ft) with 9ft ceiling and both are using the same AC.
Pulaski is pretty darn humid and plenty hot; especially for the last few days.
Highs of over 96 degrees every day, high humidity and a heat index of around 105.
I would agree that in Knoxville you are probably not quite as humid or hot as we are here.

I bought the AC last year (2014) in March. Think it was a 2013 model.
Closest one I find in todays model lineup is LRA107BU1, which is a 10,000 BTU.
The 11,000 BTU now has heat and requires 230 volt. Mine is AC only and runs on 115 Volts.
Looks like this one:

View SeanIUOE150's profile


4 posts in 1939 days

#13 posted 07-20-2015 02:18 AM

Typically with AC if the unit is undersized like it is in your case, it will not have the capability to remove the humidity in the space you’re trying to condition. It will cool the space but not remove the excess humidity. Also you’re pumping hot condenser air back into the space you’re try to condition, that’s why the humidity is going u with the unit running. A properly sized AC will cool and remove humidity all in one. Get a larger unit.

View bigblockyeti's profile


6646 posts in 2529 days

#14 posted 07-20-2015 02:31 AM

My shop is 415 sqft. and I just have a small dehumidifier that runs nearly all the time. The water bucket is feed directly to a drain so I never have to empty it. Though it runs quite a bit and only keeps the humidity ~30% lower than outside, it really helps rough sawn wood dry and keeps me a lot more comfortable that I would be otherwise.

-- "Lack of effort will result in failure with amazing predictability" - Me

View a1Jim's profile


118104 posts in 4386 days

#15 posted 07-20-2015 02:35 AM

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