# Use of an air conditioner in my shop



## jasoncarpentry (Feb 18, 2011)

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?


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## crank49 (Apr 7, 2010)

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.


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## WoodNSawdust (Mar 7, 2015)

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. eComfort.com 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.


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## Luthierman (Jun 4, 2015)

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.


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## RogerM (Oct 31, 2011)

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


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## jasoncarpentry (Feb 18, 2011)

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?


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## Pezking7p (Nov 17, 2013)

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.


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## Luthierman (Jun 4, 2015)

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.


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## WhyMe (Feb 15, 2014)

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.


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## longgone (May 5, 2009)

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


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## Crank50 (Jan 31, 2014)

> 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.


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## jasoncarpentry (Feb 18, 2011)

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?


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## crank49 (Apr 7, 2010)

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:


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## SeanIUOE150 (Apr 22, 2015)

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.


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## bigblockyeti (Sep 9, 2013)

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.


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## a1Jim (Aug 9, 2008)

This might be helpful

http://www.thisoldhouse.com/toh/calculator/0,,,00.html


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## WhyMe (Feb 15, 2014)

> 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.
> 
> - SeanIUOE150


This is the opposite of what I understand. My understanding is a unit too large will cycle too often because it reaches the cooling temperature rapidly but does not have time to remove the humidity because of running short cycles. The lessor of two evils is to have the unit too small and have it run longer, but obviously the unit should be sized correctly for the space being conditioned. Here's some info: http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/55157/Why-an-Oversized-Air-Conditioner-Is-a-Bad-Idea


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## jasoncarpentry (Feb 18, 2011)

SeanIUOE150- You refer to "pumping hot condenser air back into [the room]..." It's true that I've been testing my small AC unit without exhausting to the outside, but my long-term plan is to exhaust the air to the outside, just like crank49/50 does. Shouldn't this work to both cool and dehumidify the room?

bigblockyeti: What brand and capacity is your dehumidifier? Do you live in a humid part of the country?


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## soob (Feb 3, 2015)

So the A/C worked for a couple days then stopped? There is something wrong with the air conditioner. Duh.

I have heard the claim that large A/Cs don't dehumidify as well as smaller ones, but I don't know how true it actually is. Remember also that relative humidity goes UP when you make the air colder. A dehumidifier doesn't cool the air; it actually makes it hotter, which helps reduce relative humidity on top of pulling moisture from the air.


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## jasoncarpentry (Feb 18, 2011)

soob-

No my little AC didn't stop running. I kept it on for several hours, and it emitted cold air from one side and hot air from the other, just like an AC is supposed to do. What stopped was the flow of condensate. Once this happened, I turned it off, since my only goal was to dehumidify the room.


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## soob (Feb 3, 2015)

And it was still blowing cold air, but the humidity level did not go down?

That is very peculiar since it's not physically possible for it to work correctly, blow cold air, and not produce condensate. I would inspect it for an obstruction to the drain, make sure it's tilted correctly so it drains, and make sure it has adequate space around so the air can flow. Then try it again.

If it stops again make sure it's not frozen up. That could be caused by poor air circulation, poor drainage or low refrigerant.


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## MrUnix (May 18, 2012)

> I have heard the claim that large A/Cs don t dehumidify as well as smaller ones, but I don t know how true it actually is.


It is very true… in a past life, I worked for a large HVAC company and drew up the mechanical blueprints for the installers to use. Part of that job was to run the heat load calculations on the structure to properly size the units, ducts and CFMs required. Oversize the unit, and what you wind up with is a cool, but damp, conditioned space - usually accompanied with lots of mold and mildew. In the case of A/C sizing, bigger is not better. You need to size appropriately.

Cheers,
Brad

PS: That is not to say that a smaller unit will dehumidify better than a properly sized one for the space.


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## soob (Feb 3, 2015)

My understanding is that the theory is that large air conditioners cycle too quickly and so they don't run long enough to reach peak dehumidification efficiency. But that number turns out to just be 5 or 10 minutes.

For a shop, if you're not going to be in there very much, you don't want to cool the space all the time like a residence or an office building. IMO it's better to have a large a/c that brings the temperature down fast when you want to use it. If want to dehumidify, change the temperature differential, set minimum run times, etc., or just get a separate dehumidifier.


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## MrUnix (May 18, 2012)

> My understanding is that the theory is that large air conditioners cycle too quickly and so they don t run long enough to reach peak dehumidification efficiency. But that number turns out to just be 5 or 10 minutes.


It's not a theory. An AC reaches peak dehumidification efficiency after about 15 minutes of compressor run time. That is based on ACCA research in conjunction with the manual J heat load formulas (which has slight oversizing built in). Putting in an oversized unit will remove heat quicker, but will cost more and do nothing for humidity (which is what the original question was focused on) if it runs less than that.

Cheers,
Brad


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## Ocelot (Mar 6, 2011)

I wonder if the drain didn't just get plugged and the unit was retaining water, so that you turned it off before that water overflowed.

I suppose another possibility is that the humidity level is reduced to the point that condensate production was much less than before - possibly to the point where it would evaporation within the unit before draining out. Doesn't seem very likely but I suppose possible if rH was quite low.

-Paul


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