10,000 btu unit in 288 sq ft - too big?

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Old 07-08-10, 12:04 AM
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Question 10,000 btu unit in 288 sq ft - too big?

Before this Friedrich SS10L10 10000 Btu unit I recently installed, I was running a temporary Friedrich 5,000 btu unit and it caught up at night and the shed was crispy cold and very low humidity (couldn't feel it).

Now since I have installed the SS10L10 it doesn't seem to reach the set temperature. I set it to 67 to test it out and I put my weather channel temperature sensors in the 288 sq ft shed. Last night after a couple hours I saw it was down to about 69 in the shed & I was surprised it wasn't 67 or a tad cooler.

The compressor seems to cycle on and off a lot before ever reaching desired temperature even though I have it set for the fan to only run during cooling.

The rear of the room seems to run 1 or 2 degrees hotter too. The side walls are 6'4" tall and the peak of the gable/roof is about 8.5". The shed is 12'x24'.

I have gone to great length to seal this 288 sq ft shed I'm converting to a home office. It is so well sealed (silicone on outside, a/c foil tape at bottom edges where it meets bottom frame, spray foam in corners, edges and holes, etc..) that when I open the door I have to give it a good shove or a punch for it to open as opposed to when I had a small or large hole exposed in the side wall the door opened with ease.

There is R4 foam insulation on the walls/gables and R13 insulation on the wall facing the sun. There is radiant barrier on the roof board underside and I plan to add R4 foam insulation to the sloping ceiling sides of the shed above the sheetrock and R35 fiberglass foam to the flat top center ceiling above the sheetrock once it is in.

Currently the walls are exposed and there is no sheetrock in it yet anywhere. All edges have been silicone sealed (even in the flooring). There is no insulation under the flooring, but there is water/weather sealant painted over the flooring and the walls. I made a mistake putting thompson's water seal on the interior walls as its only for exterior use only, but that is another topic.

I had figured that I'd go with an 8000 btu unit initially, but as I played with settings on some of these btu calculator's online if I input that the room was in the sun a lot and had poor or no insulation the recommended btu's got up to 10000. Though now I am wondering if I went too high on the btu's since I have sealed everything up so well.

During the day the shed really is in sunlight half of the day time hours. The 5000 btu unit struggled during the day and always ran in high fan cooling mode and leveled off at night sometimes reaching "fan only" mode. The 10000 btu unit cools fast, but despite "paper" I have in the shed remaining crispy I swear I still feel higher humidity levels in there using the 10000 btu model than I did when the 5000 btu model was in use. During the day time the 10000 btu unit seems to struggle to keep temperature by about 4 - 5 degrees. I had left it on 67 and the front remote sensor said 71 and the rear room sensor said 72 after several hours of use.

This is a through-the-wall unit with a wall sleeve. I went to great length to seal between the wall, sleeve and between the slide out unit with spray foam and silicone. Despite this there is condensation build up on the bottom of it outside the wall closest to the the wall side.

I'm puzzled too by the water collection pan as it only seems like it would drain once it is half full in the back of the unit where the condenser coils are. This means that the condenser coils always sit in a little puddle of water up to 1 inch high before it would drain out the back.

Aside from all this, the unit arrived in factory packaging and seemed like it'd never been used before. It was packed sturdy and well, but I can tell it was handled very carelessly as the top corners of the front frame shroud were cracked and the rear corner of the condenser coil fins are dented inward about 1/2 to 3/4" inward. The coil's themselves seemed fine right there, but I thought I should mention this noticed damage.

Otherwise feeling the air the unit outputs, it seems very cold at the output sourced, but compressor cuts off before reaching set temperature, hasn't reached 67 degrees as I've seen and humidity seems like it could be lower.

Sorry so long.
 
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Old 07-09-10, 02:20 PM
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Angry

I have been monitoring the 10,000 btu unit and during the day I have it set to 75 and the room is at 80 degrees F. I guess I need to put the insulation on the roof and sheetrock on before I'll know exactly how it will perform during day time heat in de deep south Louisiana Sha.... yeaaaaa sha... cajun country.

At night the unit comes within 1 - 2 degrees of the set temperature, but never reaches it.
 
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Old 07-09-10, 03:09 PM
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I don't think it should retain any water. You sure you have it installed correctly? Maybe you should drill a small hole in the bottom for the water to drain out.

Try not to suffocate in that shed being so tight.
 
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Old 07-11-10, 12:00 AM
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Talking

The unit I have has a stale air exhaust and fresh air intake function lever, so from time to time we'll vent in and out. Thats a good point though.
 
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Old 07-11-10, 07:53 AM
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Proper sizing is very important for efficient air conditioning. A bigger unit is not necessarily better because a unit that is too large will not cool an area uniformly. A small unit running for an extended period operates more efficiently and is more effective at dehumidifying than a large unit that cycles on and off too frequently.

BTUs determine how much cold air comes out of the unit, but problems can occur if the air conditioner is oversized for the space. The more cold air that comes out – the more area it needs to circulate to avoid giving the thermostat a false reading. If the unit is oversized, the cold air bounces off the wall back onto itself giving the thermostat a false reading – causing the compressor to shut off because the thermostat reads that the set temperature has been achieved. While the unit is off – the thermostat is able to read the room’s actual temperature, which kicks the compressor back on. This process will continue, causing the compressor to cycle off and on every two to three minutes, but never staying on long enough to effectively cool the room or remove uncomfortable humidity.

Emil
 
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Old 07-11-10, 09:23 AM
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I think that in this situation I might hang a ceiling fan and turn it on to distribute the air more evenly, try that and I bet that you will be so happy.
 
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Old 07-11-10, 09:52 AM
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Originally Posted by Former Member View Post
I think that in this situation I might hang a ceiling fan and turn it on to distribute the air more evenly, try that and I bet that you will be so happy.
A ceiling fan will diffidently help circulate the air, I'm not sure if there is enough clearance; searcherrr said "The side walls are 6'4" tall and the peak of the gable/roof is about 8.5". The shed is 12'x24'."

Emil
 
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Old 07-11-10, 09:56 AM
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he might be a little person.
 
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Old 07-14-10, 06:20 PM
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Question

Good ideas and I'm thankful to all of you.

Thank you Emil for your very thorough description of how it all works with the right size unit. That makes it a lot more clear. Its funny because I had figured I would need around 8000 btu's accurately a long time ago and then when I went to buy a unit several months later I forgot almost all that research I'd taught myself about proper sizing and impulsively bought a bigger btu unit cause it was the same size and cheaper and now I may have screwed up with $$ cause it might not be the right choice for the space.

LITTLE PERSON ! LOL - well I am only 5'7".... that was funny though.

I did notice when I brought in a floor fan that it seemed more comfortable or uniformly cool in the room, at night only.

There are only 2 light fixtures in this space and the 2 ceiling slopes & level ceiling middle section are each 4' wide. I could fit a ceiling fan, but I really didn't want to have to put one in. A mobile fan seems like a better choice so we can change where the air goes as needed.

Eventually I will purchase another A/C with lower btu's for the same chassis. Currently though during the day this 10,000 btu unit (with no ceiling sheetrock and insulation in yet.... though insulation already in on the walls) is not even coming close to keeping up, but I'm hoping this is because the walls/insulation aren't done yet.

I guess I won't really know if I've sized the a/c right until I finish the walls and carpet huh? Accurate in that thinking?
 
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Old 07-14-10, 06:47 PM
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You are welcome, we all made and still making mistakes.
Keep the ceiling fan in mind though, ceiling fans are great during the summer and winter times. I use my ceiling fans during the winter more than the summer.

Stay cool,
Emil

Beer 4U2
 
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Old 07-15-10, 08:20 AM
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I agree with all the others: 10,000 BTU is quite large for 288 sq ft, even if not well insulated. You have a condition called short cycling. The unit blasts out some cold air, the unit thermostat feels cold, and turns off the unit. You have lowered the temp some, but have not removed much humidity, because that happens over time as room air is circulated through the coils.

Higher humidity makes it feel as if the room is not as cool as you wish. Running a smaller unit for a longer period of time circulates the air and removes humidity.

NOW, what can you do:
►run the unit on LOW COOL, rather than HI
►if the unit has the option, set it for the fan to continue to run in between compressor cycles. Most window units work that way, and most thru-the-wall units have an internal option switch.
 
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Old 07-15-10, 07:04 PM
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Exclamation

Low cool being "Low Fan"?

Thats a good idea about running it in lower fan mode.

Yeah, this unit has a money saver feature and a smart fan and I can turn those features both off if I want. So I'll try all that.

No one in need of a 10,000 btu unit lightly used huh? Sell for $550 plus shipping and these are over $730 new (this one is new). This is the Friedrich beginning commercial line model. SS10L10 is the model number. Its their 2009 model.

If nobody wants it i'll just have to deal with it how ya'll have said till i can replace it with a more suited btu unit.
 
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Old 07-20-10, 07:29 PM
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Arrow Use a large fan to move air...

I cool nearly 900-sq.ft with a little 6,000-BTUH Half-Ton window A/C. I use a 20" vertically adjustable fan to circulate the air through all the rooms & back to the A/C unit.

It's a 1937 farm home with a lot of windows & very little shade.
The other day a town near me had 113-F Heat Index, & my little A/C was cooling the entire floor area to perfect comfort conditions.

You can cool large areas with a small A/C by using a large fan to move air, then turn the stat down to a low temp & let'-er run...
 
 

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