AC Unit Sizing?

Robert

Stay Frosty!
Jan 2, 1999
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We are expanding our office to a 40 sq/m space underneath us.

This space needs AC, so I made a couple of calls and asked for two local companies to quote on a small split AC unit for this new space. They both sent their AC "experts" out to look at the space to see what size AC unit was needed.

Local AC "experts" are notorious for oversizing AC's, plus throw in a gringo, and you get back some pretty outrageous quotes :)

Quote 1) 36,000 BTU's (RD$64,000)
Quote 2) 24,000 BTU's (RD$46,000)

Both gave me very poor reasons why the AC unit needed to be so large.

Expert 1) "This is the Caribbean, European AC units are always smaller, plus you don't like it as cold as us Dominicans."
Expert 2) "If you want less BTU we will install whatever you want but this is our recommendation."

Experts or idiots, this should be the question!

I'm no AC expert, but I can search the web and find info sources on AC sizing :)

I looked at some AC sizing websites and they all came in at around the 12,000 BTU mark for the climate, office space (40 sq/m), windows, no kitchen and amount of people working in the space.

Australian site with a AC calculator.
I choose Cairns, which is a very tropical climate with often 100% humidity.
http://panasonic.com.au/products/easy_selector.cfm?objectID=30
They recommend a 3.6KW AC. That's just over 1 ton or 12,000 BTU's.

Florida or Texas.
http://www.hvacopcost.com/equipsize.html
They also calculate around 12,000 BTU's.

Or we can use this conversion tool and go by area code.
http://www.mini-split.com/html/mini-split.htm
Once again, just over 12,000 BTU's/

So...

Can anyone with any AC experience please tell me if my calculations are right or should I take the advice of the local "experts" ?
 

Mirador

On Permanent Vacation!
Apr 15, 2004
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40 sq/m? yes, you could do with a 12000 btu unit. However, you have to factor in energy use. A 24000 btu unit may be more energy efficient, and in a years time you could more than make up the extra capital outlay with savings in your electricity bill...

36000 btu is overdoing it...

-
 

bigbird

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May 1, 2005
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The cost estimates are they for a window unit or an outdoor compressor with the installation of duct work?
 

bigbird

Gold
May 1, 2005
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Mirador said:
40 sq/m? yes, you could do with a 12000 btu unit. However, you have to factor in energy use. A 24000 btu unit may be more energy efficient, and in a years time you could more than make up the extra capital outlay with savings in your electricity bill...

36000 btu is overdoing it...

-

A word of caution when oversizing an A/C unit. If the unit is too large it will cool the room rapidly and not run long enough to remove the humidity.
 

SKY

Gold
Apr 11, 2004
13,470
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Call this place and ask them. This is all they sell is AC and Refrigeration systems. They are in Puerta Plata, but very honest and helpful. They will be glad to tell you what you need. I bought a unit from them and they were 100% correct, good price and great product. When I asked about a very large living room also the guy told me it would be a lot cheaper to open the sliding doors with a screen! I got a unit for a large bedroom.

I know you are in the Capital but they could still give you advice for free.

Refripartes, C x A 586-3174, 586-4741. This place ia in the Plaza with Vimenca just before the circle heading to PP from Sosua.
 

Robert

Stay Frosty!
Jan 2, 1999
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bigbird said:
The cost estimates are they for a window unit or an outdoor compressor with the installation of duct work?


Wall mounted split unit, with the compressor outside.
 

Robert

Stay Frosty!
Jan 2, 1999
20,574
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dr1.com
bigbird said:
A word of caution when oversizing an A/C unit. If the unit is too large it will cool the room rapidly and not run long enough to remove the humidity.

Try explaining this to the local "experts".
I tried, this is rocket science!
 

easygoin

Bronze
Jan 2, 2005
746
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Robert said:
We are expanding our office to a 40 sq/m space underneath us.

This space needs AC, so I made a couple of calls and asked for two local companies to quote on a small split AC unit for this new space. They both sent their AC "experts" out to look at the space to see what size AC unit was needed.

Local AC "experts" are notorious for oversizing AC's, plus throw in a gringo, and you get back some pretty outrageous quotes :)

Quote 1) 36,000 BTU's (RD$64,000)
Quote 2) 24,000 BTU's (RD$46,000)

Both gave me very poor reasons why the AC unit needed to be so large.

Expert 1) "This is the Caribbean, European AC units are always smaller, plus you don't like it as cold as us Dominicans."
Expert 2) "If you want less BTU we will install whatever you want but this is our recommendation."

Experts or idiots, this should be the question!

I'm no AC expert, but I can search the web and find info sources on AC sizing :)

I looked at some AC sizing websites and they all came in at around the 12,000 BTU mark for the climate, office space (40 sq/m), windows, no kitchen and amount of people working in the space.

Australian site with a AC calculator.
I choose Cairns, which is a very tropical climate with often 100% humidity.
http://panasonic.com.au/products/easy_selector.cfm?objectID=30
They recommend a 3.6KW AC. That's just over 1 ton or 12,000 BTU's.

Florida or Texas.
http://www.hvacopcost.com/equipsize.html
They also calculate around 12,000 BTU's.

Or we can use this conversion tool and go by area code.
http://www.mini-split.com/html/mini-split.htm
Once again, just over 12,000 BTU's/

So...

Can anyone with any AC experience please tell me if my calculations are right or should I take the advice of the local "experts" ?


Rob, 1 ton per 500 ft.? in general for residential. Now you must take into consideration.... infiltration, windows and walls, doors. Also the heat load from lighting, computers, coffee urns, and BTU per person...say 150 btu active.

I would say due to computer, and other factors being your more commercial than residential you may want to go 18,000 btu's at most.
2 or 3 ton it out of hand.You need run time to remove moisture.
 

Robert

Stay Frosty!
Jan 2, 1999
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easygoin said:
Rob, 1 ton per 500 ft.? in general for residential. Now you must take into consideration.... infiltration, windows and walls, doors. Also the heat load from lighting, computers, coffee urns, and BTU per person...say 150 btu active.

I would say due to computer, and other factors being your more commercial than residential you may want to go 18,000 btu's at most.
2 or 3 ton it out of hand.You need run time to remove moisture.

I'm looking at 3 people max in this space, most of the time 2. It has one shaded window (4 sq/m), that will have a secondary double pane window for sound proofing. The space is well insulated. Lighting and 2-3 computers, how much heat do they generate?
 

easygoin

Bronze
Jan 2, 2005
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Robert said:
I'm looking at 3 people max in this space, most of the time 2. It has one shaded window (4 sq/m), that will have a secondary double pane window for sound proofing. The space is well insulated. Lighting and 2-3 computers, how much heat do they generate?


Example , a small mini tower will produce...910 btu /hr.
 

GringoCArlos

Retired Ussername
Jan 9, 2002
1,416
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18,000 BTU would do this environment with no problems, and it can also be wired as 120 volts (probably the existing wiring will do).
 

MrMike

Silver
Mar 2, 2003
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www.azconatechnologies.com
My non-espert opinion:

- Stay away from 120 volt AC's. The efficiency on the 2 I have used is terrible.

- The space Robert describes should be fine with a 12,000 BTU AC, assuming the insulation is as good as he says and the afternoon sun is not coming straight in a window everyday, and the door is not constantly opening and closing, and there really are only 2 or 3 people in the workspace.

- I think you should get a 24,000 BTU AC though, just in case and mostly because I think the price difference is much less than the benefits of the exrtra horsepower.
 

rellosk

Silver
Mar 18, 2002
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MrMike said:
- - I think you should get a 24,000 BTU AC though, just in case and mostly because I think the price difference is much less than the benefits of the exrtra horsepower.
What about the problem that others have mentioned, with the room being cooled so quickly, it doesn't have the time to lower the humidity before shutting down?
 

MrMike

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rellosk said:
What about the problem that others have mentioned, with the room being cooled so quickly, it doesn't have the time to lower the humidity before shutting down?

I just can't imagine that being a problem. I have never heard anyone say:

"Dammit my air conditioner is too good, how am I supposed to deal with all this frickin humidity?"

But I have heard alot of people say:

"Dammit it's hot in here, can't that AC unit go any higher?"

I don't think people are depending on tha AC to dry their hair or anything.
 

Robert

Stay Frosty!
Jan 2, 1999
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rellosk said:
What about the problem that others have mentioned, with the room being cooled so quickly, it doesn't have the time to lower the humidity before shutting down?

Exactly!!! And this is the point that most local AC "experts" don't get here.
 

easygoin

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Jan 2, 2005
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Robert said:
Exactly!!! And this is the point that most local AC "experts" don't get here.

Robert, that's exactly why, desired temperature and heat load, the structure of the building is very hard.

Your office should be based on commercial, which is a whole different animal.... but overkill will actually hurt the unit if oversized, due to short cycling which is very hard on your starting components, and not having a long enough off cycle to equalize the system to take pressure off of the compressor's piston on next start out due to high pressure. And also to regain oil back into the crank case that was pulled from the run cycle.