A/C split unit - what capacity

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Old 06-28-20, 09:57 AM
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Question A/C split unit - what capacity

Room size 100 Sq. Ft. needs split unit AC be installed. Condenser unit cannot be installed on top of room, but far away on the roof top of the building, some 70 - 80 ft away.

One tech guy came to inspect location, said it would need a unit of 24,000 BTU ! Isn't that too much for such small room. He reasoned, if unit (condenser) was on top of room, no problem, be way smaller unit. Yet as it will have to be located far away, and for better cooling, especially when in hot desert-like climate, no less than 24K BTU, should be installed.

Any thoughts, input, folks with expertise in split unit AC.

Thanks.
 
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06-29-20, 02:01 PM
57TinkerMan
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Just to give you some perspective, the load calc below shows (very loosely) your 100 sq. ft room in Phoenix. One door, one window (South side), guessed at construction. Increased outside design temp from 107F to 125 F. Crappy loose construction. Still not at 1/2 ton needed. Obviously this is just an exercise and the actual variables are different. Gotta think 2 tons of cooling in that small room would be quite uncomfortable.





 
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Old 06-28-20, 10:11 AM
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Welcome to the forums.

A hot climate and rooftop mounting do play an important role in capacity.
Having the condenser 75' above the evaporator also presents another challenge with liquid refrigeration. A solenoid valve may be required in the line.

Rather than specify a size I'd recommend you call and get a second price from a different contractor.
My gut feeling is the system should be closer to 18k.
 
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Old 06-28-20, 03:28 PM
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Just installing a 18000 BTU mini split 2 head unit in my shop. 500sq ft in one side and 250 other side. Don't have 75 ft th travel to condenser but have lot of humidity in S GA. 24000 sounds like a lot even with AZ heat. (Lived in Phoenix for 10 years) Love dry heat. I am with Pete get another estimate.
 
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Old 06-28-20, 08:11 PM
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I might be missing something but is the room 100 sq.ft or 1000 sq ft. I would not call a 1000 sq. ft. room "such a small room".
 
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Old 06-29-20, 11:51 AM
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No typo. One Hundred sq ft.
 
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Old 06-29-20, 12:09 PM
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You definitely won't need a 2 ton system, probably not even a 1 ton system. Best to find someone who knows what their doing. You will save on installation and operating costs with the correct size unit. I really can't believe the guy recommended a 2 ton system, that's so far off of what you need we thought your square footage was a typo.
 
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Old 06-29-20, 12:25 PM
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Thanks, but that's what he said would be needed, to accommodate for the long distance (75 ft) cooling (refrigerant) hose / pipe travel - between indoor and outdoor units..
 
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Old 06-29-20, 12:30 PM
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Distance will call for a slightly larger unit but nowhere near what you were told.
 
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Old 06-29-20, 12:33 PM
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This is just an example, not saying it's the correct size (close though). I used this one because it has the copper line-set length data easily available. The installation of this particular unit (<15,000 BTUH) has a maximum allowable length of 82' with a 33' drop in elevation. There are less expensive units out there but I don't know the line-set length data on them. You may have problems finding someone competent to install anything you might buy on line. You will probably need a condensate pump too. While you consider this, are there other rooms that you would want to cool? Multi-zone units are available.

https://www.supplyhouse.com/Comfort-...tioner-Package
 
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Old 06-29-20, 12:35 PM
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Even if temps can reach as high 120 - 130?
 
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Old 06-29-20, 12:39 PM
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It's just one room, for security guard. Adjacent to the building.
 
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Old 06-29-20, 12:48 PM
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For that size of room I would just get a window AC (if that is possible) they also make heat & cool units if you need heat also.
 
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Old 06-29-20, 12:57 PM
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Does the guard keep the window/door open all the time? Good quality construction/insulation? Lot of factors to consider. Regardless, 2 tons of cooling is a massive amount for the room you describe.
 
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Old 06-29-20, 02:01 PM
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Just to give you some perspective, the load calc below shows (very loosely) your 100 sq. ft room in Phoenix. One door, one window (South side), guessed at construction. Increased outside design temp from 107F to 125 F. Crappy loose construction. Still not at 1/2 ton needed. Obviously this is just an exercise and the actual variables are different. Gotta think 2 tons of cooling in that small room would be quite uncomfortable.





 
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Old 06-30-20, 03:19 AM
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Thanks 57TM, appreciate the info. Great work!
I'll have to get another contractor to look at it.

*Note; distance from indoor to outdoor unit location more than 75 ft, be like 83 ft. Will see accurate number from the 2nd guy.
 
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