Low AC Performance

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  #1  
Old 08-22-19, 01:57 AM
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Low AC Performance

Hello,

I have an issue with my AC, in that the house temp. never drops bellow ~79F even though outside temp is only slightly higher: ~83F. Any advice on what could possible be wrong with it is highly welcome, as I'm completely out of ideas.

For reference, I'm posting the temp measurments and specifications I have available:
-The apartment (775sqft, avg ceiling height 8ft) is equipped with a Fan Coil Unit meant to provide AC to all rooms via ducts in the ceiling.
-FCU is rated at 4.7KW (so I'm guessing about 16000BTU) - and it is using water as it's agent (in temp: 47F, out temp: 52F)
-Temperature at vents (cooled air in ducts) is ~57F
-Room Humidity 48-50%
I was unable to check duct wind-speed (I don't have an anemometer), but I've looked in the ducts with an endoscopic camera, and aside from a bit of dust, I haven't seen anything blocking, broken or that could restrict airflow.

Any advice is welcome!
Thank you!
 
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Old 08-22-19, 07:35 AM
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Is this a new issue? Your readings don't look that bad.
 
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Old 08-22-19, 11:37 PM
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Welcome to the forums.

Either there is a lot of cold water flow or you're not moving a lot of air thru the coil with only a 5 increase in water temps. I'm guessing your on the upper floor with the roof above you..... right ?
 
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Old 08-23-19, 05:36 AM
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Thanks for the welcome!

I live on the 1st floor, in a 4 story building (I have apartments both underneath and above me). Airflow issue sounds most likely, however I've cleaned both the fans and the coil itself. Filter is brand new, main duct seems clear (aside from some dust). What would a proper in/out water temp. difference be?

What I find strange, is that the 8sqft room where the FCU is getting very cold, despite the fact that I don't have an "in" vent (so in theory this would only be return air getting there).
Would it be possible that I have duct leak somewhere, causing cold air to end up ventilated directly back in the FCU room? How could I most easily verify this hypothesis if I don't have easy access to the duct system?

Also, I've did a test last night, restricted airflow to only about 400sqft (dorm, living and hallway). Temperature dropped to 77F (from 79F), so a further 2F.

airman.1994, I've only bought the place for a month now so I can't really compare. Neighbors however, told me they get it down to 73-74F with the same equipment.
I can't find the CFM rating on the FCU though. Wondering, for 775sqft, what CFM value would be nominal?
 
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Old 08-23-19, 08:28 AM
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You said air in the ducts is 57 degrees so the coil is transferring the heat so I don't think it is an airflow issue because you have around a 20 Delta T. I do agree with PJ that 5 degrees isn't much on the coil temp. I have seen an issue on a remote chiller before that is close to this. The coil size was increased so more heat can be removed. Wonder if the coil (water side) is stopped up?
 
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Old 08-23-19, 08:40 AM
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Originally Posted by JCD37
What I find strange, is that the 8sqft room where the FCU is getting very cold, despite the fact that I don't have an "in" vent (so in theory this would only be return air getting there).
Wild guess
- the system uses flexi-ducts to connect to the unit.
- somebody use a promotion for "duct cleaning"
- they cleaned the system but reversed the distribution and return ductwork
 
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