AC Not Cooling - At a complete loss as to why..

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Old 06-28-12, 07:39 PM
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AC Not Cooling - At a complete loss as to why..

All:

So I am totally baffled by what is going on right now. Here is the situation..I have a 2 story house, about 1900 sq ft on the main floor (one vaulted ceiling, family room), about 800-850 sq. feet or so on the 2nd floor. Both units are 2.5 ton units, the upstairs one brand new (less than a week old). The main floor unit is the original, 10SEER unit, while the upstairs unit is a 13SEER unit. The simple problem description is that the unit simply cannot keep the upstairs cool. During the day today (in Atlanta) is was about 86-88 degrees at the high and the upstairs would not drop below 80 by the afternoon. During the AM hours its fine, but by noon, the temp starts to creep up. By 6PM, it hits 80. This now stresses the downstairs unit, which mean the first floor starts to heat up and never drops below 78.

I am simply out of ideas. The AC thinks I need a bigger unit, but I think that is dead wrong. 2.5 tons should be more than adequate to cool a house of this size. The house is about 11 years old, good insulation in the attic. I am completely at a loss as to why the upstairs cannot be kept to 73.

Here are some things that I can say would be issues:

- The upstairs plenum is small and the builder was lazy. He has 2 x 12inch supply hoses coming out of the box and all of the upstairs supplies are split from the those 2 hoses. There are 7 supplies and 5 returns upstairs. The installer for the AC thought that it might have something to do with the way the supply hoses are setup, but he later rejected that idea and pushed to either increase the tonage (up to 4 tons! 800-850 sq ft!! crazy!!) or set it up so that the rooms are cold, but that the open foyer area where the thermostat is is warm (yes, this means relocating the thermostat).

- The attic pretty darn hot (at least to me...). I have never been given a good solid answer, but I still think the attic is overly hot during the day. I do have a power attic fan (1600CFM unit), but even during the day when the outside temp is 80, the attic hits 120-130. That seems overly hot, but no one will give me a straight answer .

This is not my first home and simply put, I do not believe that 2.5 ton unit cannot keep and upstairs floor of less than 1000 sq ft at 73 when the temp is less than 100. I have had other homes where this was not an issue, so clearly there is something being overlooked. I am fresh of out ideas so any input would be appreciated. Thanks all!
 
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Old 06-28-12, 08:06 PM
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I forgot to add a couple of items...

- airflow is good and strong. The duct work is not that long, the longest runs are around 25 feet from the blower/indoor coil (all in the attic).

- the split temp is between 19 and 21. Return is 78-79 (what the upstairs is now) and supply show between 59 and 61.
 
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