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Old 03-20-2012, 10:01 AM   #1
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AC Not Cooling - But it all seems to working fine...


Hi everyone, I am revisiting an issue I had late last summer, but put it off as the temps got cool real fast and it became a non-issue.

I bought a house late last summer and noticed that while the AC passed inspection, it wasn't cooling the upstairs very well when the temps got hotter (say over 90). This would tend to tax the downstairs unit heavily and it becomes a warmer than I would like house. The house is around 2800 sq. feet, with about 850 of that upstairs. Both units are Heil, 10 SEER 2.5 ton units (from when the house was built in 2001). I did have someone come in and look at the unit and it passed without issues. The filters is clean, unit charged and it blows cold air (about 18-20 degress cooler than the room). There are not any special environment issues (extra large windows, vast open spaces, etc.). The upstairs as 7 windows (3x5 each) and 3 face south and I live in Atlanta.

The previous owners were the original owners, so I am presuming they did not just decide to live with a hot house for 10 years . I am, of course, no wondering why the units are no longer cooling the house well. Some thoughts I have had are:

- Leakage in the house? I noticed that there are 3 medium sized (3 x 5) attic access doors scattered around the upstairs. There is, of course, the attic stairs like most houses. These could be leaking, I suppose...but the AC tech did not seem to comment on that much.

- Returns. I noticed there were 9 vents with 6 inch ducts and only 4 returns with 8 inch ducts. This sounds about right (2 to 1 ratio) but again, I am just not sure.

- ??? Have I just missed something? The filter is clean, the unit blows strong, what am I missing?

Any thoughts anyone has would be appreciated. I am just plain out of ideas...Thanks!

Allen

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Old 03-20-2012, 12:50 PM   #2
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AC Not Cooling - But it all seems to working fine...


It sounds like there are no balancing dampers on your system.If there is then they need to be set.
It could be a bad duct design.It could be a lack of attic insulation.It could be hot air infiltration because of leaky ducts.It could be because the flex ducts are slowing the air delivery to the upstairs.
Just too many things to guess about.

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Old 03-20-2012, 02:29 PM   #3
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AC Not Cooling - But it all seems to working fine...


Strategically add a return air upstairs.
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Old 03-20-2012, 03:23 PM   #4
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AC Not Cooling - But it all seems to working fine...


That's actually a 5:4 ratio of supply to return (based on area, not diameter); not perfect but not obviously a system killer. Your differential sounds OK (20 degrees). Are these heatpumps or straight ACs (if heatpumps, are they heating OK)? Sounds like you have a 2.5 ton unit cooling 1950 sq ft downstairs (and apparently working fine) and a 2.5 ton unit cooling 850 sq ft upstairs (and not doing it too well). Something's not right - time for a heat load calculation, as the caboose ain't keeping up with the engine!
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Old 03-20-2012, 03:49 PM   #5
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AC Not Cooling - But it all seems to working fine...


When you say wasn't cooling the upstairs very well. How warm was it getting. the previous owner may not have set the thermostat as low as you do.

You have 200 sq in of return duct/runs, and 250 sq in of supply duct/runs. So you don't have enough return.
Sealing those attic accesses will help a lot also. but adding return should happen first.
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Old 03-20-2012, 04:15 PM   #6
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AC Not Cooling - But it all seems to working fine...


Hi all, thanks for the great info. Clearly I have a few things learn about HVAC . I had a suspicion that there may have been fewer returns than desired.

As for the other owner, they did actually have the temp set a bit higher than I would set it (76 vs me at about 72-3). I will also take a close look at the insulation (inspector indicated a general pass, but maybe I can add another layer) and look at the seals on the attic doors.

Again, thanks to all for the info!

Allen
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Old 03-20-2012, 04:19 PM   #7
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AC Not Cooling - But it all seems to working fine...


Also check your duct work in the attic to make sure its not leaking. that includes making sure the register boots are sealed.

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