AC Just wont cool - no one can figure out

Ok guys...I am hoping one of you really smart people can help me out.
I have a 2 story house around 2900 Sq ft. Downstairs cools fine.
Upstairs I try to keep at 74, but it just goes up to 78-80 during a hot day. I have measured the attic temp and it gets around 110-120 on average during the hot days. I have good insulation and power vents in about a 20 year old house.
The AC unit was installed new in 97, it is a 3.5 ton with a 4 ton A coil. I had my AC guy out today to look at it. He says I am only getting about 6-8 degrees of cooling. It is 75 in here now and it is putting out around 67. He said everything loos good, clean and working right but it should be colling better than that. The pressures were "textbook". He has done this for a while and has said he had not seen anything like it. He was so perplexed he didnt even charge me to come out.
Anyone have any suggestions? This is the second guy I have had come look at it and not be able to do anything sort of suggesting I replace the unit.
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IMO a 6-8 degree spilt is to low. Can not see it from here. With temps like yours my unit would put out 50 F air. At 81 F inside it will reach 60 F out put air temp. But that is not what your asking.
Change the filters, check the fan for speed setting, probably on medium with any luck Check output of ducts with more that your hand. the obvious, heat rises I will bet you do not have a return upstairs.
Sounds to me like there is a problem with the volume of air to the upstairs.
As a QUICK FIX not to be done for anything other than testing take the filter out and see if there is any difference. Some units are VERY sensitive about static air pressures, installing for example a hepa air filter will lower the air volume to a point where the unit has unsatisfactory performance.
All are WAG's
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SQLit wrote:

A restricted air filter would increase the delta-T.
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Travis Jordan wrote:

If the system pressures and superheat / subcooling are OK and yet you still have a low delta-T the problem is likely to be either air bypassing the coil or too high a fan speed.
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Travis Jordan wrote:

The puzzling thing there would be if this is a new problem rather than an issue from the beginning. A change would imply either a duct work change or some other large flow distribution change.
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Duane Bozarth wrote:

Right. I hope the OP returns so we can press on with this.
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Has it always been that way or is this a new problem?
If always, its probably a design issue. Ideal for 2 stories is 2 units.
If its a new problem, well, get someone else out.

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TF wrote:

Did the tech record the temperatures and pressures or just say they were "OK"? Did he measure the delta-T (temperature drop) at the air handler / furnace? If so, how did they vary from the return and supply register temperatures?
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I think this has to do more with poor air distribution than refrigeration system malfunctioning. Cool air sinks, so even if the upstairs has vents the cool air from them is probably just cascading down the stairway. I don't know, but I'll bet many or most 2-story houses have upstairs significantly warmer than downstairs. The best way would be to have separately controlled AC systems on each floor and door at the top of the stairway. But that is probably not in the cards for existing homes. You could try closing the downstairs vents and opening the upstairs ones.
Ed
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TF wrote:

One system for upstairs and down, or two separate ones?
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OK to answer some of the questions that I left out (sorry):
1. I bought the house a few years ago after it was installed. Always had this problem since I purchased it.
2. I have two units. One upstairs (has the problem) and one downstairs that has no problem.
The tech recorded the temp at all the intakes and all the discharges. Check for leaks in the duct work.

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TF wrote:

If the system pressures and superheat / subcooling are OK and yet you still have a low delta-T at the evaporator the problem is likely to be either air bypassing the coil or too high a fan speed.
Other causes such as restrictions in air flow (i.e. dirty filter or clogged coil), wrong size piston / broken TXV, undercharge, etc. would show up in the superheat / subcooling diagnosis.
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: TF wrote: : > The tech recorded the temp at all the intakes and all the discharges. : > Check for leaks in the duct work. : : If the system pressures and superheat / subcooling are OK and yet you : still have a low delta-T at the evaporator the problem is likely to be : either air bypassing the coil or too high a fan speed. : : Other causes such as restrictions in air flow (i.e. dirty filter or : clogged coil), wrong size piston / broken TXV, undercharge, etc. would : show up in the superheat / subcooling diagnosis. : : Or a high positive pressure inside the building. That's usually the cause of poor air movement whether it's air or heat. Inside pressures should only be slightly higher than ambient.
Get a real HVAC company in there. There's no reason for the things you were told unless they either didn't want the job for some reason or don't know what they are doing.
Are you a micro-manager? That's often the cause of problems like that too.
FWIW,
Pop
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Pop wrote: ....

But for low dT, the air flow has to be high, not low...
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Wet clean the condensor. Amazing how much difference that makes. Needs a techie with the right chemicals, and technique. But a clean condensor makes a world of difference.
--

Christopher A. Young
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TF wrote:

There is no such thing as *textbook pressures*. On a TXV system the pressures can look real good, even while the system is a pound or three undercharged and providing 1/2 or less the rated capacity. Call him up and ask him what the superheat and subcool temps were running, were *they* textbook? Pressure means nothing without these. Way too many techs don't know how to check the charge in a system. It's a sad situation.
hvacrmedic
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I agree with RP. Just checking the pressures is not the proper way to check refrigerant charge. He saould also check the indoor wet bulb and dry bulb temperatures, the outdoor dry bulb temperature, and the suction line and liquid line refrigerant temperatures. Then check the charging chart from the manufacturer to verify the corect charge. If he does not do this, he does not know what he is doing.
Also the tech should check the temperatures at the indoor coil, then at the supply and return grilles. If the return temp is more than 1 degree differnbt at the retuun grill and entering the coil, I would be concerned about return duct leaks or uninsulated ducts in the attic. Do the same on the supply.
Strech
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1

I hope teh second guy was from a different outfit. :^O
My WAG is it's low on refrigerant and/or there is air in the system.
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--
-john
wide-open at throttle dot info
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Was that 6-8 degrees superheat, subcool, or delta T? Pressures mean nothing....
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