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Old 07-05-2010, 03:15 PM   #1
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please help w/ ac


We currently have gas heat and a ruud (scroll) unit outside. The a/c for the upper 2 ton unit has been checked and serviced but the air temp coming out of the vents is about 8 deg. difference between the return/outlet. Downstairs is right at 17 deg difference. So I think it is time as it's a 15 year unit. Are there certain brands I should stay away from or brands people prefer. A friend of mine will be doing the install and he got a quote for parts only right at a grand for just doing the a/c. I think he has to replace something in the attic unit to match. Not sure. Still trying to learn.
When we bought the house my friend found a leak in the system at the (schrader valve). Maybe the system got some moisture/trash in the system and hurt the compressor? The house sat for 4 years but we have been in it for about 8 months.
If I left any important info. out please let me know.

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Old 07-05-2010, 06:14 PM   #2
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May just need a good tune and clean.

Brand doesn't matter. The installation is what matters.

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Old 07-05-2010, 10:30 PM   #3
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I measured the floor space and it was right at 1200 sq. ft. All rooms are 8' except one which is 10'. We do have alot of windows and it has been in the upper 90's the last week or so. The unit was tuned up and everything was checked over. Freon is perfect, coils clean, ect. The downstairs unit is a 2.5 ton unit while the upstairs unit is a 2 ton unit. We do have a basement which the 2.5 ton unit takes care of but the vents downstairs are closed.
I will look into the attic to make sure the return duct is intact and that the duct work is insulated. I am sure it get real toasty up there. I will take some temps tomorrow. Maybe a attic fan?
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Old 07-06-2010, 12:48 PM   #4
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Not sure how your unit is installed or if it'a an upflow or downflow or horizontal. A previous house I owned had an upflow furnace which sat on a shelf with the return air below. I experenced the same thing your seeing. Turns out the slight vacuum under the shelf was pulling super heated attic air down the interior wall through some openings for pipes and electrical. Try measuring the air inside the furnace at the motor itself and compare that with the air at the return air grill. I was loosing nearly 1/2 of the capacity of my air conditioner. While return air grill temps might have been 78 degrees, by the time that air mixed with the attic air being pulled down through the wall cavity it was more like 85 degrees. I had a 15 degree drop across the coil but it was 15 degrees from 85 rather than 78. It was really a bad install. I plugged the holes and fixed the problem.
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Old 07-06-2010, 12:55 PM   #5
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please help w/ ac


Good call. I am new to a/c systems but not to any mechanical/electrical work. I will tap into the return unit as my system is the same as yours and I found some leaks at the supply joints. I consider it a sub-par install but blaming anyone won't fix it. It's pretty toasty up there now so I might wait until morning. I might even install a gable fan as it's over 130 deg. at the peak now.



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Not sure how your unit is installed or if it'a an upflow or downflow or horizontal. A previous house I owned had an upflow furnace which sat on a shelf with the return air below. I experenced the same thing your seeing. Turns out the slight vacuum under the shelf was pulling super heated attic air down the interior wall through some openings for pipes and electrical. Try measuring the air inside the furnace at the motor itself and compare that with the air at the return air grill. I was loosing nearly 1/2 of the capacity of my air conditioner. While return air grill temps might have been 78 degrees, by the time that air mixed with the attic air being pulled down through the wall cavity it was more like 85 degrees. I had a 15 degree drop across the coil but it was 15 degrees from 85 rather than 78. It was really a bad install. I plugged the holes and fixed the problem.
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Old 07-06-2010, 01:26 PM   #6
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please help w/ ac


Ok, I drilled a small hole in the main return and dropped the thermometer wire down about half way to get a good reading. Looks like the temperature in the return is at least 7-8 degrees warmer than the return grille. The temperature in the attic is about 140 degrees at the crest. I have a infared meter and so far w/ the testing I have done it is very accurate but I always double check strange readings. Anyone have a guess how much the return is being heated by the hot air surrounding the system? From the return grill is a thick insulated duct that leads to a 90 degree metal elbow which is uninsulated. It's only about three linear feet so I wouldn't think it would raise it that much. All the duct supply work is insulated. Thanks for any help.
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Old 07-06-2010, 04:10 PM   #7
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Look for air leaks allowing attic air to be drawn into the return.
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Old 07-06-2010, 04:27 PM   #8
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That is what another poster suggested as well. Either that or the three feet or so before the temperature probe are getting heat soak from the hot temperatures in the attic.




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Look for air leaks allowing attic air to be drawn into the return.
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Old 07-06-2010, 04:32 PM   #9
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I am having somewhat the same problem as well, unit really just runs all day long. One reason was the new roof did not vent the roof and I just installed a new vent, the insulation is only R20ish in the attic... so i am fixing those problems first, but before I go burying everything in the attic with fiberglass... what is the best way to look for leaks. I have 3 different return registers that pulls from the house... the returns pull from near the cieling inside, so I assume it is to draw out the heated air, but then the returns could easily be sucking in heated conduction air from the heated attic.

Any ideas to spot air leaks on the returns? or where else to look? And plug them with what? Dow foam?
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Old 07-06-2010, 04:35 PM   #10
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At all joints/connections.
And where the return box meets and penetrates the ceiling.
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Old 07-06-2010, 04:38 PM   #11
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Oh lord... all three of the returns are mostly in the drywall... this will not be easy!
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Old 07-06-2010, 05:34 PM   #12
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In addition to the problems with the install I mentioned earlier I also had a mostly unvented attic. The AC was at one end of the house and the duct that ran the complete length of the attic was loosing another 4 degrees or so because it was so hot in the attic. That coupled with the losses from the attic infiltration meant I was loosing about 75% of cooling in that one duct. Shorted ducts were less of course. . I do some data acquisition in my job and so had access to the necessary thermocouple and logging equipment to monitor many input points. I had about 20 thermocouples placed around the house. Peak temps near the roof were 140 and just above the insulation 120 to 130. What was particularly interesting was the temperature UNDER the insulation. As the day wore on this temperature continued to rise even after the sun set. In affect it became a big radiator in the evening and the AC could not keep the setpoint. Consequently the AC ran constantly after about 3 or 4PM and didn't shut off until early the next morning. This occurred on sunny hot days in Arkansas with daytime temps 90 to 100 degrees F. Cloudy days, windy days and rainy days kept the attic much cooler thus the ac could keep the house comfortable. We added a couple of rooms later and after taking care of the attic return air infiltration I was able to downsize from a 3 ton unit to a 2 1/2 ton unit without a problem. During the remodel, a sliding patio door was removed to allow access to the addition. It faced the southwest and once it was gone it also helped remove a lot of late evening heat gain.
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Old 07-06-2010, 05:37 PM   #13
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Meant to add, if you have attic air infiltration down through a wall you should be able to put your hands on the wall and feel the heat. It could be any wall surrounding the ac (or more than one wall) but most likely will be on a wall where holes have been cut into the top plate to pass utilities.
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Old 07-06-2010, 08:42 PM   #14
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Quote:
Originally Posted by Mike in Arkansas View Post
Meant to add, if you have attic air infiltration down through a wall you should be able to put your hands on the wall and feel the heat. It could be any wall surrounding the ac (or more than one wall) but most likely will be on a wall where holes have been cut into the top plate to pass utilities.
So, the AC is in the basement, and the returns run a duct into 3 separate registers. Should I be feeling the walls of the returns? Or is there another area I should look in. I am going to climb into the attic on a cooler day to see if the return register duct somehow has infiltration from the attic.

I just installed an Attic vent today since the roofer did not install a vent when they changed the roof prior to me purchasing and hoping to keep the attic away from oven temps.
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Old 07-06-2010, 08:49 PM   #15
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Not sure if your situation would be caused by return air pulling attic air down a wall. Just depends on how its put together. But yes, if your pulling down attic air into a wall you should be able to feel it's warmer than other walls and I assume it would be directly adjacent the return air grills.

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