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brand new a/c not able to cool to 72degrees when 90degrees outsid

Posted by diy-er3586 (My Page) on
Fri, Jul 11, 14 at 19:23

We just replaced a freon 3-ton 10-seer A/c and coil with a R410A 3-ton 13-seer a/c and coil. The brand new a/c is unable to cool the house down to 72 degrees when the outside temp is above 90 degrees (most summer days here in the south). Best it can do is 76 degrees and it only cuts off for a few hours at night. Filter is brand new. Outside unit is clean. The company that installed it said it was because we had a few vents closed off. We have had all of them wide open for 4 days now and no change to the problem. One bedroom reaches 80 degrees on these days. We did not change anything at all except the a/c and coil. The vents that were shut off had been shut off for 10 years with no problem. The old a/c could easily reach 72 degrees. What is going on here?


Follow-Up Postings:

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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

Are they refusing to come out and check out the system? Wow, that is scary. I'm in the market for a new system soon and hope I choose a contractor that is better than that. Is the air coming out of the vents cold, not just "cool"? Any way you can measure the air from the vents?


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

measure temps at evap coil before and after. see whats going on.


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

Did they check the duct work under the house?
BIG culprit.
Is your attic insulated real good?


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

The company that installed it did come back out, but they seemed to think that opening the vents would fix the problem. They seemed satisfied with the temps coming out of the registers. They said we might need to take one vent out of a room that gets real cool and put an extra vent in another area. I just don't understand why the ductwork and vents were never a problem before this new a/c. The guy suggested that the R410A units are less forgiving about bad layouts or ductwork. Anybody have any idea why it would be any different? We will probably have the insulation checked, but it wasn't a problem before the new a/c. Thank you all very much for responding. I appreciate the help in solving this problem.


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

You didn't mention that you have some rooms that are getting real cool and others that are warm. That suggests that maybe the CFM (volume of air) of the new system is not the same as the old and is not getting evenly distributed within the branches of the ductwork.

As for the R410A being not as forgiving, I would think that 3 tons of A/C is three tons of A/C no matter what the refrigerant.


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

brand/model of old and new system? Was a ManualJ calculation done to confirm 3 tons?

Models that say they are a nominal 3 tons may actually be something different depending on the coil/blower combination.


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

Here are the model #s:
A/C Before- Goodman CK36-1A, A/C After- Carrier 24ABB336A340. Although, for some reason when I put my serial number in Carrier's warranty website, it said the model number was 24ABB336A003 (don't know if that's a problem).

Coil Model Before- unknown, Coil After - Carrier CNPVP3617ALA.

I don't know if they did a calculation to determine the size or if they just went with 3 tons because that's what we were replacing, and it had worked fine.

Is there an inexpensive tool to measure the air flow out of each register or do just HVAC people have those?

We know we had dampers in at least one duct back when we first moved in. Do those ever get messed up in the process of replacing the coil?


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RE: brand new a/c not able to cool to 72degrees when 90degrees ou

Seems to me that your contractor should be verifying that the airflow at the coil is what it should be for a three ton system. Maybe one of the HVAC experts can chime in here, but the coil itself is an obstruction in the airflow and with the high efficiency coils with all the folds and the same blower and motor in your furnace, the airflow may not be what it was with the simpler design old R-22 coil.

Edit: One of the things I learned from the advice on this forum is that it is always risky to replace just a portion of a system.

This post was edited by Dovetonsils on Sun, Jul 13, 14 at 18:41


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