A/c in new home

I moved into a new home in Arizona in Nov 2015. The a/c worked fine until the outside temperature got over 100 degrees. When it does, my thermostat will stop at 78 degrees and can't be changed for up to 6 hrs. This summer we had days of 112+. On those days thermostat would stay at 80 degrees. We like the temp at 76-77 degrees, but can't do that when it is hot! Isn't that the point of an a/c especially in Arizona? Our a/c company has worked on the system for 3 months and still same problem, but weather is getting cooler and soon it will be hard to tell if it is fixed or not. Our house is 3360 sq ft and has 2 a/c units. The unit on the 1760 sq ft side is a 3-1/2 ton a/c with 2 thermostats. That side of the house (East side) stays as cool as we want it, even on very hot days. The 1600 sq ft side of the house has a 2-1/2 ton a/c and it is the one with the problem. The a/c company has replaced the condenser outside and the unit in the attic, but yesterday it was 106 and still the same problem. I have asked if that unit should be a 3 ton and they say no. What is the problem?
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On 9/1/16 12:44 PM, Harvey wrote:

I bet you could fry bacon and eggs on the outer body of that unit in the attic...
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They say no, so they must have the calculations for the sizing. Ask about them.
Step one is to determine what size you need. This takes a few hours as you must measure window area and type, door area and type, roof and wall area and type of construction and insulation. You have to determine the temperature drop you want to achieve.
Most houses use a 20 degree temperature differential so at 100 outside, your 80 inside is perfect. Given the construction cannot be easily changed the variable is the capacity of the AC. There are some givens. To achieve a certain temperature drop you need a unit of X capacity. You either have it or you don't. If you don't, no manner of manipulation will change the laws of thermodynamics. Only added capacity will get it cooler.
A couple of caveats. If you oversize the unit to take care of the 116 degree days it may not work so well on an 85 degree day as it will not dehumidify enough. Cool and clammy in more humid ares, probably not so much for you. Having multiple units helps but you need proper air circulation if only one is running. The other is energy costs. It starts going up exponentially but that is a decision we all make, comfort versus cost.
Get the design first and see what you should be expecting versus what you are getting. Only then can we move on.
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On Thursday, September 1, 2016 at 8:34:14 PM UTC-4, Ed Pawlowski wrote:

Might be perfect in CT, but not so perfect in AZ where temps can easily be 110F+
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On Thursday, September 1, 2016 at 12:44:08 PM UTC-4, Harvey wrote:

Who knows? Impossible to diagnose something like that remotely. I assume when you say the thermostat stops at 78 and can't be changed for 6 hours, you really mean the temperature won't go any lower than 78, even though the system is running constantly? It is running constantly, right? What is the temp difference between the air going into the system and coming out? Having the air handler part in the attic in AZ sure isn't helping any. The AC company says that capacity isn't the problem, but what do they say is the problem? The other important part of the equation is having the proper ducting, sufficient supply registers, sufficient returns, returns in the right places, etc. A lot there to work with.
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On Thu, 01 Sep 2016 16:44:01 +0000, Harvey

The long term, cheapest option is insulation. Box in that air handler to isolate it from the attic heat or hive up a closet (they pretty much made attic air handlers illegal in Florida), Then pile blow in insulation or batts over the duct work in the attic. That will buy you some long term savings. Then look at the house itself. Can you shed some of the heat load with awnings over the south windows and what is the R rating of the windows? Is there any insulation in the walls? My guess is *if* they actually computed the Manual J, they put in the wrong numbers for the calculation. A lot of time, contractors just look and say that is "X" tons without actually doing the math.
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Or, it was sized properly for maximum 100F days, and it was assumed A) there aren't that many days above that and B) everybody knows a system can't keep up above its design temperature.
It holds 80F? Only 2 degrees above setpoint? There's an argument nothing is actually wrong.
Of course, that doesn't explain spending all that money replacing stuff.
In a different climate you would rather have it a bit undersized than oversized, because you control humidity better. But in Arizona, do you even have humidity?
That 2.5 ton would be enough in Virginia to handle heat AND humidity, I would think in Arizona with sensible load only, no latent, it would do fine. Get it out of the attic though.
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wrote:

He is in Arizona. A design point should be at least 110 and maybe 115. Humidity is not an issue at all. You could not pump enough cooling in there to get to 50% so "clammy" is not an issue.
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Hot air blowing on the thermostat would make the AC run longer and the room cooler.
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Hmmm Cool air leaking "up" and "hot air leaking down". One of us was not paying attention in science class. You also said "Check behind the T-stats for intrusions of hot air in the wall cavity", hence my comment. I don't care any more ... carry on.
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snipped-for-privacy@aol.com posted for all of us...

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snipped-for-privacy@aol.com posted for all of us...

+1
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