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?
They say no, so they must have the calculations for the sizing. Ask
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.
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.
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.
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.
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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