I'll start by saying that I know the correct answer is "consult a
professional to do a load calculation", but I've consulted two
professionals who are coming up with much different answers so I'm
The house is 2076 square feet, new construction, two-story with most of
the square footage on the ground floor. R-19 insulation in exterior
walls and ground floor joints, R-30 in ceiling. One big air return,
about 12 registers. 2x4 (I'm pretty sure, a slight chance it's 2x6)
frame construction. Furnace in crawl space under house. Have some
large windows in great room and a glass slider in dining room but these
all face north, otherwise normal sized windows. Located in Flagstaff,
AZ, elevation 6900'. Average high temperature in July (the hottest
month): 82 degrees. All-time record high: 97. Humidity: tends to
be quite low.... this is the Southwest. This is a town where most
people don't have A/C, and the unit won't be heavily used. You can
usually get by with your windows open and ceiling fans. It just gets a
little toasty now and then, which is why we want the air.
Once contractor says a 3 ton unit is the way to go, another says 5 ton.
Any opinions regarding who is right?
Tom in PA
Well Tom in PA, I think that neither one of those contractors is
correct unless they went around your house taking measurements and
puting them down on a sheet of paper, or PDA, or computer. When you
find a contractor that does this, you'll be in much better shape than
the others who are just using the rule of thumb theory.
That would be to stand about 50' ft. from the house and hold up your
thumb and see how much it covers the house, if it takes 3 thumbs then
you need 3 tons. This is the kind of load calculation most of the a/c
companies do most often, or just change out the system like for like.
It's the homeowners responsibility to research the contractor as much
as it is the contractors responsibility to do the job correctly. So
call all of the contractors up and start asking questions.
Here in SW WI we have 2-ton systems on 2000-square footage homes' and
they have proper airflow and cool them perfectly!
Occasionally, we have hot 95-F muggy weather with a heavy latent load,
the 2-ton still gets by, and is optimally sized for 95% of its runtime.
The Summer Design for Flagstaff, AZ (airport), 82-F dry bulb, 55-F wet
bulb or around 13% Relative Humidity.
With that low a humidity, I am betting a 2-Ton with plenty of airflow
would handle any hot weather you have.
Someone needs to do an accurate load calc and sizing job! Never more
Air Conditioning\'s Affordable Path to the "Human Comfort Zone Goal"
Somewhere between 3 and 5 tons... LOL. Actually, you are probably closer to
5 with that square footage... figure one CFM per square foot at 450 CFM per
ton. Puts you at 4.6 tons. However, there is not a 4.5 ton unit
manufactured. Also, you are in a low humidity region, so that will lower
your calcs. Yet, there are other factors to consider. If you want to be
exact, then have a Manual J calculation performed on the structure and you
will be sure to have a properly sized system.
If what you say is true about a/c not being needed very often, then you will
probably end up being satisfied with a 3.5 or 4 ton unit with a 5 ton blower
inside. IMHO you can never have too much airflow thru the system,
particularly since you have a relatively low latent load.
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