short answer: i'd buy 5 tons and as many zones and controls as the
budget will allow.
if i had to pick a number. 5 tons x 12000 = 60,000 btu.
for a comparison, could six 10,000 btu window air conditioners do the
whole job on the hottest day? [probably a lot better than six small
6,000 btu window air conditioners, right?] just a quick example, i
know the efficiency factors vary.
long answer: with central air those unmentioned ceiling heights and big
rooms need excellent ac airflow in and out. you are asking a question
that formerly had a simple answer like ones you suggest.the better
modern answer to this incorporates exhausting desert heat, fresh air
intake into central ventilation, exchanging, heating, and cooling with
as many sophisticated controls and systems as you want to buy for as
many zones as you can afford.
in your zipcode heat pumps and swamp coolers may be popular, for
example, while here in buffalo ny neither one is helpful. we have some
really nice lake erie breezes, so get out of the summer in the
southwest and come and see niagara falls!
part of the puzzle may also be how fast you want to cool down the
house on a hot day. you mention the furnace in what may be
non-conditioned space, determine if this requires further design
attention.
many references with information as simple and as complicated as you
desire at:
http://www.buildingscience.com/resou...al/default.htm

and wiki says "Air conditioner equipment power in the U.S. is often
described in terms of "tons of refrigeration". A "ton of refrigeration"
is defined as the cooling power of one short ton (2000 pounds or 907
kilograms) of ice melting in a 24-hour period. This is equal to 12,000
BTU per hour, or 3517 watts
(http://physics.nist.gov/Pubs/SP811/appenB9.html). Residential "central
air" systems are usually from 1 to 5 tons (3 to 20 kW) in capacity."


Tom in PA wrote:
(also posted in alt.hvac... the more opinions the merrier!)

Hi!

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
feeling stuck.


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?


Thanks!


Tom in PA