A/C Size Question

(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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tom in PA wrote:

Which one did the manual calculations? If neither, keep going until you find one that does. You don't want a hack who does not do the proper calculations (or worse does not know what they are). If they had both done the calculations they should have come up with the same answer.
--
Joseph Meehan

Dia duit
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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/resources/mechanical/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:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
To know what is needed a written load calculation must be done, or you can under or oversize. Up north here humidity is the main issue and to large a unit leaves you uncomfortable even at 70f, humidity is our main concern. A smaller unit is cheaper and you say humidity is not an issue. Get a written load calculation, with a larger unit since you say its never to humid you get a quicker cool down comming home to a hot home. Up north the answer is simple, you don`t want to oversize for humidity issues.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
m Ransley wrote:

I agree with the advice to get a proper load calc done. From the description though, it sounds like a new well insulated 2000 SF house in an area that can almost get buy with just open windows shouldn't need 5 tons.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Don't agree at all. My house is about the same size and I do use window units. Total is far less than the 60k you propose. More like 38k. I think the 60k will make for a muggy house in most climates. Get a third calculation.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Tom in PA wrote:

FWIW, I have a somewhat larger, less well insulated house in a much hotter location (Austin, TX.), and 4 tons has been more than enough for us for the past 25 years.
--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form snipped-for-privacy@prodigy.net.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.