I designed my house and was active in reviewing the contractors recommendations based on the Manual J analysis. When all is said and done the old adage of 12,000 btu (a ton) for every 500 sq ft of floor space is pretty accurate, except I would modify that for any known major variable that you know would cause a problem. In my case, I had shade trees all around so there's less direct sunlight load, but I have 9' ceilings which is more air to condition. My split system has a 13 SEER that was sort of cutting edge for 1996, a single speed scroll compressor. The blower is a variable speed one, which is key to my system because I have a humidistat installed (inexpensive). During the heat of the S. Texas day it will run at higher speed for maximum cooling due to the heat load, but since it runs for like 12 minutes, humidity is totally wrung out. In the pre-dawn hours when the humidity is like 85%, but temp is 72, it doesn't run much cause there's no heat load. When it does run, the humidistat tells it its getting humid in here so it runs at slow speed so it wrings out the air, slowly getting to the selected temp setting. I have the humidistat set for like 63%, so if it achieves 64% or higher it automatically shifts to high speed. I like it alot. Last week it was 97 outdoors, the AC ran like 12 minutes then was off 18 minutes. Before you select your SEER, you should understand that number directly relates to your energy bill. Ex: Since a 16 is 33% more efficent than a 12 (diff of 4 divided by 12), an electric bill that would be $100 with a 12, will now be $67 with a 16 (of course a 16 costs more to buy, but rebates/tax credits help). Good luck.

Quote 1 (local guy): -Luxaire TCJF 24 15 SEER $2985 (2 ton) adding a 96%, 2 stage, multispeed Amana furnace would bring the total to $5285 -talked about improving my ductwork to make it more efficient Quote 2 (Home Depot): -Rheem, didn't leave model number, size, etc. $5995 plus a 2 stage furnace adds an additional $5995 -no clue about ductwork Quote 2 guy went on and on. Where did you meet? We have financing, blah blah. He kept trying to play us against each other.

Here's some good research on compressor shading...not as important as common sense would have us think and nowhere near as effective as the 10% figure I see thrown around all the time (a number someone just made up???). It seems it's much easier to obstruct airflow than we think simply because of the enormous volume of air we are actually dealing with. http://www.fsec.ucf.edu/en/publications/html/FSEC-PF-302-96/index.htm

Keep in mind that air conditioning moves heat from inside to outside, at a specific rate as per the BTU capacity of the system. It's constantly fighting against the heat load that's generated inside the house by cooking, lights, use of appliances (hair dryer, etc.) and electronics (computers generate heat) ... and heat that radiates in through walls and roofs heated by sunshine, windows, doors, etc. Other variables are involved such as humidity levels (squeezing moisture out of the air requires removing heat but the effective temp of the air doesn't drop ... latent heat). Units are sized to exceed the heat load under "normal" conditions so the temp inside the house can be reduced. There's a "balance-point" at which the indoor temp holds steady, the heat load exactly matches what the system can pull out. If the heat load exceeds the unit's capacity, as may happen during extremes of weather, the indoor temp will increase, although it will still of course be cooler inside than out. Setting the thermostat cooler will not have an effect in that situation ... it won't make the system run "harder" to remove more heat*, the system is either ON and removing all the heat it can, or it's OFF and removing none. A way to determine if the unit is functioning within normal limits is by checking the intake air temp (at the filter grill) and the output air (at the closest register). There should be a reduction in temp of about 16F to 20F ... which is the heat removed as air flows through. If the differential is less than 15F (more or less), then there may be a problem. Checked mine (5-ton, 60,000 BTU system) a few mins ago, 78F in, 57F out. *Except maybe in the case of a two-speed system if it's running on low speed and setting the thermostat cooler triggers it to high speed ... but in such a situation the system would likely already be running on high speed.