Anybody that comes up with a size without a load calc is guessing and should be guessed out of your house. The load calc is the only accurate way. If you use foam, that should cut down on your needs. Be sure if you do his calc reflects that. Oversized isn't good!

pool

what size is existing system? how did it perform?

how many sq ft are you adding? you are upgrading insulation both top, bottom, walls? New windows?

I agree with Baldloonie. Guessing is not acceptable. A reputable and professional dealer will perform a load calculation for both cooling and heating. Request this in writing on the Man J software worksheet. This is good insurance for you and the dealer for correct sizing.

Your ductwork should be evaluated as well. Obviously, it will require modifications/updates as well. Good ductwork is often an overlooked area.

This is your starting point. Equipment selection can come later.

Certainly, I would want new HVAC to qualify for the Fed Govt tax credit.

IMO
Good LucK!

You'll get a more accurate load calc & latent load figure if a Blower Door infiltration rate is performed.

Some day, that ought to become standard with a load calc.

If you get the ductwork supply & Return including oversized filter Rack(s)for more efficient airflow, you can downsize & usually get the job done.

If if doesn't handle the load, you can always do more things to reduce the heatload.

For example, in Rockford, IL a 2,400 sq./ft home with 600 sq./ft of window area; it figures, at 600-ft per ton, to take 4 tons or so to cool it.

However, a 2-Ton Unit moving 1,000-CFM of air (or 500-cfm per ton of cooling), even at 95 degrees with a blazing sun heat outside and very high humidity the 2-ton cooling equipment system still cycles! It is very comfortable at around 75-F and 50% Relative Humidity. - udarrell

Hi Guys,

Thanks so much for your help! I have a science degree and this is all very complicated! Tigerdunes, we are replacing the duct work, windows, electrical and plumbing. We have very little insulation in the house now, but we are looking at foam or a combination of foam and something else. Presently, our windows are wood with storm windows over them and the heat and air (3 or 31/2 ton, can't tell) work well. Only a few minor issues over the years. Baldloonie, I am with you, I am much more impressed with the neighborhood guy and his pre-work. He is very picky about things, especially his ductwork. He is looking at using a 4 ton. Udarrell, I will ask him about blower door infiltration rate. Again, thanks so much, I can't tell you how much I appreciate your thoughts and input.

its doubtful that they will provide a blower door test,
most companies do not, and while some do duct testing similar concept as blower door testing in measuring and identifying leakage sites...it is like udarrel says
one day....
where will you be using foam?
if you foam insulate the roofline, the benfit is that
duct work is in conditioned space, so leakage is less
of an issue. unvented attics are have more hurricane resistance to uplift and serve us well here in hurricane ally.
if you can't afford to do foam roofline install (unvented attic) you may want to check into radiant barriers.
RB's reflect heat out of home, making it easier to
cool in the hot summers.
reducing your home's air infiltration will also reduce
amount of tons of ac required, better windows (vinyl lowe)
will reduce it further.
best of luck.