If load calc was performed correctly with good numerical input, then a 3 ton would be undersized. Never have been a fan though of the XL16i condenser.

The obvious question is how did the 2 1/2 ton condenser perform on old sq footage in the summer mths. It is difficult for me to believe that 350 additional sq ft would cause a jump of a full ton, 2 1/2 to 3 1/2 ton which the new load calc would suggest. I would have another independent
calculation performed or even yet you could do it yourself for a nominal fee. I can provide a link if you are interested.

I think you would be better served with Trane's sgl stg XL 15i or XR 15 in the correct size.

This is a HP or straight AC system? I find the loads highly questionable for NOLA even the heating.

IMO

Agree generally with Tiger, but just want to remark that if the new space has lots of east/west glass, skylights, and high ceilings that may make a difference in the load profile.

Suggest having a different contractor run a load and give recommendations to you, and then post back here.

Thanks for the input.
Tigerdunes: The old unit functioned on a scale from 1 to 5 was a 3. It seemed to struggle but managed to keep the home relatively cool; please note that the system is about 15 years old and I think it's been on its last leg for a while now. Any particular reason you're not a fan of the xl16i? The XL20li is only about 9% more, would you recommend that one? As far of XL15I or XR15 anything particular about these models? Is the Manual J a complicated calculation? I found one online a while back that was free and I kind got lost in the calculation. If you don't mind sharing the link you have, I'll check it out. I have had three other HVAC companies come to my home, and have stated that 3 tons would be sufficient; however they did not provide any calculations/data. I was under the impression I could get two stage unit could be slightly over sized and work at half speed most of the time.

Juliekcmo: My home is facing West and so will be the new addition ( converting garage to a den area). I will check to see who I can get for the calculation.

For your area/climate, I would much prefer the single stage XL 15i or XR15 with a var spd blower motor that is set up for dehumidify on demand for summer AC cooling over the two stg XL16i which is known for problematic dehumidification.

You never did clarify. Is this a HP or straight AC condenser.

Here is a link for load calc software that I would recommend.

Just for info, the XL 20i is a two compressor two stg condenser that operates on a 50/100 split between high and low stage. The XL16i is a sgl stage step compressor with a 67/100 split between high and low stage.

IMO

Here is a link that might be useful: Hvac Calc load software

Thanks again.
I will ask about the xl15i and xr15. I was hooked on that the xl20i being a true two stage was a better buy. By HP or straight AC, do you mean Heat Pump or Staright AC? If so, it's just an straight AC with a gas furnace. Positioned in the attic; if it it makes any difference.
Do you think the xl20i is worth the extra investment? Or just stick to the 15 models, as you recommended. Thanks again, I greatly appreciate it.

I would want to see an independent load calc performed by yourself or a third party for comparison purposes before making a condenser mdl recommendation.

I personally would not have the XL16i myself. I assume you will go with a two stage 80% eff var speed furnace.

IMO

upgrading furnace to 90+% is a good investment.
my clients (north of N.O. near Lafayette) understand
the savings and do the upgrade.
80% will give a blast of hot air..then house
will be cool until next blast of hot air makes
house hot.
if the system adjusts to your needs then comfort
will be improved.

shopping with American Standard hvac company
will work for you.
15-17 SEER has a good performance & payback.

10 year old a/c was about 10 SEER..just an educated guess
as there are no model numbers posted. new unit will
perform much better.

4 tons on 1700 sq ft house is just crazy.
contractor is trying to sell higher tons
to minimize call backs. 425 sq ft per ton
is just outrageous. since 2.5 ton 10 year old
system did ...ok. newer more efficient system
will perform much better.

having contractor mastic seal ductwork, plenums
to furnace & supply boxes will put the air you
pay to heat & cool inside the area you live in.
30% duct leakage isn't uncommon. Entergy
charges premiun rates.

blower door testing the house & sealing air
leakage sites is another good investment.
it is expensive to try to heat & cool a leaky
structure.

where is furnace & ductwork located?
where is return air located?
getting 3 bids would be the minimum.
with nothing to compare this bid to...
how can you make an informed decision?

best of luck