What size is existing? What do you mean you don't require or need a split system? Do you live on or close to water?

IMO

my existing unit is 3 tons. but house not getting cool but could be due to system too old.

i don' t live near water.

a split system is where you put one in each room , which is not a central type.

You are entirely mistaken about what a split system is. A split system is a central system with an outside condenser and an inside air handler or furnace connected to a whole house ductwork system. You need a load calculation to confirm correct sizing for AC. You may be undersized by 1/2 ton. Only a load calc will tell you. For your location, I would want a 15-16 SEER AC condenser paired with best matching var speed air handler and a small KW heat strip for heating when rarely necessary.

IMO

As far as brand, we are American Standard and Carrier dealers, but the fact is that most of the residential systems are pretty much alike as far as components and quality. The real difference is how they are installed. A good installation will give you many years of reliable service. A bad installation will give you many years of headaches.

As far as capacity, the only correct way to tell is to have a contractor or engineer run a ACCA Manual J calculation to determine the heat gain for the home. I've made mistakes myself in the past by using "rule of thumb" or "rule of guess" estimates. Heat gain is determined by everything from building materials to building orientation, to the type of windows. Bite the bullet and have a professional run a heat load calculation. Also, have your contractor guarantee that your system will be at or below 1/2 inch total external static pressure, and have him/her prove it when the job is complete! This will insure correct air flow, which will in turn insure that your system will cool to capacity and the rated efficiency. Otherwise, you may be disappointed and just get a bad job.

If you would like more information, I've written an article on the importance of heat load calculations on my website. You can see it here if you're interested. Let me know if I can help with other information.

@ tigerdunes. I meant a duckless "mini-split" system like the fujitsu duckless minisplit, which I don't need.

Understand...confused HVAC jargon and terminology...mini split and split systems are entirely different...get load calc which one can do yourself if interested...go with 15-16 SEER AC system paired with matching var speed air handler...add thermostat that has independent dehumidify on demand feature...

IMO