How do I calculate if my central HVAC unit will efficiently heat and cool an addition to my home?

I recently bought a 1500 sq ft home with a Payne 3.5 ton HVAC unit, about 6 yrs old. I am adding a 600 sq ft addition. I need AC to handle 100+ temps in our southern summers. It is a one story 80-yr old farmhouse, with some vaulted but mostly 8-9 ft ceilings. How can I know if I really need a 2nd unit for the new part, or if the AC guy is just trying to make a sale?

Answers:
The acceptable industry standards are between 500 and 700 sq. ft. per ton.

I live in Central FL, and I usually go 1/2 more than the standard.

At 2100 sq. ft new total, you're getting far more than you need by going up to a total of 5 1/2 tons.

At the very least, if your budget allows, and considering the age, structure, and condition of the dwelling, my recommendation would be upgrade in one unit, to 4 tons.

Rev. Steven

Other answers:
3.5 ton should be fine. 1.5 ton per 1000 sq ft
3.5 ton should be fine. 1.5 ton per 1000 sq ft
Pick the brand name you want, go to thier site and they will have a question section or a calculation section to tell you how. They often even have a 800 number you can call and ask questions. some sites have most often asked questions and this one is asked often.
The best engineering guess withour paying for a manual J calculation would be 600sqft per ton of cooling.
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