sounds like from the title, you are considering replacement of a single system with zoning, or going with zoned systems.
I prefer the two system approach. that way you do have an element of redundancy in that if one of the systems would fail, you would still have a conditioned area available.
Have a proper load calculated, and a competent contractor should do that. Then, after surveying the home, he will make recommendations as to what will be both energy efficiency and cost effective.
Your use of this 2600 square foot space and how it is configured will also come into play into the proper recommendations about zoning vs. 2 systems.
If you have good air distribution system now, then it may make more sense to consider a zoned application. If you have problems with air distribution through the house, then it may be better to consider two systems, especially if the air distribution (ductwork) has to be greatly modified.
You may want to let people here know where you are, and there may be one of us in your vicinity. Othewise, the guy that actually looks at your home, and does the proper survey, is the person that needs to be making the recommendations.
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