House is 1850 sq feet total, built 1993. AC is Trane XE1000 (3 ton?), separate gas furnace is Trane XE78. Access to ducting is easy due to basement with drop ceiling and walk up attic.

Also a finished (2 years after house built) walk out basement, 500 sq ft. In summer I turn basement dampers in basement dusts to vents to a whisper for dehumidifying since basement is naturally cool, so not much of AC load. In winter open dampers more for heating and also use a electric wall heater since thermostat is on main floor and basement can be cooler. This works well for how we use the basement.

I had my AC unit checked under the service contract with a company down here. They told me the freon was low by 2.5 lbs. Last year they told me it was low 1 lbs and added freon. This time they recommended I get the unit replaced. It is a 12 year old Trane. I told him to skip adding coolant while I look into replacing the unit as I have another month until things get hot down here in NC.

We have single zone AC & heat. I adjust a splitter damper at the blower that I had installed about 9 years ago. This damper controls the flow between 2 suppplies, one for the main floor and the other for the upstairs which goes all the way up into the attic (from the basement). Prior to this damper being put in, getting enough cold air all the way to the top of the house was a no go without freezing the main floor. Now the cold air distribution is a lot better, but still somewhat uneven. In the winter I readjust the splitter to try and achieve the right balance. We get some extra heating of the upstairs when the main floor cools off during the night.

I am considering either

1) Getting just a new AC unit and all else the same

2) Getting a new AC unit and having zoning installed

I am getting some suggestions from contractors to also replace the furnace as part of the deal with the idea that its better to do all at once and have everything matched up.

Regarding the zoning, one contractor is telling me to avoid variable speed units with zoning as it creates problems.

What issues are involved with zoning. I have heard people weigh in on both sides. Does it really work well or is it just a sales gimmick. Seems like closed bedroom doors at night would negate some of the dual zoning benefit.