I have a 2800sq ft house with a 4-ton 18-yr old York system with the air-handler and ductwork in the -attic-. It's getting near the end of its useful life and I'm interested in replacing it. What I hope you can help me with is what I should be asking the contractors who are bidding on the job - in terms of system features (obviously you can't size it here). The two contractors I've selected have top-notch local reputations (one did the last installation and the system has run flawlessly)

My priorities are:

a) reliability - I'm happy to give up a bit of efficiency for fewer gadgets. I just don't want it to ever break.

b) I understand (please help me here) that the new systems are variable speed? I think it would be great to have a system that worked like a 5-ton for the 95Deg 95% humidity days, but worked like a 3-ton for the 70s-deg humid days (where my system often cools way before it's dehumidified) - what should I be asking for?

c) I have a spacegard filter which I hope to transfer to the new system, and as a result run the fan 24/7 (it -does- make a difference) - any features I should ask for to support this sort of operation?

As far as load calculation - do I need a detailed one, or can I work from the point of view that the existing 4-ton system did a reasonable job for most of its life.

Any other features I should make sure are part of the package?

Thanks very much