If you plan to move within the next 5 years, then I would be inclined to do a repair. If it is going longer, then replacing makes sense since you will eventually have to a replacement.

What is your location? A 4 ton AC for a 1700 sq.foot upper floor seems over sized. I would think 3 tons would be fine.

We live in Central VA. So we get warm summers and cold winters...but neither the heat nor the cold lasts too long.

How do I factor in the fact that the second floor will be used far less in the next 10 years (the length of time I assume we'll live here) than has been the case in the last 10 years (with the kids all off to college shortly)? Should that impact how we size the system?

What about efficiency? My uneducated view is that environmental considerations aside (and these matter too), the limited demand on the system (demand will be high only when we have kids/guests at home) means it will take much longer to realize economic benefits from a more efficient system. Of course, the price differential isn't that great.

Do the prices I've been quoted seem reasonable (for the systems quoted)? What else should I ask for in the install (assuming I replace)?

Thanks again.

The size is determined by the cooling load and not by how often you use it. If you want the upper floor to be 75 degrees on a typical summer day then, the size is the same whether you use the AC once a year or everyday.

If the AC is not being used very often, then it does not make sense to spend extra money on a higher efficiency model. You are unlikely to ever get the extra money back.

I am not very familiar with all the Trane models, but my feeling is the prices are high. The coil replacement is within the range others have posted.

Your existing system is either a 10 or 12-SEER unit; they were good performing units, & a new one, especially if oversized may not produce any savings over the older unit.

A 3-Ton properly installed & setup unit should cool your 1700-sf home at your location. You can always do cost effective things to reduce the heat-gain. It is all in how well everything is design-engineered...

In fact a 2.5-Ton unit might cool it; that is only 600-sf per/ton of cooling. I cool my farm home with a mere half-ton window unit for the entire 1st floor; that's around 1200-sf per/ton of cooling in very hot & humid weather extremes.

Slightly less than a half-ton upstairs -.cooling 2 bedrooms a bath room & hallway; usually only run it for a while before going to bed unless a real hot day & night.

A 5th grader can perform that LINKED load-calc print the instructions & follow them!

Here is a link that might be useful: Free Whole house Load-Calc

CORRECTION: In fact a 2.5-Ton unit might cool it; that is only 680-sf per/ton of cooling; not 600.