Estimating cost of future HVAC replacement

I am doing some retirement planning. I want to estimate the cost and lifetime of my HVAC system.
I have central gas heat and ac. The is 2 story and about 2300 feet. The house was built in 1991.
Also, I would like to do some planning for what to expect. How long do they last? I see from some other posts that they may want to replace all the duct work. What should I look at to prepare to replace these systems. Can I just replace the AC unit or gas furnace, only the part that fails? Should I do that?
The upstairs unit is rarely used, only when we have company.
The downstairs outside AC unit is a TRANE XE 800 TTJ730A100B0. Upstairs outside is TRANE XE 800 TTJ724A100B0. My web searchs have not even turned up the tonnage of these.
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On Sun, 29 Jun 2008 14:06:28 -0700 (PDT), " snipped-for-privacy@yahoo.com"

I think the guys in alt.HVAC (?) can help. Try that group.
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Oren wrote:

I hope you were being sarcastic. LOL.
To the OP: Don't waste your time with the psychotic burn-outs on alt.hvac. For most HVAC-related questions, the Residential HVAC Forum on the HVAC-Talk website is an excellent source of info. Here's the link:
http://hvac-talk.com/vbb/forumdisplay.php?forumid=1
At least you'll get a response from helpful HVAC professionals but, even there, don't expect a lot of info on pricing.
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It's against the posting rules at hvac-talk to even ask about pricing!
Did find something here:
http://www.oldhouseweb.com/stories/Detailed/268.shtml
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snipped-for-privacy@yahoo.com wrote:

Instead of budgeting for AC replacement, budget for continued maintenance for the entire building. Someday it'll need a new roof, or water heater, or termite remediation, or something. Some amount put away each month - and I have no clue as to the number - is a better planning tool. Retirement advisors should be able to point you toward some resources.
Good luck.
P.S. Ductwork is usually forever.
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snipped-for-privacy@yahoo.com wrote:

You need to get a qualified HVAC technician out to your home and have him do a detailed report on both of your systems. It should take about 3 hours to complete a full report to show you which areas of your systems you may have to replace or repair. In a general way of thinking if you get 10 years plus on your A/C & Heating you are doing good. If I am reading the model numbers correctly the upstairs is 2-ton and downstairs is 2.5-tons.
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Moe Jones
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Figure about twenty years, for a good ball park. I'd be looking for a package deal with the furnace and AC. Much easier to do as a set. How about call some of your local HVAC companies, and see if they do ball park phone estimates?
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Christopher A. Young
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Something else to consider: As I understand it, the manufacture of R-22 freon will cease in 2010. Existing stocks may be used until 2025, but as the stock goes down, expect prices to skyrocket. The new systems are using R-410A and are a few hundred bucks higher. If any part of your system fails, it only makes sense to replace the whole system with one using the standard freon of the future. We went thru that with the car a/c's and now it's the residential's turn.
KC
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