Since you do live on the east coast, and probably do not have to rely on the central air unit all that much, there is a good chance that you will probably get a lot more out of your central air conditioning unit than people in other areas of the country. Still, 25 years is rather old for an air conditioning unit, so I am not going to try and play air conditioner psychic here and tell you that it won't go out. I live in the south central part of the United States, and we run our central air conditioning units at just about maximum capacity for most of the summer. Having a central air conditioning unit go out in the heat of summer is a common problem. In fact, if I were to call in the middle of July and ask for an air conditioning repairman to come out, there is a good chance that they might not make it out for a week or two, leaving me without AC for several days. Around here, air conditioner service technicians are in very high demand, and most of these guys can expect to work 12-18 hour shifts 7 days a week all summer long (but the pay is very good).

I have a 2100 square foot home, and I am using a 3 ton central air conditioning unit (that is in terms of capacity). For a home of your size, you would probably need at least a 2.5 ton air conditioning unit. If you lived in an area of the country where hotter temperatures were more common throughout the year, you might consider going up a size. However, the larger the size of the air conditioning unit, the larger the size of your electric bill, so look for a more energy efficient variety.

i would not settle for a used central air unit, since many of the more modern versions are created to be more energy efficient. i think there may also be some tax incentives in place for the purchase of a new air conditioning unit that meets certain specifications. I know that they are still advertising this where I live, but it was part of Obama's big tax plan for last year. It was my understanding that this expired in April of 2010, but it may have been extended. If it is still in place, you can get back a lot of money at the end of the year, which will save you on the cost of a new unit.

The last time that my central air conditioning unit went out was about 4 years ago, and it was a 1997 model. I paid $1300 for the 3 ton Lennox air conditioning unit, but the total cost to the repairman for installation, ductwork, and running new electrical lines was close to $4000. Due to variations in the standard of living, this cost may vary in different areas, but you can call service companies in your area for a quote. In my experience, they will not give you a quote over the phone, but they will come out and take a look at what you have first. The cost also varies among different brands of central air conditioning units. American Standard offers some more affordable varieties, but there are rebates offered at certain times of the year for higher quality models such as those made by Lennox. In my experience, brands like Lennox are more reliable, last longer, and are also more energy efficient. My grandparents' home has had the same Lenno air conditioning unit for over thirty years, and it is set on 70 degrees year round.