When you begin to see your unit struggling to make your house as cool as possible, several things become evident. You will notice three primary issues that become apparent that it's time to possibly replace your air conditioning unit.  Here are just some of the signs that indicate it's time to replace your air conditioner:

You are spending a fortune on repairs.

If you are calling the AC repair man multiple times a month and bills keep piling up, you should definitely consider buying a new air conditioning unit.  You may think that a couple repair costs are no big deal; however, they do begin piling up and if you wind up buying a new unit anyway you're wasting your money on repairs. 

Energy efficiency seems to be dropping.

Energy efficiency is extremely important for many household appliances, not just your air conditioning unit.  When you believe that the house should be a lot colder and it's not producing enough cold air, you will realize the bills are higher and you will finally get that your unit is no longer being efficient for what the setting is on the thermostat.  This is also a huge clue that you should probably invest in a new AC unit.

Your system is more than 20 years old.

Even if your thermostat seems to be working okay and the AC unit itself is working great, it is only a matter of time before an older unit has a problem. At this point, you should definitely consider talking to local air conditioning experts and start pricing the units so you can get the best buy that will be most efficient for your household size. 

You want to make sure that you get quotes based on the size of your home as well as understanding the efficiency.  In a 30-year-old house, you may want to go with the ultimate efficiency, but it depends upon what kind of ductwork system you have.  Sometimes, people are ready to upgrade and then realize their upgrade is going to cost a lot more than they anticipated because of the age of their home, office building or business.

Replacing air conditioning units, with the help of a residential HVAC repair specialist, can be expensive, so making sure maintenance is done on them is something you want to do as soon as possible.  Look into maintenance and upkeep; otherwise, you may be spending several thousand dollars if something unfortunate like this happens at the brink of summer.