Your AC doesn't have to break down in order to need a full replacement. There other issues that impact your quality of life to consider. If any of the following is a problem, then it is likely time to install a new air conditioner.
1. Hot Rooms
It's frustrating when the temperature in one room is just right while another room is still hot and stuffy. If you have already verified that all the vents are open and the air is flowing, the problem is likely with your old air conditioner unit. An undersized unit, for example, may be unable to fully cool the rooms furthest away from the main unit — it simply doesn't have the power to push sufficient cool air through the vents. A larger unit may be desirable.
2. Warm Air
Even more frustrating than uneven cooling is if warm air is blowing out of the unit. This is often the result of a refrigerant leak. The refrigerant should run on a closed system, which means it never runs out nor needs refilling. If a leak occurs, then you lose the refrigerant and you end up with warm air. Although it is possible to fix leaks and recharge units, if you have an older AC then the refrigerant used may no longer be available. The best solution is to install a new AC unit.
3. Weak Air Flow
Weak airflow can also be a sign that the unit is undersized, especially if it mainly affects rooms furthest from the air conditioner. Weak airflow can also be a result of aging. As the motor and blower begin to fail, airflow becomes weaker and weaker. Depending on the age of the unit, it can be much more cost effective to replace it rather than attempt repeated repairs. Newer units are often equipped with more powerful fans as well.
4. Loud Noises
AC discomfort isn't always temperature-based. If your unit has become quite loud, then you need to get to the root of the problem. Banging, rattling, and squealing may be issues that can be easily repaired by tightening a bolt or lubricating a moving part, but they could also be a result of an aging unit on its last legs. If the noise issues can't be fixed, install a new, quieter unit so that you can enjoy your home again.
Contact an AC installation service if your old unit is making your home uncomfortable.