If your existing system is 10 - 15 years old, or older, it is likely nearing the end of its service life, and it is significant more expensive to operate than a new system would be. A system installed more than ten years ago probably has a rated SEER efficiency (Seasonal Energy Efficiency Ratio) of 10.0 or less. New Lennox air systems are available with SEERS as high as 26.0. So a new AC system reduce operating cost by 50% or more. If your existing AC system dates back to about 2004 or earlier, you system may operate with an obsolete refrigerant (R22). R22 is damaging to the environment, and can not be economically serviced in a residential application so you should strongly consider a system upgrade. A planned AC system replacement will probably save you money on the cost of the replacement system. Performing an emergency air conditioner replacement during the peak of summer heat may require overtime labor. Replacing your system in a scheduled fashion during regular business hours will ensure an efficient installation process.