This commonly asked question does not come with one hard and true answer for all homes. There are a variety of factors which go into this decision and make it a unique choice point for all home owners.
While not necessary to match your equipment by replacing both the furnace and the AC at the same time, the largest factor for consideration is energy efficiency. Equipment that is updated and designed to communicate and work well together will provide your home with the most cost-effective results.
Energy efficiency is a concern not only for environmental impact, but also due to it’s ability to decrease monthly bills. In addition to less costly monthly expenses, homeowners can save money upfront on installation. Because the process is less complicated when installing an entirely new system versus one single half, contractors are able to reduce labor costs, thus saving you money.
Finally, warranty opportunities are a consideration. When only part of the system is replaced at a time, the manufacturer is no longer obligated to honor the warranty which comes with a new system. Although not always the case, this is worth discussing with your HVAC provider to create a plan which meets your unique home needs.