It is cheaper than replacing them individually
If you will be replacing them separately, you will be paying full price for the furnace or the A/C unit, plus you will also be paying for additional labor charges instead of just one.
Matching technologies complement each other
Not to say that it is impossible to pair old HVAC units with each other, the problem is that they will not usually play well together.
You will have more options to choose from
When you are only replacing either your air conditioner or your furnace, you will need to choose models that are highly compatible with your current setup.
When NOT to replace the A/C and the furnace simultaneously
There is only one acceptable condition that makes it not cost-effective to change out your furnace, and that is when it is less than 15 years old. So if your furnace is not yet halfway through its 30-year life cycle, just replace the A/C unit on its own and just wait for it to reach fifteen years before setting yourself up with a new furnace. Besides, your furnace will still be working fine if it is just fifteen years old, it’s when it hits above twenty when it will start having problems. There is one disadvantage of replacing both your air conditioner and furnace, and that is obviously the high costs.
If your air conditioner is on its last legs, and you feel as if your furnace will follow suit in the next year or so, it is best that you replace the both of them now.