There is a fairly straightforward formula frequently used in the HVAC business to determine the best way to spend your money when considering repairs. Simply multiply the age of the item by the expense of repairs. If the amount is more than $5,000, you should replace it. If you frequently have to repair the same system, you should do the same action.
On the other hand, if your system is less than five years old, requires only the occasional repair, and functions well overall, it's probable that you won't need to replace it for some time.