Direct Answer: If your system is over 10–12 years old and the repair cost is more than 50% of the value of a new unit, it is usually time to replace it. We follow the "Rule of $5,000": Multiply the age of the unit by the repair cost. If the total is more than $5,000, a new, energy-efficient system will likely save you more money in the long run through lower utility bills and fewer "emergency" visits.