An HVAC system generally lasts 10 to 15 years but can serve you longer if you care for it properly, if the conditions under which it operates are favorable, and if it's used in the manner for which it was intended. Of course, even the best systems have a limited life span, and you've probably heard that the age of a system is one of the major factors in determining whether it should be replaced or repaired. In general, though, if an HVAC system is of good quality, if the maintenance plan has been followed with the sort of fidelity one hopes for, and if no unusual conditions have plagued the system, HVAC longevity tends to skew in favor of a high number of years.