How long should a new system last if it is installed correctly?

  The ASHRAE Systems Handbook (the air conditioning ‘bible’) says 15 years is average. While proper installation of any system may be the most important consideration there are several other factors:
a. Poor installation techniques. This should to be the #1 priority.
b. Mis-matched equipment won’t last as long as properly matched systems will. See Q10 above.
c. Poor brand. Some brands are simply built to last longer
d. Oversized units don’t last as long as properly sized units.
e. Improper maintenance
Combine 2 or more of the above factors and service lives can be as low as 2 years. When the best brands are installed correctly you can expect them to perform reliably for 20-30 years. Quite a difference.