What should you replace first when two systems are near the end of their life?

Replace the one whose failure would damage something else, even if the other is older.

  1. Age tells you when a system is likely to fail. Location tells you what that failure costs you.
  2. Anything above or beside finished space carries a higher cost of failure than its age suggests.
  3. Moving one replacement forward by a year is usually cheaper than absorbing two in the same quarter.
  4. Serial plate date codes are more reliable than a listing sheet or the recollection of a previous owner.

Reviewed August 2026