Ask a facilities director when a given split unit is due for replacement, and the answer usually comes from a depreciation schedule — a number set at the time of purchase, based on a standard useful-life assumption, that has nothing to do with how that specific unit has actually been maintained. It's an understandable default; the depreciation schedule is the number that's already in the system, tracked automatically, requiring no additional effort to reference. It's also, mechanically, close to meaningless — a ten-year accounting assumption applied uniformly to units that might have anywhere from six to twenty years of real service life left in them, depending entirely on how they were maintained in the years before anyone thought to ask.
The two clocks diverge for a specific reason: depreciation schedules are set once, at purchase, using a conservative industry-standard assumption that has to hold across every possible maintenance scenario, from excellent to neglected. Mechanical life isn't set once — it's the outcome of every service decision made over the unit's operating history, and it moves in response to those decisions in a way the accounting schedule never will. A unit that's had its refrigerant charge verified and corrected at every visit, that's had coil cleanings thorough enough to restore full heat exchange, and that's had electrical connections checked for the resistance increases that precede a failure, will often run years past its depreciation schedule with no meaningful increase in failure risk. A unit that's had none of those things will sometimes fail years before its depreciation schedule runs out.
This matters most at the exact moment when a facilities team is deciding what to do with an asset that's approaching or past full depreciation. Treating the depreciation date as the mechanical end-of-life date leads to one of two costly mistakes: replacing capable equipment on a schedule that has nothing to do with its actual condition, or — more commonly — assuming a unit still has years left simply because the schedule says it should, without ever checking whether the maintenance history supports that assumption.
The alternative isn't complicated, but it does require treating the two clocks as separate questions with separate evidence. The depreciation schedule answers a tax and accounting question: what's this asset worth on the books this year. A measured condition assessment — current refrigerant charge compared to commissioning baseline, amperage draw compared to baseline, a review of what's failed and why — answers the mechanical question: how much real service life is actually left. Only the second question should drive a replacement decision, and only the second question can be answered with any confidence, because it's based on this specific unit's actual history rather than a generic industry assumption applied at time of purchase.
For a portfolio of any size, this reframing has a direct budget consequence. Instead of a wave of “scheduled” replacements arriving in whatever year the original purchase invoices happen to cluster around, replacement becomes staggered — driven by which units are actually showing end-of-life indicators, rather than which units happen to share a purchase date. That's a more level, more forecastable capital plan than one built around an accounting artifact that was never meant to predict mechanical failure in the first place.
FIELD NOTE
The next time a unit crosses its depreciation date, don't schedule its replacement — schedule a measured assessment. Let the readings, not the invoice date, decide what year it actually belongs in.