Two-inflection curve
Components hold steady, then decline, then drop off a cliff
A component’s condition does not fall on a straight line from new to failed. It holds roughly steady until about half of the rated lifespan (the first inflection), declines faster through the next quarter, and past 75 percent (the second inflection) drops off a cliff. Two bends in the curve, which is what the name says.
The mechanism: the curve is why age alone predicts so little, and why failures feel sudden when they are not. A 12-year-old boiler serviced every year can have more life left than an 8-year-old one nobody has touched, because servicing flattens the early part of the curve and pushes the cliff back, while skipped servicing pulls it forward. The curve also runs one way only: a repair stops further damage, it does not restore condition already lost.
Worked example. Karin’s house outside Gothenburg has a 30-year roof covering in year 24. The second inflection fell around year 22, so the covering is on the steep part of its curve. Nothing has failed yet, and that is the point: this is the window to price and schedule the replacement as a summer decision with competing quotes, rather than as a January emergency after a storm finds the weak spot. The curve turns “when it fails” into “when you decide.”
The distinction matters at three moments. When buying a house, a component past its second inflection is a near-term cost regardless of how it looks at the viewing. A repair quote late in a component’s life is the second: the years a repair buys shrink as the cliff approaches, which changes the annual-cost arithmetic. And a service visit that feels skippable is the third: skipping it does not save the fee, it moves the cliff closer.
Sibling frame: lifespan is the rated range for a component type; the curve says where within that range your unit’s condition sits. The practice built around the curve, pushing each component to its full lifespan and planning the replacement before the cliff, is lifecycle maintenance. The heating-system-lifespan guide shows the curve applied to the component it matters for most.