CapEx underwriting
Every system in the house, its wear-out window, and the monthly contribution that funds replacements before they fail.
Monthly contribution
Computed
The window
Fund the next
3 is a clear wear-out curve. 1 would be memoryless, which understates an old roof.
A one-year window is a catch-up figure — what it costs to be ready by next winter. A five-year window is the sustainable contribution. Both are honest; they answer different questions.
The systems
Central air condenser
7–15 years expected
Gas furnace (forced air)
15–25 years expected
Split heat pump
10–15 years expected
Tank water heater
6–12 years expected
Architectural shingle roof
20–25 years expected
Kitchen dishwasher
8–10 years expected
Refrigerator or freezer
9–13 years expected
What the fund is for
99% chance of failing inside 5 years
33% chance of failing inside 5 years
43% chance of failing inside 5 years
94% chance of failing inside 5 years
40% chance of failing inside 5 years
60% chance of failing inside 5 years
87% chance of failing inside 5 years
Next three, by median date
Lives are InterNACHI's service-life ranges; costs are 2026 replacement ranges. Both are inputs — replace them with the quote you were given.
The lever for each system, and the reason it usually fails early, is in the table behind this page.
A water heater does not fail at year nine because a table says nine. It fails on a wear-out hazard that climbs steeply once the unit is past its median. The engine models that curve, which is why an eight-year-old water heater asks for more than a new one and a two-year-old roof asks for nothing.
A one-year window is a catch-up figure: what it costs to be ready by next winter. A five-year window is the sustainable monthly contribution. Both are printed because a household that has just bought an old house needs the first, and one that plans to stay needs the second.
The engine prices the same replacements on an 18-month card. That interest figure is the actual cost of having no fund, and it is usually larger than the inconvenience of the monthly contribution.
F(age) = 1 − exp(−(age / λ)^k)
P(window) = [F(age + T) − F(age)] / [1 − F(age)]
contribution = Σ(replacement × P(window)) / months in window
median failure = λ × (ln 2)^(1/k)
Service lives are InterNACHI ranges and replacement costs are 2026 ranges. The shape parameter is exposed on the page: raising it makes the timing later and sharper, lowering it spreads failures out.