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A $900 repair on a 12-year-old system scores 10,800 on the rule — and the 4-year ledger says keeping it costs $7,312 vs $2,167 net to replace.
The $5,000 rule (repair cost × unit age; over 5,000, replace) is a useful screen but a lazy verdict — it ignores energy, refrigerant, and what fails next. The honest ledger prices both paths over the old unit's remaining life: keeping a 12-year-old HVAC after a $900 repair means $3,892 of expected future failures (they escalate ~25%/yr on aging systems) plus $2,520 of efficiency penalty — $7,312 total, against $2,167 net for the new unit after crediting its residual value. But repair IS right when the fix is a wear part (capacitors and ignitors are $150–450 of maintenance, not decline), when a warranty covers the part, or when you're selling within two years. This calculator runs the screen and the ledger together, with the refrigerant and timing traps priced in.
Homeowners use it when the tech quotes the fix: a $900 repair on a 12-year-old system scores 10,800 on the rule, and the 4-year ledger — future failures plus the energy penalty — says keeping it costs $7,312 vs $2,167 net to replace.
Three scenarios survive the ledger. Wear-part repairs: capacitors, contactors, ignitors, and thermocouples are $150–450 of routine maintenance on systems with years left — replacing a unit over a $200 capacitor is how $6,500 disappears. Warranty coverage: a compressor or heat exchanger under parts warranty changes the math completely — but confirm LABOR coverage too, because "covered part" with $800 of labor is half a repair bill, not a free fix. And the short-horizon sale: if you're selling within two years, buyers discount old systems but not dollar-for-dollar — a working repaired system plus a disclosure beats a $6,500 investment you won't recoup at closing. The flip: an off-season replacement quote runs 5–15% under July emergency pricing, so the repair that buys you to October can be worth doing even when replacement wins the ledger.
It breaks old-unit repairs by price escalation — ask what's in the system before approving anything. R-22 (pre-2010 systems) has been phased out so long that reclaimed refrigerant runs $100+/lb, turning a $400 leak repair into $1,200+. R-410A (most 2010–2023 systems) entered its phase-down under the AIM Act, with production cuts driving the same curve — repair quotes on R-410A systems will climb through the late 2020s as supply tightens. New systems use R-454B or R-32, which aren't backward-compatible. The practical rule: a refrigerant-involved repair on an R-22 system is almost always a replacement trigger regardless of the rule score, and on an R-410A system it shortens the old unit's honest remaining-life input by years.
Same ledger, different constants — and the calculator takes them as inputs. Water heaters: 10–12 year lifespan, repairs under $300 favor fixing a young tank, but a leaking TANK is always replace (it's the one failure that isn't a repair). Appliances: washers/dryers/refrigerators run 10–13 years; the repair-is-right zone is a young unit with a sub-$300 fix, and the rule score works as-is. Vehicles are the special case: use the same rule but price "replacement" as the annual cost of the next car (payment + insurance delta + tax), not the sticker — a $2,000 repair on a paid-off car beats a $600/mo payment almost every time the frame and drivetrain are sound. In every case the honest inputs are the same three: what fails next, what efficiency buys, and how long the new thing lasts.