When a big-ticket system fails, use two numbers: the cost of the repair versus a replacement, and the age of the item against its expected life.
The 50% rule
If the repair costs more than half of a full replacement, and the item is past about two-thirds of its expected life, replacing usually wins. Under both thresholds, repair.
Typical lifespans
- Asphalt roof: 20–25 years
- HVAC system: 15–20 years
- Water heater: 8–12 years (tank)
- Major appliances: 10–15 years
The other factors
A newer, more efficient HVAC or water heater can cut energy bills enough to offset part of the cost. Check whether the failed part is still under warranty. And weigh how long you'll stay: if you're selling soon, a clean repair and disclosure may beat a big spend you won't recover. For anything structural or safety-related, get a second written opinion before deciding.