A furnace has a ten-year lifetime, lasting up to 15 years, provided the homeowner maintains it with proper precautions. However, if your furnace is not energy efficient and has caused a substantial increase in your electricity or fuel bills, you might need a replacement or emergency repair.
The furnace needing constant technical care indicates that you need to replace it since the expenditure on repairing it might be the same as that for the new HVAC unit.