The most important variable in gauging the useful life of a furnace is the quality of the installation. Not only does the unit need to be properly sized but the ductwork/airflow design to and from the furnace must be able to accommodate the size of the furnace. Sadly, in many cases, the furnace is too large for the home it is intended to heat because a proper load calculation of the home’s needs was never performed. This oversizing of equipment is often accompanied by a lack of sufficient airflow/ductwork which is the most vital variable in a proper HVAC installation.
If these variables are present, there will be frequent costly furnace repairs like the major motors failing and the furnace heat exchanger will often not last even 12 years before it becomes faulty. While all of this is happening there are also usually comfort issues in the home due to improper installation.
A properly installed furnace can last 20-25 years.
You should replace your furnace if it is improperly installed and around 10-12 years old with a major repair needed if you are in a financial position to do so.
If your furnace heat exchanger is cracked that normally happens later in the life of the furnace where it is no longer under warranty a standard parts warranty and the cost to try and repair the furnace is almost the cost of just updating the system. If the furnace heat exchanger fails it is not long before another major repair follows behind it.
If you are simply not happy with how the furnace is heating your home or how the air conditioning is performing with major temperature differences throughout the home and you are in a financial position to do so you may also decide to “start over” with a better installation and replace.
