How long does a furnace last?

A furnace wears with use and with heat. A furnace in a cold northern climate runs for much of the year. One in a mild climate may run for a few weeks. Fuel type, installation and maintenance add more spread.

The age figure with a clear public source comes from ENERGY STAR, which suggests you consider replacement when a furnace or boiler is more than 15 years old. That is the age to start comparing repair bills with the cost and efficiency of a new unit. It is not a failure date. A furnace that passes its yearly safety check and heats the house evenly can stay in service.

• Oversizing. A furnace that is too large heats the house fast and shuts off, over and over. That repeated heating and cooling stresses the heat exchanger.
• Restricted airflow. Dirty filters, closed registers and undersized ducts make the furnace run hot.
• Skipped maintenance. Dirty burners and a weak flame sensor cause hard starts and shutdowns.
• Moisture and corrosion. A damp basement, a leaking humidifier or a blocked condensate drain can rust parts from the inside.
• Poor installation. Wrong gas pressure or bad venting shortens the life of a new furnace from the first day.

Change the filter on schedule. Keep supply registers and return grilles open and unblocked. Keep the area around the furnace clear. On a high-efficiency furnace, check that the condensate drain line is not clogged.

Everything else belongs to a licensed technician. An annual check before the heating season covers burners, ignition, the heat exchanger, venting and carbon monoxide readings.

• Repairs are more frequent, and fuel bills are rising. ENERGY STAR lists this pair as a reason to replace.
• Some rooms are too hot and others too cold.
• The furnace has become noisy, with banging, rattling or a loud blower.
• A technician finds a cracked heat exchanger or heavy rust.
• Several major parts have failed in a short span.

When a large repair lands on an older furnace, compare it with a replacement quote. The guide on whether to repair or replace a furnace covers the decision, and the repair or replace calculator scores it for you.

Turn the furnace power switch off, then look for the data plate. It is usually on the inside of the cabinet near the burner door or on the side of the unit. The plate lists the model and serial number and sometimes the manufacture date. If you see only a serial number, the manufacturer can tell you the date from it. An installation invoice, a permit record or a home inspection report will give you the install date.

Write the age down with the model number. A contractor will ask for both, and they tell you whether the parts warranty could still apply.