A well-maintained gas furnace commonly lasts about 15 to 20 years, but that is a planning range rather than an expiration date. Installation quality, usage, maintenance, and repair history all matter. An older furnace that heats evenly and runs safely may still be serviceable; repeated breakdowns, rising costs, or safety concerns deserve a closer look.
If your furnace is approaching that age, ask a technician to compare its condition and repair needs with a properly sized replacement. Stanley offers furnace installation for Dayton-area homes.