Before you need to use it! Although we are enjoying a gentle transition into sweater weather, now is a great time to have a professional out to your property to evaluate your heat source and air filtration systems.
Overall life expectancy for a home furnace is 15-20 years–if you keep up with maintenance. What does that mean? Annual checkups, filter cleanings, and possibly replacing parts. Once your furnace is over ten years old, you may want to bump up service appointments to twice a year.
Servicing your home's critical infrastructure, like your heat source, is something you want to include in your annual budget because if your furnace is going to fail, it will happen when it's working the hardest (aka when you need it the most).