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The vast majority of gravity furnaces don’t have filters. There are a few reasons but the main reason is that gravity furnaces were built before modern filters were invented. The furnace uses hot air to move heat around (unlike forced-air systems that we use today), and filtering the hot air simply wasn’t common practice. Because of advancements in air studies, it’s known now that having an air filter in your furnace is essential to maintaining air quality when heating your home.

Inexpensive filters do an effective job of keeping your HVAC system clean and running properly. However, they usually aren’t the best at filtering out allergens and other pollutants. So, if you have pets, several people in your household, or allergies and asthma, you’ll want to invest in a higher-quality filter.

Most modern furnaces made within the last 15 to 20 years should have some safety features that automatically shut off the unit if a problem is detected, but older furnaces likely don’t have these same features. Some issues that could cause newer models to shut off automatically include an increase in carbon monoxide production, poor airflow through the system, and overheating. Regardless of your furnace’s safety features, you should always take additional steps to protect your family and your home, like having annual inspections and maintenance done, and installing carbon monoxide detectors throughout your home.

You need to check your furnace at least once a year, especially before winter arrives and temperatures drop. This inspection should accompany other regular maintenance tasks, like cleaning the furnace every year and changing the filters at least every few months. It’s common to schedule furnace inspections in late summer or fall and air conditioner inspections in the spring to keep the entire HVAC system running optimally.