Replacing the filter in your furnace regularly keeps equipment working properly and your energy bills low. It's also a task that many homeowners forget about for months at a time, which can send your energy bill skyward and potentially contribute to other maintenance issues down the line.
Typical recommendations range from every 30 days for cheaper fiberglass filters (which often don't do a great job of filtering), to as long as 6 months for higher-end pleated filters. These estimates assume average use and take into account the type and size of your filter.
A general rule of thumb for high-end pleated air filters is to replace your filter every 90 days, typically at the beginning of Spring, Summer, Fall and Winter. As your filter traps more dirt, dust, and allergens from the air, filter efficiency decreases. You should at least check once a month to see if you should be replacing your filter more often.
If you live in a temperate climate and use your HVAC system only a few hours a day, a single filter can last you from a whole season to an entire year. If your HVAC system is running on a near-constant basis, however, change your filter every few weeks (especially if you're using a cheap fiberglass filter).
There's no strict guideline, but if your filter shows only a subtle layer of dirt under which the filter's material is still visible, your filter is in fine working order. If there's a visible coating of dirt and dust covering most of your filter's surface that is thick enough to block the light, that's a sign your filter has been in your system far too long.
Every few weeks, take out your filter and give it a look! Is there a musty odor? Is the filter covered in dust? If in doubt, it's always best to err on the side of replacing your filter. They're inexpensive, and that small cost will be worthwhile when your energy bill comes due.From U.S. DOE Energy Savers, and various NEO and filterbuy publications.