# How often should the air filter in the furnace be changed?

Does the period of change of this air filter have an affect on the longevity of the heating and a/c system? What is a good rule of thumb to require replacement?

• I change mine when it looks quite filthy. – Matthew Ruston Jul 28 '10 at 20:48
• Can someone answer this in runtime hours rather than in months? I run my furnace about 15× as much in January as I do in October, and not at all in the warm months, so the typical "every n months" normally given don't make much sense to me. Thanks! – mattdm Jan 26 '11 at 22:57
• Also, my thermostat, like a lot of digital thermostats, keeps track of the number of hours the furnace is run. So that's easy. – mattdm Jan 27 '11 at 3:59

Dirty filters not only make the HVAC equipment not last as long, but the low air flow makes them less energy efficient. A dirty furnace does not conduct heat into the air as well and the dirt sticks to the fan blades making it not last as long or spin as easily.

There are two main types of air filters:

The 1" thick ones that look like this:

And the 6" thick filters look like this:

There are also 2", 4", and 5" filters available but generally not at the local hardware store. Some folks who have 6" filters use a 2" + 4" filter and replace the 2" one every other month and the 4" once a year. This seems like a hassle to me, but it might save you $50 a year if you have 6" filters. • My furnace manual says that I should replace my 4" filter once a year. My local furnace repair guy says to go by that, but the filter gets filthy enough after about 6 months that I feel the need to just replace it then. :-) – Brian Knoblauch Aug 23 '10 at 11:38 The rule of thumb is every 3 months but it does depend on the filter and your air quality. And yes, if you don't keep the filter clean it will shorten the life of your unit and could cause other problems like a frozen coil (where the air handler coil freezes because there isn't enough airflow). I like to use the cheap air filters as they allow the best airflow. Some units are made for the better filters but those can cause more problems than the solve. It depends on the filter. Go to Lowes or Home Depot and they will have a chart with the different filter qualities and the recommended length of time they can be in service. In general, the cheapest filters that cost only a couple of bucks have to be replaced once a month. As you go up to more expensive filters, you can replace them every 3 or 6 months. I currently buy the ones that need to be replaced every 3 months. 1-2 months for a 1" filter, and 6 months for a 6" filter is a good rule of thumb. However, that is for average conditions in an average home. You may want to check yours more frequently if you have any pets since fur will tend to dirty up the filter quicker, and reducing the filter's usable life. You can also compare a new one (or photo of a new one that you took before putting it in) with the current one to determine how "bad" or "dirty" the current one looks. If it looks bad, change it. If you install a GeneralAire G99 Air Filter Gage, it will tell you exactly when your air filter needs to be changed. It can be found for <$20 online. I don't have one, but I've thought about it...