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?

  • 1
    I change mine when it looks quite filthy. Commented Jul 28, 2010 at 20:48
  • 5
    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
    Commented Jan 26, 2011 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
    Commented Jan 27, 2011 at 3:59

6 Answers 6


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: alt text

And the 6" thick filters look like this:

alt text
(source: filterscanada.ca)

Generally the 1" filters are good for a month or two. The 6" thick ones are good for 6 months.

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. :-) Commented Aug 23, 2010 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...

After doing a bit of research, I've decided to change mine every 3 months. I don't use the cheapest filters, but I don't get the incredibly expensive ones, either.


i would say that rule of thumb must be different for different places. e.g if you're living in industrial area like where you have alot of pollution in the air , you must go for change before the time mentioned in the manual but if you're living in healthy environment like well planned societies than it could lasts more than specified time. In short you must check the quality of the filter time to time.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.