# How often do HEPA filters really need to be changed? **URL:** \ **Category:** Factual Questions\ **Created:** [October 19, 2015, 6:06am UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108 "2015-10-19T06:06:45Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![SlackerInc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slackerinc/32/275_2.png) [@SlackerInc](https://boards.straightdope.com/u/SlackerInc)\ **Post date:** [October 19, 2015, 6:06am UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/1 "2015-10-19T06:06:45Z")
The “change filter” lights (we have three units) went on long ago, and a Google search finds me very dilatory. But every hit seems to be from places trying to sell replacement filters. I clean the pre filter periodically; I want to replace the HEPA filter when it is really needed, and preferably no sooner, as I am working on a very tight budget. At this point I have had the units long enough so that I don’t really begrudge getting new filters for them; but I would like to know how long I can wait until next time. ---
**Author:** ![jtur88](https://avatars.discourse-cdn.com/v4/letter/j/e9c0ed/32.png) [@jtur88](https://boards.straightdope.com/u/jtur88)\ **Post date:** [October 19, 2015, 1:05pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/2 "2015-10-19T13:05:34Z")
The timing of changing a filter is a pretty subjective exercise, like any other household cleaning chores. There is no precise schedule that applies to cleaning the crumbs out of the drop-tray of your toaster. What are the consequences of not changing the filter soon enough, and are you prepared to live with them hanging over your head? By the way, what kind of device are you using these filters in? Air conditioner? Vacuum cleaner? ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [October 19, 2015, 1:30pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/3 "2015-10-19T13:30:29Z")
I’ve heard to wait till it is ‘puffing’ which is noticeable unstable air flow. ---
**Author:** ![SlackerInc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slackerinc/32/275_2.png) [@SlackerInc](https://boards.straightdope.com/u/SlackerInc)\ **Post date:** [October 19, 2015, 1:42pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/4 "2015-10-19T13:42:34Z")
They are big room air filters. The largest home units I have seen, roughly eighteen inches in diameter and two feet high. No puffing yet! ---
**Author:** ![Balthisar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/balthisar/32/2825_2.png) [@Balthisar](https://boards.straightdope.com/u/Balthisar)\ **Post date:** [October 19, 2015, 2:04pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/5 "2015-10-19T14:04:23Z")
I kind of have this question, too, especially given where I live. A complete set of filters for my IQAir system are worth a quarter of the original purchase price. There’s no huffing and puffing, and the interior air smells pretty nice. Mostly my problem is the filters are PITA to get. ---
**Author:** ![bump](https://avatars.discourse-cdn.com/v4/letter/b/7c8e57/32.png) [@bump](https://boards.straightdope.com/u/bump)\ **Post date:** [October 19, 2015, 2:53pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/6 "2015-10-19T14:53:07Z")
AFAIK, the filters only get more efficient (i.e. filter out more crud) as they go, up to the point when the fan can’t pull/push air through the filter at all. This is true for any sort of air filter- home furnace, vehicle, etc… as they go, they get more effective, but it requires more oomph to get the air through the filter. In the case of automotive filters, it limits the rate at which air can make it into the engine, reducing power and efficiency, and in the case of the rest, it puts undue strain on the electrical fan motors. That’s really why you change the filters, IMO. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [October 19, 2015, 6:03pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/7 "2015-10-19T18:03:33Z")
As the filters load up (get dirty) the pressure drop across the filter get higher, at the expense of airflow (and fan energy costs). If you have air conditioning coils in this system, the reduced airflow across the coils can cause problems with your air conditioning system. Change or clean the filters regularly. ---
**Author:** ![SlackerInc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slackerinc/32/275_2.png) [@SlackerInc](https://boards.straightdope.com/u/SlackerInc)\ **Post date:** [October 20, 2015, 3:15am UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/8 "2015-10-20T03:15:13Z")
What if I don’t have air conditioning coils? These are just standalone electrical appliances that plug into the wall and sit there and pull air in from the sides and shoot the filtered air out the top. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [October 20, 2015, 5:57pm UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/9 "2015-10-20T17:57:57Z")
Then the downside is simply reduced airflow. You’ll be filtering less and less air as time goes on, defeating the purpose of having the filter in the first place. ---
**Author:** ![SlackerInc](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/slackerinc/32/275_2.png) [@SlackerInc](https://boards.straightdope.com/u/SlackerInc)\ **Post date:** [October 21, 2015, 12:31am UTC](https://boards.straightdope.com/t/how-often-do-hepa-filters-really-need-to-be-changed/735108/10 "2015-10-21T00:31:28Z")
Okay, so as long as I can feel a good volume of air moving out the top, I’m good.