Hot Topics: When to Replace an Air Conditioner Filter

A man replacing an hvac air filter in a ceiling vent.

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There's a lot of information out there on how often homeowners should replace their HVAC air filters, but no real industry standard. The forum gives its opinions on when this simple task should be done, with consideration towards lifestyle and climate conditions.

Original Post: When to change a/c air filter, really?

angelsix Member

I want to know when to change the air filter in my central heating/cooling system. I'm feeling frustrated by the advice I see when Googling this because the advice mostly makes no sense to me.

The most common advice seems to specify some fixed time frame: every 20 days, once per month, every three months, etc. This makes no sense to me because it fails to take into account how often the system is used (hardly ever in spring or fall here), how dusty the location is (central desert vs. shoreline), what kind of filter is used (simple fiberglass, 1" pleated, 5" pleated, etc.). Advice that recognizes these factors is remarkably imprecise as to how much of an adjustment should be made for them. The people giving this type of advice mostly seem to have an incentive to recommend unnecessary air filter changes.

Why change air filters at all? As far as I can figure out, the answer is that if they get so dirty that it impedes the air flow, then the heat/cool exchange becomes inefficient and could even be damaged. (Or, sometimes odors build up in the filter so it should be changed, especially if there are pets.)

Sensibly, considering how expensive HVAC systems are, they would have a little pressure or air flow sensor and when it detects that the filter is clogged enough to impair the operation, a signal would be sent to the thermostat and a little icon or red light would come on, labeled "change air filter." I have seen add-on gauges that provide objective evidence of the air pressure drop caused by the filter, however, installation and access would be problematic for me as my air return (with filter) is on a high ceiling, directly below the rooftop HVAC installation. (I did see a review by someone who had successfully installed one of these aftermarket gauges, celebrating that there was no meaningful drop in air pressure after her air filter had been in place for a year.)

I saw one answer that seemed to make sense, especially as it is similar to how I make the decision about when to change my car's air filter: visible dirt load. Has anybody out there tried this approach? The advice was to have a new air filter on hand and then occasionally remove the current air filter and hold them both up to sunlight. If the current air filter is visibly darker than the new one, then it should be discarded.

Another approach (which I don't trust because I just made it up) is to use a handheld anemometer (air speed gauge). I'd check the air speed at either the return or a supply vent with the old filter in place and then replace the old filter with a new filter and see if the air speed changes. If it doesn't, then the old filter can't be slowing the air very much and could go back in. Anybody ever heard of this approach? Would an inexpensive, handheld anemometer likely be sensitive enough to measure whether there's enough of a drop to warrant changing the air filter?

PJmax Group Moderator

Some people need a set time so they can put it into their maintenance schedule. It's far better to change the filter more often than less often. There doesn't have to be any fancy indicators or science involved. Just change it when it's dirty. You should know by now when that approximately is.

Gunguy45 Super Moderator

Unless I'm mistaken (I often am), some thermostats can be set up to how often to change the filter. Seems like it's based on run time with some user configurable parameters?

In answer to your actual question, the recommendations all have to be adjusted based on your particular lifestyle and even where the filter is located. For instance, a low mounted wall filter will get far more pet hair than a ceiling location, in my experience. I live in a very hot, super dusty area (for about six months of the year), so in the winter I don't have to change it nearly as often as in summer since it's not as dusty, the solar heating of the home helps, and there are relatively mild outside temps. Even a fancy thermostat wouldn't be much help to me. Also, no kids or dogs going in and out all day. It all makes a difference.

Norm201 Member

Your normal method is the correct one—visible observation. Like everybody says, local conditions will dictate how often and only you know what your conditions are.

Of course, now this begs the question and long-standing argument about what kind of filter: the cheap fiberglass that you can see through or the high-end, antibacteria type. Everybody seems to be an expert on this subject and there are as many "right" answers as there are experts; one says use the cheap ones so as to not block the air flow while another says use the high-end to filter the air, while the manufacturers say their specific brand is the best because it costs the most!

I get this question all the time at the store. Here is my answer: if you don't have a dust or allergy problem, then the cheap ones are fine. They will trap large particles like cat and dog hair, but not much else. If you have a dust or allergy problem, I suggest to move up by increments to the higher filter types until you find the one that works. And as far as efficiency goes—don't worry about it. The machines are made to handle the air flow even under some restrictions. Check the filter once every three to four weeks and most likely change it about every two months. Your mileage may vary depending on conditions.

If you're looking for a detailed scientific answer, then talk to the filter manufacturer and I guarantee you will spend lots of money on filters.

guy48065 Member

Manometers and differential pressure gauges have been used to monitor filter efficiency in industrial settings for decades, probably because filter changes are expensive on large industrial systems. But, deep pleated home filters are $40 or more and are increasingly common. I agree it's time for measurement to replace blanket interval recommendations.

Using an inexpensive wind speed gauge to measure flow through a suspect filter is an interesting idea, but falls flat when you have to reach in with it and disturb the normal flow. There are thin probe-type sensors (hot wire) that perform this function, but they aren't cheap.

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