When to change a/c air filter, really?

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Old 09-04-17, 07:26 PM
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Question When to change a/c air filter, really?

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, from every 20 days, or once per month, or 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 unneeded 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 detected 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, labelled "change air filter." Since I have never seen or heard of this, obviously that basically doesn't exist (and, certainly it isn't part of my house HVAC). I have seen add-on gauges for sale online to provide objective evidence of the air pressure drop caused by the filter, however installation and access would be problematical 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 ... by visible dirt load. Has anybody out there tried this approach? The advice was:
#1--Have a new air filter on hand.
#2--Occasionally remove the current air filter, then hold them both up to sunlight. If the current air filter is visibly darker, it should be discarded.

Another approach (which I don't trust, because I just made it up) is to use a hand-held anemometer (air speed gauge). I'd check the air speed (at either the return or a supplyl vent) with the old filter in place, 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, hand-held anemometer likely be sensitive enough to measure whether there's enough of a drop to warrant changing the air filter?

Thanks for any assistance you can provide.
 
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Old 09-04-17, 07:38 PM
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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.
 

Last edited by PJmax; 09-05-17 at 08:43 PM. Reason: typo
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Old 09-05-17, 02:28 AM
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Pete has given you the standard quick and dirty response, which from your initial post I suspect is NOT sufficient for your inquiring mind.

This question requires one of my infamous long-winded responses but since I am just a few days out of the hospital I cannot do it at this time. I will try to get back to you in the next day or so.
 
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Old 09-05-17, 10:31 AM
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Furd -- Thanks. I'll look forward to reading your response -- get well soon. I am sorry my inquiry was so wordy, but (as you sympathetically acknowledged) the answers to concise questions like "How often should I change my A/C filter?" are almost invariably unsatisfying to me.

One thought that keeps coming back to me about these short interval recommendations is that I think hardly anybody does follow them, so these are just more of those recommendations that people live with, ignore, and feel vaguely guilt about, like saying everyone should drink 8 glasses of water every day (which, coincidentally, turns out to be untrue).
 
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Old 09-05-17, 06:46 PM
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Unless I'm mistaken (I often am), some thermostats can be set up to how often to change the filter. Seems like it is based on run time with some user configurable parameters?

If there isn't, there should be...hmmmm.

In answer to your actual question, the recommendations all have to be adjusted based on your particular lifestyle and even were 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 6 months of the year), so in the winter I don't have to change it near a often as summer since it's not as dusty, solar heating of the home helps, and 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.
 
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Old 09-05-17, 07:34 PM
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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 micro-bacterial type. Everybody seems to be an expert on this subject and there are as many "right" answers as there are experts. One says the cheap ones so as to not block the air flow, another says the high end as to filter the air, while the manufactures say there specific brand will be the best because it cost the most!

I get this question all the time at the store. Here is my answer. If you don't have a dust problem or an allergy problem then the cheapo's are fine. They will trap large particles like cat and dog hair but not much else. If this is what your tarping and you don't have a problem then you're OK. If you have a dust or allergy problem I suggest 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 3 to 4 weeks and most likely change it about every two months. In winter more often than in the summer for Northern climates. 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.
 
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Old 09-06-17, 03:48 AM
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If you have a dust or allergy problem I suggest 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.
We have one of those high filtering cleanable filters. I like the fact that all I have to do is clean it once a month and it seems to help with my wife's allergies. Over the last 20 yrs my air handler has had 3 blower motors I've been told that the restricted air intake is why.
 
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Old 09-06-17, 04:53 AM
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Marksr, Is the filter the one that was originally specified for the unit? The motor is either too small (or the wrong type) or the filter is too much for rated blower. For those units that require special and/or specific filters (as opposed to the off the shelf Filtret Filters) should only use the OEM filter as specified by the furnace/AC mfg.

And Cizzi, I'm not making any blanket statements.
 
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Old 09-06-17, 05:41 AM
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I don't know, I went with what the installer recommended. Another HVAC tech recommended I go with a larger motor but only the motor specified will mount in place. I could probably modify it to make a larger [cheaper] motor fit but the air handler is in the crawlspace and orientated in a manner that makes it difficult to access. .... plus the motor has yet to go out when I had the time to figure out how to do more than a simple replacement.
 
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Old 09-06-17, 07:20 AM
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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 becoming 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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