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My condo has a central heat pump AC/heater system that takes a 20x30x1 air filter. My area is a very mild climate and the HVAC system rarely gets run.

Checking my usage data over the past 12 months, there has been 140 hours of use for the A/C and heat combined.

Unfortunately, all the recommendations I can find are for 3 months, but I cannot seem to find any hour interval recommendations. If I replaced the filter every 3 months, each filter would get very little use.

Where can I find a recommended air filter replacement interval in hours?

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    Varies way too much. Forest fire upwind? Dust storm? Short hours. Rain keeping the dust down - long hours. Pets shedding - short hours... etc.
    – Ecnerwal
    Dec 6, 2022 at 2:35
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    A 20 by 30? JFC. You don't. Ever.
    – Mazura
    Dec 6, 2022 at 17:23
  • @Ecnerwal can you provide a ballpark estimate for what a short hour change interval would be and a long hour interval would?
    – James
    Dec 6, 2022 at 18:09
  • This will not answer your question. But I encourage you to give a try, for a few months of always running air circulation. Even when it's not cooling or heating. You'll get more uniform temperature and humidity along with much less dust. Can make the air feel much fresher. The downside is that the filters then get their full use :-)
    – Jeffrey
    Dec 7, 2022 at 14:06
  • @Jeffrey The other downside is "non-trivial to extremely non-trivial power use, depending on how inefficient the furnace fan is. Some are shockingly bad. I'm happy to report that the fans in Mitsubishi min-split heads are quite efficient, particularly at moderate speeds. Of course, part of that is - no ducts.
    – Ecnerwal
    Dec 10, 2022 at 22:56

3 Answers 3

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I like Greg Hill's method if you want a data-driven approach that does not require eyeballing the filter.

I also really like my washable electrostatic filters, as they mean I can clean my filters for pennies whenever I like, rather than having to worry about when to buy & throw away a disposable filter. Hose them out, let them dry, and good to go, even if they were not very dirty when I thought to look at them. I gather that those may not be compatible with some systems - my current heating system came with a set built in. 20x30x1 versions are certainly available. If your system is adaptable to thicker filters, 20x30x2 is also available.

But if you really want to spitball hour figures....

90 days 24 hours a day is 2160 hours. So that's an absolute limit on "90 days expressed as hours."

Figure a typical system in constant use is running 50% time (unless the circulating fan is running 100% of the time, as some people do,) call it 1000 hours to make a nice round number - somewhere in there might be a "long interval for low loads."

My car recommends changing its "dust and pollen filter" (cabin air filter - the equvalent of a household furnace filter) every 15,000 miles if driving in dusty or polluted conditions, which I'll equate to 300 hours at an average of 50mph. So that might be a short interval, but it's still rather situation-dependent.

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The "every 3 months" recommendation has an unknowable set of assumptions built into it. There are so many variables, for example:

  • How dusty is your outdoor environment? How well-sealed is the house?
  • Do you keep pets? How many? Large or small? Do they shed?
  • How many people live in the house? How often do they bathe? Do they exfoliate?
  • Hard floors or carpets? Sweep or vacuum? How often? Central vac or portable? How effective is its filtration?

A general hours-based interval is far too precise considering all the variability at play.

Qualitative inspection is a fair strategy for many people. (Though, if we're being honest, I'd wager that many people don't have good intuition about how a dirty filter would look.)

If you want data-driven filter changes then get a differential manometer or pressure gauge. You'll still have to figure out for your particular system what "time for a new filter" looks like on the gauge, but once that's done, it'll be easy to know when it's time.

This Dwyer MARK II pictured below is one example (photo: www.globaltestsupply.com). It can be mounted to the equipment and plumbed so that it measures the pressure drop across the filter. Yes, I actually have one of these.. for me it made sense because my system uses "lasts up to 1 year!" filters that cost US$40+ at each change, but in my environment and usage they're overloaded with dust in as little as 7 months! I'm prone to leaving a dirty filter in place too long. Although that's somewhat good for the equipment (a dirty filter cleans air even better than a fresh clean filter does), it's not good for my utility bills.

Dwyer MARK II

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  • With all the smart sensors out there now this should be built into a standard filter box, would be pretty easy to set up a pitot tube configuration that connects to an app
    – redlude97
    Dec 6, 2022 at 18:27
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    Give me the dumb ol' analog tube.
    – Ecnerwal
    Dec 6, 2022 at 22:43
  • I put a different model on my furnace: G-99 Air Filter Gage (google it). It works fine. Dec 7, 2022 at 15:24
  • Note: The Air Filter Gage only works when the fan is running. I leave my fan on 24-7 so it's fine. (Gee, I never knew we couldn't edit our own comments after a short period of time). Dec 7, 2022 at 15:46
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Inspection is your friend here. Take the filter out and look at it. If it's covered with dust and dirty. Time to replace. If you can see lots of light thru it, it's good to go for another 3 months or more.

To me it's like a car or truck you don't drive much, you don't have to change the oil every 3 months when you've only put on 500 miles during that time.

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