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I've been struggling to understand air filter sizes and the frequency of changing them out.

We have a Rheem 4 ton AC unit. Below is the filter (20x25x4) we currently have in our attic and some info from our warranty registration.

  • How frequently is it recommended to change this filter? These are fairly expensive (compared to a different place where I only used 1in depth filters) and some say to change every 3 months which adds up. I suppose it depends on several factors, but we live in Texas and it's just 2 people, no pets.

It seems like I should just check it visually to see if it's ready to change - I can see this lasting easily over a year. Please correct me if I'm wrong.

Model Serial Registration Date Install Date

RA1648AJ1NA 16 SEER CUBE A/C - W221838882 8/9/2018 8/8/2018
R801TA075421MSA 80+ UPF GAS FURNACE RHEE - W221845259 8/9/2018 8/8/201

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Change when it is blocked or just before...

Dusty seasons will cause more rapid filter blocking.

You could make a U tube manometer to see the difference between clean and getting too dirty.

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On expensive filters I suggest magnehelic gauges to know when to replace the filter but not many folks want to purchase a “pressure gauge” that cost 75$.

A u tube manometer can also be used to measure the differential pressure across the filter for 20-30$ or can be made for about 10$ with some tubing and a couple of fittings when the pressure across the filter gets to the max MFG specification it is time to change the filter no guessing.

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