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

enter image description here

2 Answers 2


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.


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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.