Is Extra A/C Filter needed?

FWIW, Paul LaGrange, forensic building inspector, who hosts a “The Home Improvement Show” radio program on WWL radio, and answers numerous questions on HVACs, says that he uses a low MERV 1 inch filter (MERV 4) that he changes monthly in the return (intake) plenum, in addition to the 5 inch filter in the air handler in the attic which he changes every 6 months.

I suppose if you used a high MERV filter in the intake plenum you could possibly restrict air flow so you might not want to do that. Would probably be best to check with company who installed it.

Definitely don’t do anything that restricts the airflow to your unit. I’d just follow the installer’s advice or when you have time call the technical help desk of the HVAC manufacturer and ask them. Without getting technical there is a certain static pressure your system needs to operate at and the filters play an important role in that. Hopefully someone with better knowledge on this will weigh in on your post.

You do one or the other - not both

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Is Extra A/C Filter needed?
You do one or the other - not both

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+1

Just had a new a/c unit installed that has a 5 “ filter in the cabinet in the attic. I have always had one at the intake point. Should I remove that one ?
no, leave it in and consider it as a backup secondary filter.

use your regular filter and clean/replace it as normal but the 5" filter in attic should be monitored and only clean/replace it as needed which i would expect is only around once a year or maybe even less.

if it gets dirty sooner then that then you have leakage in the return air passage and its sucking in dirt and dust through wall spaces.

Does having two make the unit work harder ? yes but only by a small amount and if it came with it, then its designed for that added restriction so you are fine