HVAC question: I have two units that both have Aprilaire filters at the unit in the attic

This is what is connected to my system in the attic.

LINK

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you dont "have to" double filter but the ones at the unit are there to catch anything that gets passed the other filters or leaks in the return air system

the ones in the units should only need replacing once a year at most unless there are air leaks in the return system

think of the filters in the units as cheap insurance so you wont need to go through the cost and down time to have the indoor coils cleaned every 5 yrs

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So you recommend using the 1'' filters as well?

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The regular 1'' filter is what I mean by the kind you change every month at the ceiling. I should definitely keep using those right?

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yes use and replace the ceiling ones every month

the ones at the units should only need to be replaced if they get dirty. i have seen those last 5 yrs sometimes before getting dusty. if the ones at the units need replacing more then once a year then you need to find and seal up the air leaks in the return air shafts. its not a fun job if you need to do it and i once had to pay a regular customers kid to crawl in there and do it because adults could not fit in there where the leak was lol

better check your static pressure before double filtering. That extra filtration can kill your blower motor (if ECM blower) or your compressor (PSC blower). the risk with this is no different than those who try and use extra high efficiency filters that are too restrictive.

Personally I would only use the filter at the units. The filters at the return grills will create more vacuum in the return plenums and increase the chance of pulling in outside air from different cracks/nooks and might cause the aprilaire filters to get more dirty even faster. using only the filter at the unit will help reduce the amount of negative pressure in the returns. The strongest negative pressure area will be between the filter and the blower.