HVAC Filter question

I asked the same question before, definitely want to keep the larger one then follow manufacturers recommendation which you can probably find online. Some units may not need both may make the unit work less efficient.

Not a HVAC professional but have done extensive reading on the subject from HVAC sites and gathering opinions of HVAC techs, and I’m not sure that even HVAC professionals would have common agreement with an appropriate answer.

First and foremost the primary purpose of the filter is protection of the HVAC equipment and not air equality for human health. With your set up as is, probably the best answer I heard from a individual with knowledge of HVAC systems, would be to use a cheap “see through” spun fiberglass filter (not a pleated filter) in your return vent grill changed monthly, at least during the summer, and change your thicker pleated filters at 6 month intervals. You don’t want to restrict the air flow so much that it raises the static pressure in the system causing issues with air flow and having the long term consequence of reducing the life of your HVAC equipment associated with high static pressure.

Here's some food for thought. You can buy permanent filters that can be hosed out monthly or whenever you would change out your filters in the return air grill.

They do cost more up front, but last for years if you don't bang them around. I ordered 2 for where my wife works when they first opened the office in an old building that was refurbished.

One cost $75 and the other cost $50 to fit the units they had installed. Once a month I'll pull them out and just hose them with the garden hose and let them drip dry and replace them in the grill. You will save money in the long run doing this.

Do not use filters at the return grills. The filters at the AHU do a much better job, especially 4"-5" deep filters.

Doubling up will only kill your AHUs.

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Really? That advice sounds unusual. Where you do you get AHU filters?

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Does your air handler have a slot for AHU filter? Not all do - mine doesn’t. If that’s the case you need one at the return vent grill - assuming that how your HVAC is set up.

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Really? That advice sounds unusual. Where you do you get AHU filters?

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AHU filters are essentially the same but they are located at the AHU instead of return grills.

AHU filters are much better when you have 4" to 5" deep filters like the picture in the original post. They filter more contaminants, have lower pressure drop and last longer. Their housings also tend to seal much better where as return grill filters let a lot of unfiltered air by.