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My HVAC air filters are of an odd size, hard to find and very expensive. Can I stop using them altogether and substitute an adequate standard size filter on the return? Sorry, I wasn't clear enough. My HVAC is in the garage and has an air filter located at its base, just before the fan. Can I remove said filter, stop using it altogether, and replace it with a larger, better filter at the 20" x 30" return grille located inside the house?

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    Many forced-air systems are designed with a return air grill that accommodates a filter; others have the filter housing incorporated into the air path elsewhere, often at the unit itself. Would you please edit your question to clarify your existing filter setup? Oct 19, 2023 at 17:00
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    Pictures of your current filter installation and your return where you'd like to put the filter would be very helpful. Where is your HVAC filter located now? It seems unusual that it's not already at the furnace return, as that's the typical location for it. If your return has more than one location, the filter is usually located at the furnace.
    – Milwrdfan
    Oct 19, 2023 at 21:07
  • My filter has been on the return side of the furnace for the last 30 years. Are you talking about putting a filter at the return air inlet?
    – FreeMan
    Oct 20, 2023 at 18:12

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The air filter must be on the return side. Aside from providing you with cleaner air, it also reduces dust buildup in the heat exchanger, evaporator, and whatever other air treatment gubbins you have.

Sometimes it is installed in the air handler cabinet just before or just inside the entry point, or just outside the blower housing. That's a convenient place to put it but it's even better to put it in the frame of the return air grille. Then it also keeps your return air ducts cleaner, and depending on the location it may be even more convenient for you to change the filters, and this can result in your doing that more faithfully, which will improve your air quality and the life of your system.

The only caveat is that if you have more than one return air grille, you must install filters on all of them, and you may need to replace the grille housings or improvise something to do that. If you have more than 2 or 3 (eg in every bedroom) that isn't practical. You will then not change the filters more faithfully, and when you do it will be expensive.

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The point of the air filter is to protect the air handler. It's not to increase air quality, that's just a side effect. The way an air conditioner works (at least in the context of removing an air filter) is that it pushes/pulls air through narrow channels of metal fins. If dust is allowed to pass through, they become clogged and the AC will become ineffective.

To be clear, the narrow channels I'm referring to are on the air handler's coils which look like this:

enter image description here

The air has to go through those fins which can very easily accumulate dust. Keeping dust off of this is the reason you need an air filter on the intake side.

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  • Thank you for your answers. From what you tell me I can only draw the conclusion that the answer is NO because there isn't a single air return grille, but several. At least one in each room, and installing a filter on each is highly impractical. In conclusion, I am stuck with my odd sizes, hard to find, expensive filter... Oct 24, 2023 at 15:45
  • @GabeGozzoli see my edit. The return air grills aren't the narrow channels I'm referring to. Those are quite wide channels by comparison. Oct 24, 2023 at 17:08
  • Gotcha, and thank you. Now I understand that keeping those fins clean is job 1. I'm glad I got your input before doing something stupid. Sometimes the best answer to a question is simply, NO. Oct 25, 2023 at 19:41

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