I have 3 returns in my house. I was told that I do not need an air filter
in my main unit (which is in my attic) if I have a filter in the returns.
However, when I had a filter in the main unit, I noticed it was
dirty....even though I had filters in all the returns. So currently, I
replace all 4 filters regularly. Is this ok? I use MERV 8 pleated filters.

I want to start using MERV 10 filters. Is it okay to still use one in the
main unit?

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Thanks.