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
When filter media accumulates dust the pressure drop across the filter
increases. This increase in air velocity
then increases the amount of dust entrained in the air.
And a dirty filter will be indicated by dust coming out
of entrainment at the diffusers.
In a practical sense having a filter on the main unit will
not effect the system to any great degree however the
filters in the returns will require more frequent cleaning
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