return air filter

I have a blue fiberglass looking air filter in the return air grill.I also have a filter in the unit itself that is replaceable. do I need both?
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Are they part of the original design? Are they supposed to be t here? Etc.
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I would just use the filter in the unit itself...get a decent pleated filter instead of those blue fiberglass filters.
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Stick to the blue fiberglass filter. Do not use a pleated filter unless you have your air flow tested and verified as adequate. The pleated filters catch more dirt, but they are very restrictive to air flow. I have measured air flow for 18 years and find pleated filters reduce system air flow by around 10 to 20 percent. Most people have very low air flow to begin with. If you MUST use a pleated filter, change it every month as they load up fast.
A pleated filter has 2.5 times the resistance to air flow that a fiberglass filter has. In 90 days, that doubles or triples.
The Only exception is if you have a variable speed blower in your furnace. Then a pleated filter is OK, just don't leave it in 90 days.
Stretch
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The filter that have is not pleated.It is washable and reusable

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