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Community and Q&A

Air filter MERV “sweet spot”?

Roy Goodwin | Posted in Mechanicals on

I’m looking at air filters for my new house and was wondering if anyone had found a “sweet spot” in the tradeoff between filtration efficiency and cost, or some way to calculate that, or maybe a rule of thumb?

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  1. Nick T - 6A (MN) | | #1

    I have had a differential pressure gauge on my home filter for my past two homes and in both cases it seems like anything more then a Merv6 or 7 and the amount of pressure drop (restriction of flow) really shoots up. Quickly gets past typicall 'design' filter pressure drop... which is typically Merv3 or so (if taken into account at all....)

    In each step up in Merv rating that it is the equivalent of a very dirty filter. So a very grey dirty Merv6 filter had the same airflow restriction as a clean Merv7.

    I am able to get low cost TrueBlue Merv6 or 6 at Menards in my area. I tested the Merv-less Filtrete and even their lowest rated one had a higher pressure drop then a 7 (by a good amount) IIRC.

    If you don't have a pressure meter or gauge - make sure you carefully watch furnace temperatures (return and discharge) to ensure proper flow is being maintained (make sure not to high of a difference, don't want past the specifications). Also once you put in a high eff filter it will change the dynamics of the system, and effect your flow vs design. Might need to change motor lead to a faster speed to overcome additional pressure drop...

    Would be possible to test multiple filters if you had an amprobe to put on fan motor.... adjusting fan lead to maintain a constant differential air temperature (or airflow sensor or static pressure). That would give you a cost for that flow restriction. Would be a good bit of tinkering...

    Often an additional return can be added to the other side of the furnace - which will help increase total surface area of filtration. This will reduce the velocity of air across the filters and reduce pressure/flow loss.

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