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Carbon Filters Restricting Return Air Flow

suect | Posted in General Questions on

I had an energy assessment done on my home a month ago.  An additional 330 cfm of return was advised.

I had the company who installed the system test the air handler.  He mentioned it was my carbon filter.

The previous filter used was a Honeywell 11 MERV, I installed a Nordic Pure 10 MERV with carbon thinking it would be equal to the Honeywell.  The numbers shot up 3 times the amount of the prefilter on the post filter evaluation.  Quite surprised.  Is this commonly known? Any filters to remove odors without hindering the system?

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  1. GBA Editor
    Kiley Jacques | | #1

    Allison Bailes, our resident HVAC expert, has numerous articles that might help you troubleshoot your situation. I’m no expert but I wonder if it’s more of an HVAC system design matter than a filter type problem. Allison says, “Many (most?) systems with high-MERV filters decrease the air flow because of poor design.” Take a look at this article: How to Ensure a Low Pressure Drop Across a High-MERV Filter

    1. suect | | #2

      I read the article, thank you! If I am reading correctly, a 2” vs 4” filter should be used? Any thoughts on how this helps?

      1. Expert Member
        Akos | | #3

        The thicker filters have significantly less restriction. 2" is about 2/3 the pressure loss of a 1" filter and 4" is about 1/2. Since they also last much longer, thicker filter is well worth it.

        I've had issues with carbon filters. Lot of manufacturers don't publish specs on these, only find out after install when the furnace locks out on you. They seem to be much more restrictive than even Merv13 ones.

      2. Expert Member
        BILL WICHERS | | #4

        Thicker filters offer less restriction (I mentioned this in your other thread too). The reason for this is that thicker filters have more surface area of filtration media. Imagine the filter like an accordion: for the same number of folds (pleats), the deeper/thicker filter will "unfold" to a much longer piece of materail. More area = less restriction to airflow. More area also means it takes more dirt to plug up the filter, so the thicker filters tend to last longer before they need to be replaced.

        Standard residential HVAC filters are 1" thick. These aren't any kind of "high performance" in terms of restriction to airflow. 2" is better, 4" is better yet. That Aprilaire system I mentioned uses a custom type of filter that has even more filter media in it than a 4" thick cartridige-type filter has.

        Higher MERV filters can filter finer (smaller) bits of dirt out of the air. That means the "holes" in the filter media are smaller. Smaller holes means more retriction to airflow, and they plug up easier too. That's one of the reasons that you really need thicker filters when using higher MERV numbers -- you need to get AT LEAST the airflow through the filter that a less-restrictive low MERV filter would have offered in the 1" size.

        Carbon filters can be issues as they are notorious for plugging up. I only recommend using carbon filters for smelly things (or bad tasting things in the case of water filtration). If you don't have some bad smell you're trying to get rid of (like smoke, etc.), the carbon filters don't offer much benefit. Higher MERV numbers are better for allergens and the like. I run MERV 13 filters in my own home since my wife has chronic sinus issues, and they do help. Aprilaire now has MERV 16 filters available which I'll probably try at my next filter replacement.


      3. JC72 | | #6

        In this article filter placement had an impact on the pressure drop. The more air moving through the filter the greater the pressure drop so for example 2" filters placed in every return can exhibit less pressure drop vs a 4" filter mounted right next to the blower.

        In any case the issue is both the density of the filter media as well as the surface area of filter media.

        A filter media of a deeper filter is physically larger vs one which is not as deep. The increase in filter area affords more pathways for air to move through it which decreases the resistance. Hence lower resistance. Filter density (MERV 10 vs MERV 14) add another variable so a 2" MERV 10 may have similar resistance vs a 4" MERV 14

  2. ranson | | #5

    Depending on your filter holder, you might have luck with Practical Pleat filters, which let you get a thicker & higher MERV filter into a 1" housing. I'm using them now because my installer didn't put in the filter box he was supposed to.

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