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Building Science

Seven Reasons Your Filter Isn’t Cleaning the Air

Address these problems and you could have better indoor air quality

Just having a nice filter doesn't guarantee clean air. In this case, a lot of air bypasses the filter.

Indoor air quality (IAQ) researchers have established that little bitty invisible pieces of stuff that float around in the air are bad for human health.  Of course, being scientists, they don’t call it little bitty invisible pieces of stuff.  They call it particulate matter (PM) or particulates.

And it turns out that the really small pieces — the stuff that’s 2.5 micrometers (0.0000025 m, or 2.5 µm) or smaller, abbreviated PM2.5 — is worse than the bigger stuff because it can penetrate deeper into the lungs and more easily find its way into your blood than the bigger particulates.  The chart I included in my last article shows that PM2.5 is probably the worst indoor air pollutant overall.

And how do you deal with it?  One of the primary methods is to filter it out of the indoor air.  In most homes, the only place where filtering of the air occurs is in the heating and air conditioning system.  But if you think you’re covered just because you have a forced-air HVAC system which has a filter, let me give you a few reasons why that filter — the filter that you so dutifully change — may not be helping your IAQ.  (You do change your filter, don’t you?)

It’s true.  If you don’t have a filter, you’re not gonna get much filtration.  And yeah, it really happens.  Sometimes someone removes the filter because it’s in a difficult spot to reach, like a crawlspace.  Sometimes they take it out and forget to put it back in.  Sometimes… well, who knows!  All kinds of things happen.  But if you don’t have a filter, not only are you not improving your indoor air quality, you’re also getting your ductwork, blower, air conditioner coil, furnace heat exchanger, and everything else in there dirty.

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  1. Canada_Deck | | #1

    My forced air electric furnace uses a hammock filter. The furnace is old but still running fine. However, the hammock mechanism has seen better days and the filters are a serious pain to replace. Aside from inspiring a lot of cursing, it leaves bypass gaps and I haven't been able to find high quality filters in that format so I question how well it cleans the air even if it was possible to install it well.

    I'd like to eliminate the hammock filter and install a box that will allow me to use the standard 16"*20" filters. Has anyone seen that retrofit done before?

    1. User avater GBA Editor
      Allison A. Bailes III, PhD | | #2

      Canada_Deck, my first suggestion is to replace the electric furnace with a heat pump. Unless you live in Miami, Florida or Harlingen, Texas or some other place with little heating load, it's costing you a lot of money. If your pen name here references where you live, you definitely should get rid of the electric furnace. Electric resistance heat may be 100% efficient at turning electricity into heat, but a heat pump would 200-300% efficient by the same measure.

      Now, the hammock filter also should go. I'm sure you're correct about bypass and you're also limited on how much actual filtration you can do with that type of filter. I suggest installing either a filter cabinet at the return plenum or see if filter grilles can work for you. In either case, aim for a size of 1 square foot for each 200 cfm of air flow.

  2. Armand Magnelli | | #3

    In my experience, the filter housing cover is often leaky, damaged, or removed completely. This product is a relatively inexpensive way to reduce the air bypass at the opening of the filter housing for some systems, although it will not work in all instances.

    Allison, I'm looking forward to your next article and the issue of high MERV filters and the potential for reduced air flow.

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