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Air Conditioner Filters

  1. Apr 16, 2006 #1
    I have 2 places to put air filters on my home system. One is a 16x20x1 at the wall intake and the other is 4ft up the same duct at the unit (16x25x1). The filter in the wall is a pain in the ass to change in comparison to the one at the unit. But, they are both usually dirty at 3 months so I have continued to replace both.

    The repair man that came out recently told me I only needed one filter, either the wall or at the unit. Is this correct? I am worried about the trash that has been collecting on the second filter up line from the first just going into the unit. Or, could 2 filters be placed against each other at the unit or would this be extra strain on the unit having them directly up against one another?
  2. jcsd
  3. Apr 16, 2006 #2
    I would guess that two at the larger opening would be less stress than one large and one smaller since the air is moving slower there.
  4. Apr 16, 2006 #3
    I didn't know if having them back to back would be any different than having a 4ft separation between 2 of them, in terms of pulling required by the motor. I guess it should not make any difference.
  5. Apr 16, 2006 #4


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    I would caution againt having two filters. I learned something similar last winter with my forced air system and having too much back pressure against the blower. I was using one of the 3M super duper filters that are pushed by Home Depot. It caused a lot of problems with my unit. The repair guy that I had to have come out told me never to use the 3M's because of that very reason.

    I would think that your situation would be similar. However, if you are already doing it with no troubles, then perhaps your unit can handle the pressure drop.
  6. Apr 16, 2006 #5
    Thanks for the advice. Coincidentally I just ordered a 3M filter from Home Depot online about 15 minutes ago because my brother was saying it was stupid for buying a cheaper one at this time of the year because of the pollen. The furnace has been pulling through 2 cheap filters for several years, the only problem has been a coil leak and I replaced the coil.

    I am not really sure what to do now. I have the cheap filter installed at the unit and I have the expensive one ordered for the wall. I definitely won't run them both since you said that.
  7. Apr 16, 2006 #6


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    The 3m ones are good, but you do have to watch them - because they are good, they load up quickly.
  8. Apr 16, 2006 #7


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    Same experience with the 1" thick 16x25 $10 and $15 filters from Home Depot hurting the flow.

    In fact, did some testing and found it was so restrictive that lots of air made it past the filter's edges because it was causing that much of a restriction. It was mounted like junk too, but even with some new brackets it would flex and leak, very frustrating.

    Instead, went on ebay and found an AprilAire electrostatic 5" thick paper filter. It was only $400 at the time, and that is so much cheaper than paying a shop markup+labor it seemed worth it to try it. Price has since went up, and you're not supposed to be able to buy them alone and put them in yourself. In fact, if you're not up to electrical code and wiring and the sheetmetal (or fiberboard) work you maybe shouldn't try to put it in.

    It was an incredible amount of work to put in, not so much rigging up a sheet metal brake and making the connecting metal box to space up the unit so I could slide it out and clear my gas line that was haphazardly ran, but just building a 2x4 platform in the confines of the attic trusses and braces with an extra foot of insulation blown in was a nightmare. Had it been like a house in the northern lattitudes where the furnace would be in the basement it would have been 1/10 the work without the confined area.

    Anyways, to the point, after installation the difference was very impressive. It was like the blower was on low before, you'd have to put your hand in front of the regsiters to see if it was blowing air. Now you could just walk by them and you could feel it on your face and arms! Seemed to cool a lot better, no hard data or measurements but it seemed to make the system much more efficient. My neighbor does home remolding, he took a step back when I turned the system on and he felt the air coming from the register in the dining room, so he was very impressed too.

    Try taking out the filters completely and see if there is a difference in flow that you can feel. If so, in the future you might want to try to install a holder for a 4" thick filter from Home Depot, you get a lot more surface area from the same sized opening. This means less restriction and will get close to the flow rate without a filter.

    It might be all in my head, but with putting in a new filter in my unit just recently I feel much less of the pollen effects here in Atlanta now then I did before replacing the filter (recommended every 12 months but it was so dirty I think I'm going to do 6 months).

    Maybe one of the HVAC engineers who post here regularly could post data on how much those filters affect the units performance instead of just anecdotes.

    For now, you might just want to experiment with one filter and see which works best. Fancy filter to knock out pollen, cheapie filter if it flows better and takes out enough pollen to be liveable.
  9. Apr 16, 2006 #8
    I will try without the filter and check the flow. I am also in Atlanta and this pollen has been hurting me. It is even worse where I work than at my house. The past 3 weeks I cough and sneeze as soon as I walk in, I need to complain about it.
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