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  1. #1
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    New Lennox System - Couple Questions & Filter Replacement. HELP!

    First off—new here. And I'm coming in with limited HVAC knowledge. So I appologize up front. Though really interested in HVAC systems and the science behind them.

    I just recently had a new Lennox furnace and A/C unit installed. Along with having all but one of my supply runs replaced and an additional air return added. Right now, I've got three questions that I'd love to get some feedback on:

    - PRIORITY My best option for a 1" replacement air filter in order to run the most efficient system? I'd love to have cleanest air possible, do have some worries about IAQ—but above that I really just want my HVAC system to meet it's fullest potential and be as efficient as possible. *Before* reading through posts on this forum, I purchased Filtrete Ultra Allergen MPR1500 MERV12 filters to try out. My mistake. Once installed I would hear a high-pitched rhythmic humming noise from my main air return (12x12 I believe). Not sure if it's due to pressure drop/velocity and/or grill itself? I next tried installing Filtrete Micro Allergen Extra MPR1085 MERV11. Again the humming noise. Not sure what to do. The filter that came installed with the furnace (without noise) looks to be made by Columbia(?) and says MERV8. So... I figured I'd give the Clean Living MPR600 MERV8 a try—I know it's still Filtrete, but seems like some people here mentioned the 600 is at least a decent option from them?

    - The best option available for air filtration with my system? I know the air filter paired with the furnace should not be expected to be the primary source for clean air in my home, but instead largely there to keep my furnace functioning properly. Just want to know what the best product/setup out there to replace a standard 1" drop-in filter.

    - As a home owner, what I can look for to make sure my system is running efficiently? Are there certain test/diagnostic numbers I can look out for to make sure everything is performing as expected? I can view all this data through my thermostat (Lennox iComfort S30).

    Home info:

    '57 Daylight ranch home in Portland, Oregon. Just over 2,000 sq ft.

    System info:

    Furnace: Lennox M/N SLP98DF090XV48C-11
    Air Conditioner: Lennox M/N 13ACXN042-230-17
    Ductwork: 9 total supply runs (1-5"/6-6"/2-7"); 3 total air returns (newly installed 12x12" through attic); believe the original main trunk line is 8x22 to 8x17 and return I believe to be 8x25 to 8x14

    I can provide any other information needed. Would be great to hear your thoughts. Thanks!

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  2. #2
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    If you can check static pressure. As close to .5" as possible when the A/C is running(under .5" is good also).

    You are still probably short on return. Are all of your returns in the floor?
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  3. #3
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    @beenthere: There are two returns (original) at the floor. Both have grilles that are roughly 8x25" and 8x30" in size. The grilles' fins are also pretty tightly spaced, not sure if that makes a big difference. Then I've got the newly installed I believe (12" dia?) return running through the attic which drops into the ceiling at 12x12" opening.

    UPDATE:

    I placed the Clean Living MPR600 MERV8 filter and am still getting noise, although maybe a little less intense. Very frustrated. Not sure why for the last three months or so it's seemingly operated normally and even now with the clogged up original filter makes no noise. Side note, all three Filtrete filters are about 1/4" larger on one side when compared to Columbia filter. New filters fit a bit more snug into the compartment.

    Also, when the new ceiling return grille is pressed with some force in middle of grille a good portion of the humming sound is alleviated. Again, not sure if this matters—I doubt it has anything to do with the grille itself as haven't had issues beforehand.

    Below is image of new main air return grille:

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  4. #4
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    If that a 12x12 grille, then its only a 10 flex return.

    Remove your air filter completely. Run the system, see if noise gets better or worse.
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  5. #5
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    @beenthere: Yeah, that makes sense being 10". When I get home from work this evening I'll pull out the filter and see how it reacts. Thanks for your help. Is there any reasonable explanation as to why I'm getting a different system response between a brand new filter vs 3-month old dirty filter? So strange to me.

  6. #6
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    Most likely, the newer less restrictive filter is showing you how under sized your return is.
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  7. #7
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    But if that's the case wouldn't I have experienced the same noise when the original filter was new and in theory, also less restrictive?

  8. #8
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    Quote Originally Posted by jeffcwheeler View Post
    But if that's the case wouldn't I have experienced the same noise when the original filter was new and in theory, also less restrictive?
    The no filter test tells us better what is going on. My other post is a guess, because I'm not there to check it myself.
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  9. #9
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    Gotcha. Alright, well I'll give it a try this evening and see what happens. Thanks again—appreciate your time.

  10. #10
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    @beenthere: Just ran the test without any filter installed. Did in fact get the same rhythmic humming noise. Are the any ways to resolve this? Like I said, I haven't experienced this for entire 3 months or so that we've been using the new setup. Just now that I've tried to install a new filter. I couldn't find static pressure readings in the testing & diagnostic sections of the thermostat. I could just be missing it—but may not be a data set that's included via thermostat?

  11. #11
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    Is this new equipment greater in capacity than your old equipment? Upsizing often results in more air noise because of the larger blower.
    1" pleated filters are very restrictive and can lead to noise when customers are led to believe by marketing that a "better" filter is needed.
    Did the system operate quietly in heating mode but become noisy in cooling mode? If so, then the noise can be attributed to a change in blower speed.
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  12. #12
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    Harmonics of the grille. A slight re-angling of the fins will usually stop the noise. Why did it start now, can't tell you/don't know.
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  13. #13
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    @kdean1: The noise isn't air noise. The system has operated as expected while both heating and cooling. Until I tried swapping in a different 1" pleated filter. The harmonic humming only shows itself when a) new filter is installed b) system is run without any filter in place. The a/c operates as normal without any high pitched humming when dirty 3-month old MERV8 filter is being used.

    @beenthere: Alright. I guess I may just remove the grille (or if I can hack it without it looking like garbage) for the time being. At least then I'll be able to replace the very dirty filter with the new Filtrete MPR600 MERV8 filter. Would this be my best option for system efficiency until I find a more permanent solution? I just want to stress the system a little as possible. Thanks!

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