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  1. #1
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    4" media filter with lowest pressure drop

    I currently have a Honeywell 20x20x4 Merv 11 media filter. Part no. FC100A1011. I'm getting .21wc pressure drop across the filter, with a brand new filter. So the filter alone is consuming 40% of my allowable static pressure (.5 total), and thats when its perfectly clean. Does anyone know if there are other brands that have less pressure drop than honeywell? I see some merv 8 filters out there in this same size, but I just cant find a ton of info on any pressure drop comparisons. I'd be willing to go less Merv if i could save on pressure drop.

  2. #2
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    Quote Originally Posted by a_hvac_newbie View Post
    I currently have a Honeywell 20x20x4 Merv 11 media filter. Part no. FC100A1011. I'm getting .21wc pressure drop across the filter, with a brand new filter. So the filter alone is consuming 40% of my allowable static pressure (.5 total), and thats when its perfectly clean. Does anyone know if there are other brands that have less pressure drop than honeywell? I see some merv 8 filters out there in this same size, but I just cant find a ton of info on any pressure drop comparisons. I'd be willing to go less Merv if i could save on pressure drop.
    unit size? I would not put (1) 20x20x4 on anything larger than a 2 ton, we do multiple 4"

  3. #3
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    Quote Originally Posted by lkapigian View Post
    unit size? I would not put (1) 20x20x4 on anything larger than a 2 ton, we do multiple 4"
    2.5 ton. I was thinking the same thing, that 20x20x4 is not large enough. It seems it would have been better off with a 20x25x4 - though not sure that would fit? It seems the existing filter box on the side of the air handler (maybe a honeywell F100) already is a bit large compared to the air handler/return plenum, so not sure its possible to go bigger? though I have heard of a dual filter setup like you're talking about. What about taking the filter out of the media box, and just put the same style filter behind the return grill (I only have one return grill). Like the honeywell FC40R, which is a grill media filter. My return grill is 20x25, so 100 more square inches. It seems the extra surface area would help the pressure drop.

  4. #4
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    Quote Originally Posted by a_hvac_newbie View Post
    2.5 ton. I was thinking the same thing, that 20x20x4 is not large enough. It seems it would have been better off with a 20x25x4 - though not sure that would fit? It seems the existing filter box on the side of the air handler (maybe a honeywell F100) already is a bit large compared to the air handler/return plenum, so not sure its possible to go bigger? though I have heard of a dual filter setup. What about taking the filter out of the media box, and just put the same style filter behind the return grill (I only have one return grill). Like the honeywell FC40R. My return grill is 20x25, so 100 more square inches. It seems the extra surface area would help the pressure drop.
    we use filter grills so we can install multiple filters , not a big fan of 1 filter on the side of an AH, smallest single filter we install is 20x30

  5. #5
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    Quote Originally Posted by lkapigian View Post
    we use filter grills so we can install multiple filters , not a big fan of 1 filter on the side of an AH, smallest single filter we install is 20x30
    So maybe I will try out the honeywell 25x20 grill filter, and see where that takes me. And if i need more air, think about installing a second return somewhere, also with a grill filter. As it is, my total static pressure is exactly .5, with a brand new filter. So once it starts getting dirty, pressure will be too high.

    One question on static pressure. The readings I took are at nominal speed. But in order to take the readings, i set the speed tap on nominal. I normally keep it on low to help with moisture removal. Pressure readings on low are obviously below .5 since theres less airflow. Should I be concerned with too high static pressure at nominal speeds, since i dont ever use nominal?

  6. #6
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    Why be concerned with something you dont use? Before changing your return setup, measure your static with no filter, this will give you an idea of how much air that duct can carry, as you may need more than one or just a larger one. Dropping to merv 8 should lower it noticeably, much less restrictive

  7. #7
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    Quote Originally Posted by Makeitcold View Post
    Why be concerned with something you dont use?

    Makeitcold, I dont understand your question here. Do you mind explaining the question?

  8. #8
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    Sorry, I meant I wouldn't be concerned if your static pressure was above 0.5" wc on a speed tap you dont use

  9. #9
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    Quote Originally Posted by Makeitcold View Post
    Sorry, I meant I wouldn't be concerned if your static pressure was above 0.5" wc on a speed tap you dont use
    Ah ok, that makes sense now. Yeah that was one of my questions. I guess I was under the assumption that it should be .5 or lower at "normal" speeds for the system. In reality i might should run it at full speed in the winter and maybe reduced for the rest of the year for humidity control. I suppose I should see what pressure i have when I use the lower speed tap, which is what its been set at for the last 6 years or so.

  10. #10
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    "Normal speed" is subject to where you live. Las Vegas 450 cfm/ton is going to be normal, New Orleans is going to be 350 cfm/ton or less. Unfortunately they dont design equipment based on regional climate, so we get a one size fits all approach to efficiency. My area 400 cfm/ton is normal but mines set to 350 as I've got a creek behind me that spikes the humidity, If i remember right it was a 2% capacity loss in cooling by doing that, not worth me getting onto the roof to change it back between seasons, imo

  11. #11
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    Quote Originally Posted by Makeitcold View Post
    "Normal speed" is subject to where you live. Las Vegas 450 cfm/ton is going to be normal, New Orleans is going to be 350 cfm/ton or less. Unfortunately they dont design equipment based on regional climate, so we get a one size fits all approach to efficiency. My area 400 cfm/ton is normal but mines set to 350 as I've got a creek behind me that spikes the humidity, If i remember right it was a 2% capacity loss in cooling by doing that, not worth me getting onto the roof to change it back between seasons, imo
    Oh ok gotcha. I should have said "nominal" speed when i said normal above. Nominal is 350cfm/ton with this system. So I've been testing pressure at that speed since I thought thats what i was supposed to do. The low speed fan tap, which is what i normally run it on, is -10% according to the service guide, so i guess that would be 315cfm. Im in NC, where we definitely get the heat during the summer. So humidity is not usually a problem in the middle of summer with the AC running so much. Its more a problem like in the last several weeks when the AC doesnt kick on much as we're heading into fall. We have a creek too, and springs in the yard. In fact its 63% RH in the house right now. Tomorrow I'll set the tap back to Low speed and see what pressure I have. Maybe the whole question this post was about...(finding a filter with less restriction) might be void now on low speed. We'll see.

  12. #12
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    So i set the speed tap on Low. I tested total static pressure before and after. Before: .53, After: .41. Here are the full readings:

    Nominal Fan speed tap:
    1. Before Filter: .15
    2. Between Filter and Coils: .37
    3. Between Coils and Fan: .48
    4. After Fan, in supply plenum: .15
    5. Pressure Drop Over Filter: .22
    6. Pressure Drop Over Coil: .11
    7. Total static pressure: .52

    Low Fan speed tap:
    1. Before Filter: .12
    2. Between Filter and Coils: .29
    3. Between Coils and Fan: .39
    4. After Fan, in supply plenum: .12
    5. Pressure Drop Over Filter: .17
    6. Pressure Drop Over Coil: .10
    7. Total static pressure: .41

    So it does seem that if i keep the tap on low, i should have nothing to worry about as I have .09 wiggle room for the filter getting dirty. It looks like if i ever decided to run at nominal speed instead of Low, then i should consider adding another (smaller) return line somewhere, such as to the master bedroom and maybe consider a merv 8 filter as well.

  13. #13
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    Running another return is a good idea on it's own merit, mixes the air up much better. I added one to our MB when I changed our gaspack, going to install additional runs to the other bedrooms this winter. One central return in the house can work mechanically just fine, but as soon as doors get closed those rooms become isolated from the tstats ability to sense temperature. Looks like you got the static pressure squared away, time to test for duct leakage. DOE says 33% is average!

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