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Thread: Home furnace filter - Is this right?

  1. #16
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    Quote Originally Posted by Prashun Patel View Post
    Bill, I did, but they maintain that it's right. They contend that because it's in the return path, it does not require to be sealed; all it means is that some return air may come in from the furnace room. The argument seemed logical.

    They were not, by the way, the cheapest option out there. I've used them for other work and they have been professional.
    It seems logical, but-- . The dirty unfiltered air from your furnace room is drawn in on the sides and bottom thus unfiltered. I'd want it sealed. I'm not an hvac expert, but I'm not selling anything either.

    I have to unscrew a cover that has a rubber gasket on mine. It's a hassle but it's airtight. I like that because my furnace room is my shop!

    Furnace filter info gets so complicated. From thick filters that must slow down air flow (just like a dirty thin filter). That's hard on the system, but picks up more dust. For me a thin cheap filter changed or vacuumed every month or two works well.
    Last edited by Andrew Joiner; 02-16-2018 at 3:03 PM.
    "Whether you think you can, or you think you can’t - you’re right."
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  2. #17
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    Thank you, Brad. I will get this resized properly...

  3. #18
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    Quote Originally Posted by Brad Adams View Post
    The issue is that it is too small a filter. I gaurantee that the total external static pressure on the furnace is above what it is rated at. Does the furnace whistle or change air flow sounds with the filter in? I size, design, and install these things for a living. IT IS TOO SMALL.
    Where do you see that the filter is to small. It goes from one side to the other and if it is pushed in all the way then it is the size of the return duct. Are you saying the duct is to small. Mine is like the picture and it has rails that you can't see when the filter is in place. No whistling and it fits tight
    Last edited by Jerome Stanek; 02-16-2018 at 5:51 PM.

  4. #19
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    It doesn't really have to be "sealed"--the contractor is correct about that, but that's a darn small filter/air return...I didn't notice the dimensions previously.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  5. #20
    Quote Originally Posted by Jerome Stanek View Post
    Where do you see that the filter is to small. It goes from one side to the other and if it is pushed in all the way then it is the size of the return duct. Are you saying the duct is to small. Mine is like the picture and it has rails that you can't see when the filter is in place. No whistling and it fits tight
    I'm guessing Brad is saying the size of the filter is too small relative to the size of the duct in terms of static pressure vs. volume (not just that it physically doesn't fit the space). A 4" thick filter, for instance, would "fit" in the same cross-sectional area but have much greater filter surface area.

  6. #21
    Quote Originally Posted by Jerome Stanek View Post
    Where do you see that the filter is to small.
    The filter is too small in the sense that it is not the correct size to allow full airflow through the furnace. Not that it fits in the space. A filter can only flow a certain cfm at a given fpm. From experience I can tell you that the pressure drop across a small pleated filter like that is at least .8" of static pressure. A furnace is normally rated at .5" static total external pressure drop. This will cause issues with a high temp rise across the heat exchanger and much decreased motor life. The duct is fine.

  7. #22
    Quote Originally Posted by Dan Friedrichs View Post
    I'm guessing Brad is saying the size of the filter is too small relative to the size of the duct in terms of static pressure vs. volume (not just that it physically doesn't fit the space). A 4" thick filter, for instance, would "fit" in the same cross-sectional area but have much greater filter surface area.
    +1 ^^^^ and give a lot less pressure drop across it

  8. #23
    Because it's a gas furnace, unless it has a sealed combustion system, then filter area should be sealed to prevent exhaust gases from being drawn back down flue pipe. Since I can't see filter size, I making an assumption on my recommendation. Use and "A" filter rack in return duct. This will double filter area, and get you to what Brad recommends. You can't fit a 10" filter into a 10" duct that has interior insulation. If this is the case, then installer should have replaced this section of duct with some with exterior insulation.
    Last edited by Bruce Wrenn; 02-16-2018 at 9:51 PM.

  9. #24
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    To get a bigger filter wouldn't you need a larger return. There are many unknowns such as how big is the house and is the return the correct size. just from a picture you can't tell if it is to small you would need to check air flow and balance of the system. If everything works as it should then it is the correct size. I can not tell if that filter is a mesh one or a pleated one it looks to be mesh
    Last edited by Jerome Stanek; 02-17-2018 at 6:23 AM.

  10. #25
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    I’m no HVAC expert, but I jusr had our furnace replaced as well. They installed a box filter, which they highly recommended for the size of our furnace. I’d look into having something like this installed.

    C82EC7A8-18A9-48EA-B084-872AF38769EC.jpg F6DE017D-2F97-48DD-837E-1FC388495277.jpg

  11. #26
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    My setup is similar to Phil's, except instead of a single filter, I have 4 filters stacked--a washable/reusable filter is hit first by the incoming dirty air, then two somewhat inexpensive disposables, then a premium allergen filter. My (and my family's) sinuses are thankful...

    As for the filter in the OP, I've seen plenty of that type of setup, but I would agree that it should probably be moved to the section of duct that is horizontal--the larger cross-section means the air velocity is much lower and more particles will drop out of the air and cling to the filter there, and the filter will be larger to boot (less static pressure drop across the larger, higher-flowing filter).
    Last edited by Jason Roehl; 02-17-2018 at 8:47 AM.
    Jason

    "Don't get stuck on stupid." --Lt. Gen. Russel Honore


  12. #27
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    Quote Originally Posted by Brad Adams View Post
    The filter is too small in the sense that it is not the correct size to allow full airflow through the furnace. Not that it fits in the space. A filter can only flow a certain cfm at a given fpm. From experience I can tell you that the pressure drop across a small pleated filter like that is at least .8" of static pressure. A furnace is normally rated at .5" static total external pressure drop. This will cause issues with a high temp rise across the heat exchanger and much decreased motor life. The duct is fine.
    It's not clear what you are recommending be done here. Could you please elaborate? Are you saying they should have a thicker filter, or bigger ductwork, or What? My own furnace setup is similar except the filter drops in vertically at the bottom of the duct, right where it enters the furnace.

  13. #28
    Quote Originally Posted by Jerome Stanek View Post
    To get a bigger filter wouldn't you need a larger return.
    No, it could merely be a thicker filter (like the one Phil pictures).

    Think about it: If you had a flat piece of filter material that was 10"x15", you could fold it up ("pleat" it) to be, say, 1" in thickness (measured between the peaks of the pleats), but fit in a 10x10x1 frame. You have 10*15=150sq in of filter material. If you start with a piece of flat filter material that's 10"x60", you could fold it up to be 4" thickness, and have it fit a 10x10x4 frame. This filter has 4x as much surface area, so 4x LESS pressure drop for a given air velocity.

  14. #29
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    Quote Originally Posted by Dan Friedrichs View Post
    No, it could merely be a thicker filter (like the one Phil pictures).

    Think about it: If you had a flat piece of filter material that was 10"x15", you could fold it up ("pleat" it) to be, say, 1" in thickness (measured between the peaks of the pleats), but fit in a 10x10x1 frame. You have 10*15=150sq in of filter material. If you start with a piece of flat filter material that's 10"x60", you could fold it up to be 4" thickness, and have it fit a 10x10x4 frame. This filter has 4x as much surface area, so 4x LESS pressure drop for a given air velocity.
    I would see this as 4x as much restriction.

  15. #30
    Quote Originally Posted by Pat Barry View Post
    I would see this as 4x as much restriction.
    Then by your logic, shouldn't every filter be as small as possible?

    If you have the same amount of air moving through a larger surface area, the volume of air per unit area is less.

    See this chart:
    http://www.homeenergy.org/UserFiles/...-6_p36bbig.jpg

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