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  1. #1
    Join Date
    Feb 2009
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    110

    Filter Box Size and Furnace Cutout Size?

    If you have a 20x25 filer box and media filter butted against the side of the furnace, are you losing filter capacity if a typical furnace cutout is 15x22?

    In all of the pictures, you usually see the filter against the side of the furnace, but what's the point of a 20x25 when a 16 x 25 covers the whole cutout area? Would it be better to move the filter cabinet away from the furnace and have a metal transition piece reducing down to the furnace cutout?

  2. #2
    Join Date
    Nov 2000
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    1,251
    You're the first person I've seen notice this. The instructions from the filter manufacturer say that there should be a transistion of no more than 30 degrees downstream of it.
    "You do not merely want to be considered just the best of the best. You want to be considered the only ones who do what you do.".... Jerry Garcia
    "You just have to decide if you're a Tigger or an Eeyore,..I think you know where I stand on the great Tigger/Eeyore debate." Prof. Randy Pausch

  3. #3
    Join Date
    Feb 2009
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    110
    Quote Originally Posted by lbf View Post
    You're the first person I've seen notice this. The instructions from the filter manufacturer say that there should be a transistion of no more than 30 degrees downstream of it.
    At 30 degrees for 4" height, then about 5" width for the transition should do it.

    Anyone know why they butt those 20x25 filters against a 15x22 cutout in the furnace? Doesn't seem to make sense.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,308
    Because it looks nicer then this.
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    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Mar 2009
    Location
    Indianapolis, IN
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    80
    New member, long time lurker, first response.

    Usually the job goes to the lowest bidder or a "handy man" instead of trained, caring professionals. The customer usually don't know any better and the system is forgiving enough to get by with this type of work. Most people will either change the filter long before it is necessary (not many) or will not change it nearly often enough (most people).

    Do what is correct and sleep well at night.

    Just a thought.

    shephvac

  6. #6
    Join Date
    Feb 2009
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    110
    Quote Originally Posted by shephvac View Post
    New member, long time lurker, first response.

    Usually the job goes to the lowest bidder or a "handy man" instead of trained, caring professionals. The customer usually don't know any better and the system is forgiving enough to get by with this type of work. Most people will either change the filter long before it is necessary (not many) or will not change it nearly often enough (most people).

    Do what is correct and sleep well at night.

    Just a thought.

    shephvac
    Yup. People should know. I wouldn't mind paying a bit more for it, but we need to know. I asked a couple of contractors about this and they asked me why would I want a transition between the filter and furnace? I had to explain about the area of the filter, etc. Virtually everyone talks about equipment numbers, features, etc, but no one wants to review duct work in detail. I guess it doesn't pay to talk ductwork and installation when people just compare prices on pure equipment.

    The "cost less than this" in BT's drawing is the way to go.

  7. #7
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,308
    Seen a lot of those type installs.

    Next thing I've heard over the years. Is that the size of the opening of the furnace is X dimensions, so you don't need bigger then that from the air cleaner. So mounting it against the furnace is fine.
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  8. #8
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Take at look at what beenthere posted and consider mounting the air filter/cleaner/purifier in the vertical drop of the return,with appeopriate transitions,then a 90 degree elbow with turning vanes int the furnace.

  9. #9
    Join Date
    Feb 2009
    Posts
    110
    Quote Originally Posted by dash View Post
    Take at look at what beenthere posted and consider mounting the air filter/cleaner/purifier in the vertical drop of the return,with appeopriate transitions,then a 90 degree elbow with turning vanes int the furnace.
    Unfortunately my return is 12x20 and would require quite a bit of work to do that.

    Virtually everyone I asked if they butt the filter beside the furnace said yes. When I asked about the cutout size, they said they would build a transition, but it would be extra. I told them to put it the quote. I guess that most people look at the bottom number and it's tough for these companies to put this stuff in and lose by a few hundred. It forces them to do what everyone else seems to be doing.

    Well, at least I will have it done correctly

  10. #10
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Quote Originally Posted by noobs View Post
    Unfortunately my return is 12x20 and would require quite a bit of work to do that.

    Virtually everyone I asked if they butt the filter beside the furnace said yes. When I asked about the cutout size, they said they would build a transition, but it would be extra. I told them to put it the quote. I guess that most people look at the bottom number and it's tough for these companies to put this stuff in and lose by a few hundred. It forces them to do what everyone else seems to be doing.

    Well, at least I will have it done correctly
    Will the elbow have turning vanes,or a p0roperly sized inside radius?

    If not air flow will likely be uneven over the filter,which reduces the effectiness and increases the presure drop(resistance 0 thru the filter.

    Often there's not enough space to and a transition ,without moving the return drop away from the furnace.Can you post a pic of the furnace and drop?

  11. #11
    Join Date
    Feb 2009
    Posts
    110
    Here are some pictures. The return is about 13.5 inches from the furnace. There is a picture of the filterbox and supply duct. The existing furnace is narrow, only 17" wide.

    Do I have enough space?

    P.S. I bought that filtrete filter before I started reading posts here. It will be the last one
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  12. #12
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Quote Originally Posted by dash View Post
    Take at look at what beenthere posted and consider mounting the air filter/cleaner/purifier in the vertical drop of the return,with appeopriate transitions,then a 90 degree elbow with turning vanes int the furnace.
    Here are photos of my 20x25 filter installed in the vertical drop of the return as reference by Dash. (Before and After photos).
    Key1cc
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    In the land of the blind.....the man with one eye is king....

  13. #13
    Join Date
    Nov 2000
    Posts
    1,251
    Quote Originally Posted by noobs View Post
    Here are some pictures. The return is about 13.5 inches from the furnace. There is a picture of the filterbox and supply duct. The existing furnace is narrow, only 17" wide.

    Do I have enough space?

    P.S. I bought that filtrete filter before I started reading posts here. It will be the last one

    You have a lot of space, a good sheetmetal company would have no trouble making a nice installation.
    "You do not merely want to be considered just the best of the best. You want to be considered the only ones who do what you do.".... Jerry Garcia
    "You just have to decide if you're a Tigger or an Eeyore,..I think you know where I stand on the great Tigger/Eeyore debate." Prof. Randy Pausch

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