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  1. #1
    Join Date
    Jun 2009
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    147

    External Static Pressure Question

    My return static:

    before filter -.10
    after filter rack, in blower compartment, with no filter in place -.15
    with filter (cheap blue fiberglass) -.35!

    Why in the world does my drop over a cheap blue fiberglass filter go as high as -.20?? I'm using a magnehelic 1.0 wc pressure guage with a probe. I'm inserting the probe in a preexisting hole on the side of the blower compartment.

    The supply static before coil is .35, so my total ESP is .70.... But, I'm still puzzled by why the filter is causing so much pressure. Could it be that I'm putting the probe in the most turbulent part of the blower compartment?

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,058
    Thats possible.
    Could also be the air filter is too small.
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  3. #3
    Join Date
    Jun 2009
    Posts
    147
    Quote Originally Posted by beenthere View Post
    Thats possible.
    Could also be the air filter is too small.
    You're probably right about the filter being too small. The contractor told me to use a 20x20, but a 15x20 would cover the opening to the furnace.

    Is it normal for there to be a pressure drop after the filter rack even without a filter in place? I have a drop of -.05 even when there is no filter... is that one of the signs that the opening is too small?

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,058
    Opening tpp small.
    Turbulance where your measuring.

    Either one.
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  5. #5
    Join Date
    Jun 2009
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    Quote Originally Posted by beenthere View Post
    Opening tpp small.
    Turbulance where your measuring.

    Either one.
    Would you suggest drilling a hole in the middle of the blower door to get a more even measurement?

  6. #6
    Join Date
    Jun 2005
    Location
    SW Wisconsin
    Posts
    4,985
    I had a reply ready to send & the power went off.

    So, this will be short.

    What tonnage & CFM are we talking about?

    Is there a knockout to use a larger filter?

    Beenthere answered your questions.

    A manual D might be advisable.

    It apparently needs more filter area!

    Probe areas - use your own judgment. - Darrell
    Last edited by udarrell; 08-14-2009 at 10:01 PM. Reason: Typo - probe areas

  7. #7
    Join Date
    Nov 2005
    Location
    India
    Posts
    46
    Check velocity across filter if it higher than 400-500 feet per minute then also static will be high.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,058
    Quote Originally Posted by david8370 View Post
    Would you suggest drilling a hole in the middle of the blower door to get a more even measurement?
    I don't suggest any DIY actions.
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  9. #9
    Join Date
    Jun 2009
    Posts
    147
    Thanks guys.

  10. #10
    Join Date
    Jun 2009
    Posts
    147
    Quote Originally Posted by udarrell View Post
    I had a reply ready to send & the power went off.

    So, this will be short.

    What tonnage & CFM are we talking about?

    Is there a knockout to use a larger filter?

    Beenthere answered your questions.

    A manual D might be advisable.

    It apparently needs more filter area!

    Probe areas - use your own judgment. - Darrell
    Thanks udarrell. There is actually about 2 inches more of knockout on the side that the contractor did not open because the return duct was not wide enough. I guess I'm kind of screwed for now until I can afford to get my return duct modified.

  11. #11
    Join Date
    Jun 2009
    Posts
    147
    Quote Originally Posted by beenthere View Post
    I don't suggest any DIY actions.
    I hear ya, man. Thanks.

  12. #12
    Join Date
    Apr 2008
    Location
    Princeton, NJ
    Posts
    581
    Post 44 in the attached thread links you to a video illustrating static pressure testing.

    Key1



    http://hvac-talk.com/vbb/showthread....light=upstairs
    In the land of the blind.....the man with one eye is king....

  13. #13
    Join Date
    Jun 2009
    Posts
    147

    Hmm

    Quote Originally Posted by key1cc View Post
    Post 44 in the attached thread links you to a video illustrating static pressure testing.

    Key1



    http://hvac-talk.com/vbb/showthread....light=upstairs
    Thanks Key1. That's the video I initially used for guidance. The only difference is that I put the probe on the side of the blower compartment through a hole that already exists instead of in the middle of the blower door. Wonder if the location makes that much of a difference.

    In the video, the guy says to put the probe 90 degrees to the airflow, but I've also seen where the instruction is to put the probe pointing into the airflow... which one is correct?

    I realize this is a bit DIY, but, in fairness, I've had 2 different contractors out who didn't know the proper way to measure static pressure.

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