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Thread: Static Pressure

  1. #1
    To get the amount of static pressure , Do I add the drop across the

    Return Ducts +filter +return grilles+ Static Pressure

  2. #2
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    huh?

    you may want to give that question a second shot.

    start with terminology:
    e.g. "total static" or "static available" ???

  3. #3
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    The number I find useful is the External Static Pressure (ESP) of the air handler. This identifies the workload the air handler sees, and it being too-high can be a strong hint of duct deficiency for a given amount of airflow. This is measured by the pressure difference between the vacuum in the return plenum just before the air handler, and the positive pressure in the supply plenum just after the air handler. Since one is negative, it's best to just ignore the +/- signs and add the two numbers together. For example, if the return plenum is -0.4 inch water column and the supply plenum is 0.3, then the ESP is 0.7.

    I do believe a purpose of Manual D is to calculate a number for ESP before you actually build the system. I myself have been measuring numbers in an existing system. Maybe some of the pros can critique what I say, however I must say this is a question for the "Residential" board, not ideally the "Indoor Air Quality" board.

    Am I even talking about the question you wanted to answer?


    Hope this helps -- Pstu

  4. #4
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    "This is measured by the pressure difference between the vacuum in the return plenum just before the air handler, and the positive pressure in the supply plenum just after the air handler"
    I've often wondered and haven't read or seen anything on it, the pressure measured in the return plenum, is that with or without a filter in place?

  5. #5
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    I believe you want to measure that with your choice of filter in place. This is what one or two pros affirm is the right way to measure. I would have thought taking out the filter is a good way to make more reproduceable results, but they don't recommend that way to measure.

    If you have highly restrictive filters (e.g. the infamous 3M Filtretes with a couple months usage) you may find the ESP is too high and you have a problem. Putting less restrictive filters on, is one way to somewhat reduce ESP. It may solve a problem, if and only if you have a marginal situation in the first place.

    A lot of pros on this board will badmouth the 3M filters and rightfully so, but what they seldom say is... they are trying to give advice without measuring ESP. That measurement is something they can professionally do, and I wish they all would before giving such advice. In the absence of a measurement, they tell you to avoid restrictive filters always, because *sometimes* they are harmful. The hazard is real and might kill something expensive, but only if adding that filter raises ESP above tolerable levels. In *my* system I am convinced it will not.

    I measure my systems' ESP constantly, using a permanently mounted Dwyer manometer on a closet wall. Some AC techs think it's undesirable to do business with a customer who knows this kind of stuff. On the other hand some AC techs I have met, don't own the tools that can do this measurement.

    You can purchase the ACCA Manual D in book form and in theory, learn how to calculate ESP, airflow, all the important things on a duct system on paper. In this book you will find some attention to filters, grilles, etc which will relate to your original question. I own this book but in my opinion it takes a really dedicated person to learn it himself. If you are really a numbers guy, perhaps an engineering type, you just might be the right kind of person.

    Hope this helps -- Pstu



    [Edited by pstu on 07-27-2006 at 01:32 PM]

  6. #6
    Originally posted by acdcacdc
    To get the amount of static pressure , Do I add the drop across the

    Return Ducts +filter +return grilles+ Static Pressure

    Call Rob Falke at the National Comfort Institute, 440-949-1850. He'll be happy to give you an easy to use formula to determine static pressure.
    Ron Rajecki
    Contracting Business magazine
    HPAC Engineering magazine

  7. #7
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    804
    Originally posted by pstu
    I believe you want to measure that with your choice of filter in place. This is what one or two pros affirm is the right way to measure. I would have thought taking out the filter is a good way to make more reproduceable results, but they don't recommend that way to measure.

    If you have highly restrictive filters (e.g. the infamous 3M Filtretes with a couple months usage) you may find the ESP is too high and you have a problem. Putting less restrictive filters on, is one way to somewhat reduce ESP. It may solve a problem, if and only if you have a marginal situation in the first place.

    A lot of pros on this board will badmouth the 3M filters and rightfully so, but what they seldom say is... they are trying to give advice without measuring ESP. That measurement is something they can professionally do, and I wish they all would before giving such advice. In the absence of a measurement, they tell you to avoid restrictive filters always, because *sometimes* they are harmful. The hazard is real and might kill something expensive, but only if adding that filter raises ESP above tolerable levels. In *my* system I am convinced it will not.

    I measure my systems' ESP constantly, using a permanently mounted Dwyer manometer on a closet wall. Some AC techs think it's undesirable to do business with a customer who knows this kind of stuff. On the other hand some AC techs I have met, don't own the tools that can do this measurement.

    You can purchase the ACCA Manual D in book form and in theory, learn how to calculate ESP, airflow, all the important things on a duct system on paper. In this book you will find some attention to filters, grilles, etc which will relate to your original question. I own this book but in my opinion it takes a really dedicated person to learn it himself. If you are really a numbers guy, perhaps an engineering type, you just might be the right kind of person.

    Hope this helps -- Pstu
    Pstu:

    What model of Dwyer manometer do you have?



    [Edited by pstu on 07-27-2006 at 01:32 PM]
    Bill

  8. #8
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    Dwyer Model 25

    It is a Dwyer Model 25 with a range up to 3 inches w.c. (water column) using red oil.
    It looks like Ebay items 320011489555 or 190012354216 :

    http://cgi.ebay.com/DWYER-MARK-II-MA...QQcmdZViewItem

    http://cgi.ebay.com/Dwyer-Mark-II-ma...QQcmdZViewItem

    A tube goes thru the wall or attic, to reach the AC supply and return plenums.
    A pro would prefer a Magnahelic gauge because in a toolbox, oil might leak.
    Several times the price but worth it professionally.
    For my semi-permanent wall installation, oil is no problem.
    A nice device with basically no moving parts.


    Hope this helps -- Pstu

  9. #9
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    I think that maybe you have mentioned this before but what are you using it for or what's it telling you about your system?
    Bill

  10. #10
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    Why it matters

    For awhile I had been told (by a pro) that an AC system only delivered 723 CFM,
    at that time we thought it was 3.5-4.0 tons and should have delivered 1400-1600 CFM.
    I was told it had 0.7 inch ESP (External Static Pressure measured in inches water column),
    and we made plans to add return capacity since return vacuum was clearly too high
    at 0.45 inch in that plenum alone. That made me highly aware that ESP could be
    a tool for testing system malfunctioning. Since the Dwyer 25 was available
    for well under the cost of a service call, I bought it.

    There are two AC systems in my house, the second one was *not* diagnosed as having
    such a problem. But I connected one manometer to its return plenum anyway,
    since it was easy and I was curious. From that I experimented with
    different filters and concluded that on a well designed duct system, it *was*
    possible to have a high MERV filter and not see too-high ESP. That was a
    valuable lesson right there, one contrary to many statements you will read on this board:
    Those infamous 3M filters are potentially poison if your ESP is already too high,
    but if you monitor this measurement you may find they are OK for your air system.

    The Carrier Infinity control will tell you when a filter change is needed, the algorithm
    based on the ESP rising as the filter loads up. Sounds simple, elegant, and powerful.
    With this manometer installed, I can visually monitor the same thing.
    I find they load up far slower than I had expected, so I need to buy fewer filters.


    Subsequently I learned that the problem system had *better* ESP with 2 added returns,
    but it wasn't as great as I had hoped. I saw about 0.1 inch improvement in ESP.
    By watching the different fan speeds on a 2-stage furnace, got a pretty good
    idea how much ESP declines when airflow (CFM) is reduced. That was valuable info
    that served to troubleshoot a problem when a dehumidistat was installed incorrectly
    -- could see the airflow was not reduced as it was supposed to, and
    since the tech was right there it made the problem easy to fix.

    That is a summary of why it pays to know your ESP under various conditions.

    Hope this helps -- Pstu

    P.S. FWIW there was a problem with the original 723 CFM diagnosis. I think the flow hood was used incorrectly, but also
    the tech failed to even see the air handler was not set to its high speed.
    Another tech set the air speed in about a minute, and retested airflow at 1130 cfm.
    Never forget the basics!! It made me sad but convinced me I needed to part ways with that original AC tech.

    [Edited by pstu on 07-31-2006 at 11:00 AM]

  11. #11
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    Re: Why it matters

    Hmm, managed to delete my old post on a edit!

    There is a rule of thumb, 400CFM/ton, correct? The house belonged to my parents and my Father always kept all the paperwork. I was digging and found out that the AC is a 2 ton unit. Never seen any nameplate with static pressure as yet!

    This old relic has a Honeywell F50A EAC that works better than it's supposed to. All comments about particle counts receive no comment because of these newer units on the market today.

    I was thinking about goiont to a media filter but if the PD is too and raises the ESP too high, that's not good!

    I think that a manometer is in my future! Thanks for the reply. Can homeowners get technical outside the Pro sections??

    [Edited by wptski on 07-31-2006 at 12:42 PM]
    Bill

  12. #12
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    Look for "Maximum External Static Pressure" on the nameplate. It should be on there.. often quite abbreviated. "MAX ESP" or "MAX EXT STC" or something like that.

    And yes, 400CFM/ton is a general rule of thumb for airflow. There are lots of reasons to change the airflow per ton in specific situations, but 400CFM/ton is the standard.

  13. #13
    Join Date
    Dec 2002
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    143
    PSTU,

    Could you drop me an email.... look on my profile, I want to pick your brain about something off the forum.

    Thanks,
    Matt

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