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Thread: Static Readings

  1. #1
    Join Date
    Apr 2007
    North Ga.
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    Static Readings

    My A/H has a Dwyer Model 2001AV magnehelic differential gauge installed on it permanently.

    The high pressure side is connected to the S/A plemun---and--- the low pressure side is connected to the R/A plemun via. small aluminum tubing. see photos

    The A/H blower is set on the enhanced mode---and---the CFM is set on low--> 350cfm per ton--it is a 2 ton unit. ( 700 cfm at 100% )

    The readings with both little plastic valves on the magnehelic gauge opened and the A/H blower at 100% ( 700 cfm ) are about .14"wc ...see photo

    Going with the information I have read on this site this seems low. Seems like the average I should be looking for would be around .50" wc

    Any ideas if I am looking at this thing wrong---or that I may have a issue with static/duct configuration?


  2. #2
    Join Date
    May 2007
    Naples, Fl
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    Fig 2 Looks like you are up stream of the filter. The probe needs to be downstream of the filter. That'll get that static up for you.

  3. #3
    Join Date
    Jan 2004
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    Yea, looks like your on the wrong side of the air filter.
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  4. #4
    Join Date
    Aug 2006
    Dallas (Plano), TX
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    Quote Originally Posted by Bpawb View Post
    ... The readings ... with the A/H blower at 100% (700 cfm) are about .14"wc ...

    Going with the information I have read on this site this seems low. Seems like the average I should be looking for would be around .50" ...
    If you're trying to measure your unit's ESP, and it's being accurately measured, and it's consistent with whatever footnotes AS publishes, then it's an excellent number, probably representative of not a lot of existing duct the blower has to move air through.

    If your system is a heat pump, where the evap coil is part of the unit, there's probably a footnote that says something like "max ESP includes allowance for wet coil."

    If not, then you need to check if your supply side measurement point is between the blower's output and the coil's input.

    Since your return side measurement point is on the incorrect side of the air filter (should be between the filter's output and the coil's input (if HP) or blower's input (if not HP)), while you could get it moved, another alternative would be to instead leave it there and look up the static pressure resistance of the filter. You'd then always add this number to whatever your dial reads to arrive at your ESP.

    For example, using your reading from your posting, and if we assume your filter offers 0.10" of static pressure resistance at 700 cfm, then, your ESP would be about 0.24".

    If your unit has a common nameplate rating of "max 0.50" ESP," and if your unit's footnotes for the 0.50" ESP account for the things in between your 2 measurement points, then 0.24" as compared to 0.50" would be excellent, and an indicator that your air distribution system is operating pretty efficiently.

    If, due to a low ESP, you have too much air being distributed throughout your residence, then that probably means you have a PSC (non-variable speed) blower. And you thus have room to have your blower speed reduced if indeed that's an option.

    Best regards,


  5. #5
    Join Date
    Jun 2005
    SW Wisconsin
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    That low a reading could indicate major supply air duct leaks.
    With an E-Coil, it is difficult to believe you could get that low an ESP.

    On low speed with large duct sizes & register outlets it might be possible, but very questionable. An oversized duct system situation would produce low velocities & not much throw from SA registers.

    Check all the supply air register FPM velocities & convert 'em to CFM, total 'em up & see if you get that 700-CFM.

    Formula for finding CFM Airflow from Velocity in FPM
    If you can measure the air velocity coming from a known size duct or open area of a SA register, here is a rough ballpark formula to get the CFM:

    CFM = (velocity in (FPM) Feet per Minute times the Square Footage of the duct area). To convert sq.ins. multiply by 0.00694 for sq.ft., or divide sq.ins. by 144.
    - Darrell

  6. #6
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    "The readings with "both little plastic valves" on the magnehelic gauge opened and the A/H blower at 100% ( 700 cfm ) are about .14"wc "

    You are reading the differential between the two.

    Try reading the positive and negative separately,then add them ,ignore the + - signs.
    And yes move the probe to the other side of the filter for the ESP.

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