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  1. #1
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    What strategy does a constant CFM blower use to protect against being overworked?

    I have TRANE/AMERICAN STANDARD air handler TEM8A0C48V41DBA.

    Via the control board, installer set constant CFM mode (versus constant torque mode) and CFM per ton to 350.

    Fan-only speed is set to 100%.

    I measure supply duct static pressure (which is high) with DWYER MAGNEHELIC in fan-only mode.

    I remove the HONEYWELL F100 filter and leave the door off, effectively increasing return airflow significantly.

    Supply duct static pressure increases by .07.

    I think this tells me that the blower was not achieving full requested airflow in normal configuration (with filter and door in place).

    What sets the upper limit on airflow (from the blowers point of view)?
    Is the blower module using some algorithm on watts/rpm/static pressure to limit effort?

    Obviously, I am most interested in the answer for my specific blower assembly, but for future reference I am also interested in how constant CFM blower assemblies in general protect themselves from being overworked.

    I have several discussions underway to explore ways to reduce static pressure. I don't need to hear about that.

    I want to understand in more detail the risk of overworking the blower by understanding what protection strategies are used.

    It seems to me that manufacturers would put some logic in the blower control module to maximize the probability of achieving a 10-year life, the typical warranty period.

  2. #2
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    High static pressure is what causes ECM motors to fail. Manufacturers state the maximum static pressure on the ratings plate though it will operate at a higher static up to a point. It is up to the installer to ensure that the external parts of the system don’t contribute to excessive static pressure.
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  3. #3
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    Most modern variable speed ecm have a rpm speed limit set by the manufacturer.

    However long before the blower reaches the rpm speed limiter, the motor is already outside the recommended static.

    The higher the static the higher the rpm till limiter is hit. Also as rpm increases against static, amperage also increases.

    In the old variable speed unit that had a poor rpm limiter, variable ecms used to huff, huff, huff, till the motor Burned out.

    Don’t think the rpm limiter, is enough protection for the motor or unit.

    Total external static pressure is recommended to be .5 or under in most cases. I have found ecm motor issues are guaranteed if TESP is above about .7-.8

  4. #4
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    Thread Starter
    Quote Originally Posted by kdean1 View Post
    High static pressure is what causes ECM motors to fail. Manufacturers state the maximum static pressure on the ratings plate though it will operate at a higher static up to a point. It is up to the installer to ensure that the external parts of the system don’t contribute to excessive static pressure.
    I have read words to this effect hundreds of times before. It does not address my question.

    I think the test I did shows that the blower limits airflow in response to some detected or calculated situation.

    In this case, the air handler information plate does not refer to static pressure in any way.

    The installations instructions left with the equipment have no statement about static pressure goals.

    The only reference to static pressure anywhere is in the fan performance tables, where values for airflow and watts consumed are given to static pressure values of .1 .3 .5 .7 .9.

    If I were the manufacturer, and I didn't want to replace constant CFM blowers under warranty due to installer oversight/error, I would put some logic in the blower control module to limit the top end, and devise a way to raise an alert.

  5. #5
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    Thread Starter
    Quote Originally Posted by Advanced Response View Post
    I have found ecm motor issues are guaranteed if TESP is above about .7-.8
    If manufacturer decided that .9 static pressure was guaranteed trouble, why would they put a column for .9 in the fan performance tables?

    Do manufacturers ask questions about static pressure readings when blower replacement under warranty is requested?

  6. #6
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    If there are repeated blower failures then yes, they will ask for a commissioning report or atleast a static pressure test.

    The chart is just that a chart.

    In the literature or sometimes on the data plate, the manufacturer list the recommended maximum or recommended static pressure. Those charts always go over the maximum number listed.

    Some manufacturer charts also show the amperage at differant static’s to show how much the amperage increases above recommended static.

    Short term high static is always possible, such as when the filter starts to get clogged.

    It used to be that most manufacturers used to grey out the boxes above a certain static but still state the numbers in the box, with a note at the bottom that grayed out boxes are not recommended.
    Now I think they just assume the tech read the whole manual and know the prior listed recommended maximum static.

  7. #7
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    Thread Starter
    Quote Originally Posted by Advanced Response View Post
    In the literature or sometimes on the data plate, the manufacturer list the recommended maximum or recommended static pressure.

    ...

    Now I think they just assume the tech read the whole manual and know the prior listed recommended maximum static.
    Not in this case. No mention of static pressure on the information plate. No mention of static pressure goals/limits in the installation documentation.

  8. #8
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    VS blower motor moduless have a governor in them.
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  9. #9
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    Quote Originally Posted by ANALYST1946 View Post

    What sets the upper limit on airflow (from the blowers point of view)?
    Is the blower module using some algorithm on watts/rpm/static pressure to limit effort?

    A designed combination of torque and speed will deliver a certain air flow. So, VS mode will monitor both.
    Constant torque only monitors torque, therefor air flow reduces as resistance increases.


    I want to understand in more detail the risk of overworking the blower by understanding what protection strategies are used.
    Temperature limiting, current limiting and voltage limiting.

    It seems to me that manufacturers would put some logic in the blower control module to maximize the probability of achieving a 10-year life, the typical warranty period.
    Well, Trane/AmStd. technicians are supposed to be trained. They offer many free classes to their dealers in order to help ensure the products are installed properly. Ducting is one of the design criteria that is heavily discussed in these classes. You can lead a horse to water...
    .
    "The only true wisdom is in knowing you know nothing" Socrates

  10. #10
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    I have not worked with them.

    So:
    Not sure about this, but I think that in the instance of a constant torque motor, if you chose the highest speed, and this was on a clean , brand new system , and then later the system static increased.... the filter wasn't changed , there's some coil dirt, that blower will still, up to a point, try to maintain its RPM and amperage calculation for required air flow. After that point it will fault so as to warn you that it is in a condition detrimental to it's life.

    I would not be surprised if there is some chip in there that when the motor gets sent back in for warranty they can download its data and find that it was run in an abused state, thus negating the warranty.


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  11. #11
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    The Rheem Mods, at least the older ones, flashed a code of 66 if the rev limiter was holding the speed back due to restriction. Saw a job where the furnace was 66 and the DATS of the zone panel had the fire shut down. Air Bear plugged with construction dust

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