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  1. #1
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    Running a blower in reverse

    Ive been told running a forward curve blower in reverse will yield roughly 50% flow

    How accurate is that?


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  2. #2
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    I think it would be somewhat difficult to calculate. A forward curve fan is pushing the air towards the outlet of the squirrel cage. Were you to reverse the rotation, the fan would be throwing the air in the opposite direction of the squirrel cage outlet. The result would be the air moving thru the outlet because it has nowhere else to go. I imagine there would be a ton of turbulence making it difficult to accurately measure performance.
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  3. #3
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    For some reason, about 70% comes to mind. But I don't remember where that number came from.
    In honor of RichardL: "Ain't 'None' of us as smart as 'All' of us".

  4. #4
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    Quote Originally Posted by heatingman View Post
    Ive been told running a forward curve blower in reverse will yield roughly 50% flow

    How accurate is that?


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    That's an interesting question @heatingman. Where is this going?

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  5. #5
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    Quote Originally Posted by icy78 View Post
    That's an interesting question @heatingman. Where is this going?

    Sent from my SM-G950U using Tapatalk
    It came up in a conversation with a coworker about a burner I was having strange performance issues with.

    With power burners, airflow capacity has a large influence over burner capacity. IE the more the airflow, the larger the volume of gas you can consume and still maintain combustion quality.

    The issue I was having was that the gas manifold pressure was much lower then stated pressure, like .6” compared to 3.2 rating yet the combustion was running very low O2 with a wide open damper.

    Its most likely the wheel is very dirty reducing air, and the burner had been derated on fuel to compensate.

    But he meantioned that the blower could possibly be running reverse, then he said doing so would cut airflow in 1/2.


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  7. #6
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    Airflow with reverse rotation is around 60%. With the bigger fans you can normally tell by the sound if the fan is running backwards.
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  8. #7
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    I once replaced a furnace blower with a universal motor. Running in reverse, it felt like 25% or so. It was not much airflow. I wish manufacturers could agree on which end of the motor they use to determine rotation.

    Anyway, it took a long, long time for the blower to come to a rest after running backwards. This amount of time vs the amount of time to come to rest after correct rotation, along with some math, would probably be a way to get an idea of percentage of output. It's not going to be simply a matter of division, though.

  9. #8
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    Quote Originally Posted by Rovert View Post
    I once replaced a furnace blower with a universal motor. Running in reverse, it felt like 25% or so. It was not much airflow. I wish manufacturers could agree on which end of the motor they use to determine rotation.
    I'm not implying this about you. If a technician doesn't know which way the fan blade is supposed to rotate, the motor rotation being cwse or cwle is a moot point.
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  10. #9
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    Quote Originally Posted by 2sac View Post
    I'm not implying this about you. If a technician doesn't know which way the fan blade is supposed to rotate, the motor rotation being cwse or cwle is a moot point.
    I knew very well which direction the blower needed to spin. The universal motor, however, only had cw/ccw settings, no arrows or indication as to shaft end or drive end as the reference for direction. It was quickly and easily remedied.

  11. #10
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    My comment was concerning commercial fans. I don't know how much air a backwards resi will move. Never measured airflow with one of them running backwards.
    No man can be both ignorant and free.
    Thomas Jefferson

  12. #11
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    Quote Originally Posted by Rovert View Post
    I knew very well which direction the blower needed to spin. The universal motor, however, only had cw/ccw settings, no arrows or indication as to shaft end or drive end as the reference for direction. It was quickly and easily remedied.
    Again, wasn't implying my comment towards you. There are a number of techs that have issues discerning rotation, primarily on paddle type fans and backwards curve fans.
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  13. #12
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    40 - 60 % when running backwards .

  14. #13
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    Fans running backwards have low airflow and high current draw.
    No man can be both ignorant and free.
    Thomas Jefferson

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