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Thread: Running a blower in reverse

  1. #21
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    I tested one boiler fuel oil pump that was running backwards but all I remember is the flow was really low.
    No man can be both ignorant and free.
    Thomas Jefferson

  2. #22
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    Thread Starter
    The blower style in question is similar to a blower found in a residential furnace.

    Just alot more robust. Similar housing and wheel style.


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  3. #23
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    Centrifugal pumps will load up because of check valve(s). Fans do not have checks so the blade Unloads and amperage drops

  4. #24
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    I'm not sure why anyone would need to know how many cfm a forward curve fan will move when it's rotated backwards? But I am sure why a tech needs to know how to determine proper rotation, and why. First, don't be fooled by rotation marks often found on a fan housing, or forcing cones. The knuckleheads at the factory occasionally get it wrong. A forward curve fan will appear to many techs to be rotating the wrong direction when it is actually moving the correct direction.

    Here's one example of why forward curve fans must rotate the proper direction...…..

    On variable volume air handling systems, series fan powered variable air volume terminal units use fractional horsepower single phase motors and forward curve fans. If the system air handler is started before the fans in the vav's, the forward curve fans will be forced to rotate backwards. If you manually rotate a small single phase motor backwards, and then energize it, it will run backwards. Running a single phase motor backwards will generate high amps and heat, and if forced to run this way, will eventually tend to end, or greatly reduce, the service life of the motor. This can get expensive, not to mention the system comfort issues, and customer complaints.

    For another discussion...…. Remember there are many different fan designs. They each have their own uses, purpose, and characteristics.

  5. #25
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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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    Just for clarity, are you talking the motor should be going CW and it is running CCW or are you saying the wheel was removed and put in backwards? 2 different situations but don't know if the change in air flow is different between the 2 and realize that the difference from normal is drastic.

  6. #26
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    The subject is the motor running backwards. The fans will deliver 50 to 60% of design airflow.
    The purpose for knowing this is when airflow is below design the reason needs to be known so the problem can be corrected.
    No man can be both ignorant and free.
    Thomas Jefferson

  7. #27
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    Quote Originally Posted by WAYNE3298 View Post
    The subject is the motor running backwards. The fans will deliver 50 to 60% of design airflow.
    The purpose for knowing this is when airflow is below design the reason needs to be known so the problem can be corrected.
    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?


    Sent from my iPhone using Tapatalk
    Actually nothing was said about the motor, it was said the fan was running backwards which is why I asked the question. Also I understood the reason for the original question, just wanted clarity on which of the 2 scenarios the OP was talking about as either will result in similar symptoms/effects. On a burner as the OP was talking unless it was 3 PH I don't believe they are reversible so what would cause the motor to reverse? It is not like the blower on a furnace where if something causes it to start turning backwards and is then energized it will usually continue in that direction. There is however the possibility that someone removed the wheel to clean it and put it in backwards as the screw is accessed through a notch in one blade to access the set screw. Where as with a furnace blower if you put the wheel in backwards you can seldom access the set screw.

  8. #28
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    They do manufacturer reversible single phase hp motors. There is no hard and fast cfm, or percentage that you can quote for the diminished performance of a FC fan rotating in reverse. Too many variables, ductwork, size, application, etc.,...…. You need to learn what the correct rotation for a forward curve is, and take a look at the fan when it's energized. Yes, depending, it is possible that someone thought the fan was turning backwards and turned it around on the shaft. For the inexperienced, it's easy to think that this particular fan style is running backwards.

  9. #29
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    Most often when a fan is running backwards the motor is running backwards. I'm not saying that has to be the case but it is predominate.
    I have tested thousands of fans and when a forward curved fan is running backwards the airflow measures around 60% of design if there are no duct issues. I have never seen that written down but have tested several fans running backwards with % flow results very close to each other. They also draw a lot more current when running backwards.
    With pneumatic controls it was common for series FPB fans to run backwards. There were measures at times to prevent this buy no method I know of could for sure total prevent backward rotation. DDC controls employ control strategies that will and do prevent the backward rotation.
    No man can be both ignorant and free.
    Thomas Jefferson

  10. #30
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    The solution, (pneumatic, or DDC), (inlet vanes, or a VFD), to preventing series fan powered boxes motors from running backwards, is to never allow the AHU fan to pressurize the main ductwork before you have the systems VAV's up and running. Back in the day, you used to see those fans running backwards because the people there before you didn't understand, or care to learn how the equipment was designed to operate. So.... we had unnecessary comfort issues, and equipment failures.
    Today the issue is eliminated, for the most part, by programming start delays and auto zero functions into the system. That is, if the person programming the system understands the system they're programming. Professionals understand the differences between a series, or parallel FPB. And they understand those differences from a straight VAV, to a variable volume fan on a variable volume terminal unit.

    Professionals use the word sheave...…..not pulley.

  11. #31
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    My experience is somewhere in the 40 to 60% airflow range. I would round both of those to 50%. But most TAB is roughly +/- 10%.
    If "I have always done it this way" is a good reason to do it again, how many times do I have to do something wrong - before it becomes right?

  12. #32
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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.
    I once had a motor with direction referred to a ccw or cw OPE with OPE meaning opposite power end. Kind of ambiguous as to whether the power end is the shaft or lead?
    -Marty

  13. #33
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    I have been told 33% flow, felt like 25%. It varied with fan design.

  14. #34
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    Trane, in their fan book, lists forward curve fans at 60-68% efficient and Backward incline fans at 75-80%.
    I don't think that would necessarily mean a FC fan was comparable with a BI in terms of calculations because a BI has to run at higher speeds to develop required static pressure even while using less HP..
    If BHP was calculated in normal rotation and the CFM was known, the BHP in reverse could be measured and the CFM calculated using fan laws if curious. Maybe. Pie R Round.
    Many pump impellers are like BI fans but if run in reverse there is a lot less flow and high power use.
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