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  1. #1
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    Parallel fan power box

    I should probably know this, but at the moment I am at a loss.

    On a fan power box - parallel (or at least thats how it looks)configuration on a VAV system, should the fan run continuos during occupied mode, or should the fan only run during reheat operations?

    If it should only run in heat, is there supposed to be a damper to prevent back draft during cool mode?


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  2. #2
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    A parallel FPB is designed for the fan to be off during cooling. Yes there has to be a backdraft damper but if you pull the filter it is easy to tell if the box is parallel or series. If series you can see the supply duct terminate just inside the box which means the fan has to run continuously or the air will exit the filter. The supply on a parallel box is ducted all the way thru the box to the discharge and without a backdraft damper most of the air will blow back thru the fan when the fan is not running.

  3. #3
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    The filter blew out. Behind the filter all I see is the reheat coil, but right next to the reheat coil is the medium pressure supply connection. Not much access to internals to see exactly how it's laid out inside. Perhaps I could have removed the feed from the medium pressure supply for a better look. But, did not think of that till right now.

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  4. #4
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    And as usual with terminal units, there is only a worn out unreadable hand written data tag.

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  5. #5
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    If the filter blew out it sounds like the box is series unless you have a parallel with a stuck back draft damper. When I ran into stuck BDD's I just converted the box to operate as series. They work fine that way and the owner will love you unless you have an inherently noisy box. The filter being in front of the reheat coil sounds like you have a hot water coil. If you can't see the inlet duct the easiest way to tell what is going on is to command the fan off and open the box damper wide open. If air exits the filter convert the controls to series operation.

  6. #6
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    Parallel fan power box

    Yes, hot water reheat.

    My guess is the stuck or not fully closing BDD.

    My first inclination was that it would make more sense to have the fan run all the time during occupied mode.

    But, then I had a debate in my head whether when in cooling it would mix too much plenum air or not.

    My original thinking was that if the fan ran all day, when the cooling damper closed, it would recirc plenum air, and in cooling it would take the vast majority of air from the cooling supply as that pressure is higher then the plenum air...energy being lazy and all, but that theory only holds if the blower is in series with the supply air, and not in parallel.

    So just to be clear - either way you would just re-control it and call it a day?


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  7. #7
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    You have to set up the controls and the FPB as a series box. The first thing I did was to drill a hole and make sure the BDD was fully open and made sure it stayed that way. Then you drive the box primary damper to full cool and adjust the fan so the filter is neutral. Put a piece of 8.5 x 11 inch printer paper on the filter and speed the fan up until the paper just slides off. The box will not pull in plenum air unless the stat is satisfied and that is exactly how a series box works. It is not worth the time and effort to fix the BDD unless the owner wants it done. I always advised the owner what I was going to do and made sure they were OK with it but not one of them wanted to fix the BDD.
    Some boxes will make the fan run backwards unless when started up the fan starts prior to opening the primary damper so make sure the controls start it that way anytime you go from unoccupied to occupied.

  8. #8
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    Quote Originally Posted by WAYNE3298 View Post
    You have to set up the controls and the FPB as a series box. The first thing I did was to drill a hole and make sure the BDD was fully open and made sure it stayed that way. Then you drive the box primary damper to full cool and adjust the fan so the filter is neutral. Put a piece of 8.5 x 11 inch printer paper on the filter and speed the fan up until the paper just slides off. The box will not pull in plenum air unless the stat is satisfied and that is exactly how a series box works. It is not worth the time and effort to fix the BDD unless the owner wants it done. I always advised the owner what I was going to do and made sure they were OK with it but not one of them wanted to fix the BDD.
    Some boxes will make the fan run backwards unless when started up the fan starts prior to opening the primary damper so make sure the controls start it that way anytime you go from unoccupied to occupied.
    Great info Wayne. I will look into it, the controls is the tricky part. We put the stuff in, but its a little touchy with the billing - full coverage on the contract except for controls.


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  9. #9
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    Normally the controls part is to simply select which kind of box you have. That selection should be all you have to do.

  10. #10
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    Just wondering if you have gone back to this box.

  11. #11
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    Quote Originally Posted by WAYNE3298 View Post
    Just wondering if you have gone back to this box.
    No, I have been back to the building putting out other fires though.

    One of those back burner issues at the moment.


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