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  1. #1
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    Drop Ceiling as RA Plenum

    Hi all,

    Curious of any and all opinions about using drop ceiling as a return air plenum. I know it can and has been done but curious whether certain situations would not allow.

    Long story short my company was asked to move several ducts in order to install a higher drop ceiling. Concrete slab ceiling above drop ceiling which is around 20% of store floor space and contacts only one 4" block exterior wall. Single story building and is fairly well sealed. Flex was used for RA ducts and is not equal area to original return plenum. Currently moving 2720 cfm on an 8.5 ton RTU.

    Using drop ceiling is not my only option but would be the quickest fix.

    Thanks in advance





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  2. #2
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    The biggest problem I see is that the registers closest to the RTU will flow much more than the furthers ones. You'll probably get calls for lack of cooling/heating in the areas furthest from the RTU.

  3. #3
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    Quote Originally Posted by HVAC_Marc View Post
    The biggest problem I see is that the registers closest to the RTU will flow much more than the furthers ones. You'll probably get calls for lack of cooling/heating in the areas furthest from the RTU.
    Store's layout is wide open. Existing return plenum that was removed landed in one place with no takeoffs in the center of the store. We have not had temperature complaints at this site in the past but predict temperature issues in the future for reasons I dont want to get into.

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  4. #4
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    I see drop ceilings and plenum return all the time... yesterday as a matter of fact. There may be code issues with it though... Wiring and things may need to now be "plenum rated". It's definitely doable, just check into it before there may be any trouble.
    -----Stop, step back, relax and have another go at it.-----

  5. #5
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    Quote Originally Posted by Guyinfield View Post
    Store's layout is wide open. Existing return plenum that was removed landed in one place with no takeoffs in the center of the store. We have not had temperature complaints at this site in the past but predict temperature issues in the future for reasons I dont want to get into.

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    Somewhere my reply vanished...

    The store layout doesnt matter. The inlets closest to the RTU will ge the most air, the ones furthest away, the least air to the RTU. The supplies furthest away from the RTU will supply the least as well due to the lack of negative static pressure there. You'll probably have complaints of hot/cold spots in the store.

    What Benny said, too. AND, you cant have any other flue vents in that space at all. So, if they have a hot water tank, it has to be changed to direct vent. The same goes for any other fuel burning appliances.

  6. #6
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    Quote Originally Posted by BennyD View Post
    I see drop ceilings and plenum return all the time... yesterday as a matter of fact.
    Stop shopping at Dollar General for your contraceptives and you wont see that anymore.

  7. #7
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    Quote Originally Posted by HVAC_Marc View Post
    Somewhere my reply vanished...

    The store layout doesnt matter. The inlets closest to the RTU will ge the most air, the ones furthest away, the least air to the RTU. The supplies furthest away from the RTU will supply the least as well due to the lack of negative static pressure there. You'll probably have complaints of hot/cold spots in the store.

    What Benny said, too. AND, you cant have any other flue vents in that space at all. So, if they have a hot water tank, it has to be changed to direct vent. The same goes for any other fuel burning appliances.
    Thanks for the clarification. Luckily electric water heater. I will try to post pictures but basically the drop ceiling in question is only a small portion of the store's ceiling. The majority is open ceiling with exposed ductwork. For better or worse we can not bring additional return duct into that area per request of the customer. Any return, ducted or through the grid ceiling would have to be within 15ft of the roof penetration for the RTU.

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  8. #8
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    For better or worse we can not bring additional return duct into that area per request of the customer.
    Then put it in writing that due to their requests that the performance may not be the same as a ducted system. After that, it's on them.

  9. #9
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    This is how it goes when the budget guy acts as the executive GC

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  10. #10
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    Quote Originally Posted by HVAC_Marc View Post
    Stop shopping at Dollar General for your contraceptives and you wont see that anymore.
    Really?! Lol
    -----Stop, step back, relax and have another go at it.-----

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  12. #11
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    This is a cheap construction technique that is less effective.

    The roof heat radiation heats up the entire air mass above the drop ceiling, and the unit has to heat that air continuously, and this raises the supply temp, and slows the entire cooling process, to the point where the system has difficulty keeping up.

    And a HUGE waste of power.
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  13. #12
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    This is not even legal in southern cal...for good reason

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