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Thread: Question about secondary supply plenum

  1. #1
    Join Date
    Feb 2021
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    Confused Question about secondary supply plenum

    Hello Folks,
    I just had the Heat Pump installed in my house recently. The installation included all the ductwork for the house. Unfortunately, I am experiencing uneven air flow in the house. It is a split level house (2500 Square Feet, I have 3.5 ton, Variable speed Air Handler). The Air handler is located on the attic. I have two attics and it is connected through a space big enough of for a window (3.5 ft X 5.5 ft). Air handler is placed in the attic above the bedrooms and the connected attic has the kitchen, dining room and living room underneath it. The uneven air flow is experienced for the supply lines from the connected attic. I am getting great deal of air for all the bedrooms/bathrooms and hardly getting any air for dining room, kitchen and living room. I had gone through all the ductwork. It seems that I have two supply plenum. One is attached to the air handler and is feeding all the bedrooms/bathrooms. There is a branch coming out of the supply plenum air handler and going to a secondary supply plenum which is placed in the second attic. The second plenum has four supply lines coming out and is supplying the air for dining room, kitchen, and living room. I had never seen a setup where there is two supply plenum, and I had quested the contractor and he said it is perfectly fine. Is this normal?

  2. #2
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    I'm not quite picturing what you're describing but it's not uncommon to not have perfectly even airflow but it should be reasonably close. Was the design engineered or did the contractor just wing it? There should at least be manual dampers for you to balance the airflow and if it's new ductwork that should have already been done. You are going to really feel that bad airflow when the cooling season gets here.

  3. #3
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    Feb 2021
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    Thread Starter
    Quote Originally Posted by Sooty View Post
    I'm not quite picturing what you're describing but it's not uncommon to not have perfectly even airflow but it should be reasonably close. Was the design engineered or did the contractor just wing it? There should at least be manual dampers for you to balance the airflow and if it's new ductwork that should have already been done. You are going to really feel that bad airflow when the cooling season gets here.
    Thank you for responding back to me. I do not know if they had the design engineered or wing it. It seems like they just winged it. Anyway, I had a drawn up a rough diagram, please take a look and let me know if that helps. The diagram only shows the supply lines.
    Attached Images Attached Images  

  4. #4
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    That's a new one for me I don't see how it would work properly. Maybe someone else has run into this two attic problem before and I'm wrong but to me there doesn't seem to be any directing of the airflow at all.

  5. #5
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    Jun 2003
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    similar to some flex duct designs here but there wouldnt be that many straight runs off the ah plenum the trunk going to the second attic prolly needs to be bigger and or the straight runs dampered down. obviously wasnt designed correct.

  6. #6
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    Flex line to trunk with the 4 runs, is probably too small.
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