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Thread: Ductwork Flex

  1. #1

    Ductwork Flex

    My old unit was a 2.5 and I recently upgraded to a 3.5 ton unit, my contractor recommended going to a 3.5 Ton unit to sufficiently cool the second floor and it does!

    But I hae noticed when the blower shuts down it sounds like the ductwork is flexing? It is possible the ductwork is slightly undersized if so what effect will this have on the system?

  2. #2
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    Yes its most likey under sized.
    Undersized ducts can cause liquid refrigerant to return to the compressor and damage the compressor.
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  3. #3
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    Yes its most likey under sized.
    Undersized ducts can cause liquid refrigerant to return to the compressor and damage the compressor.
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    How many times must one fix something before it is fixed?

  4. #4
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    Did they make any changes to the ductwork or just exchange the old unit with the new one? Alot of times even in new installations the installer will undersize the ductwork to cut costs so the 2.5 could have been slightly undersized to start with. Now you've added 40% more airflow need so I would have it looked at ASAP.

  5. #5
    Undersized ducts can cause liquid refrigerant to return to the compressor and damage the compressor.


    Can you please expand on this I do not follow the logic?

    The ductwork is all original

  6. #6
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    Your ductwork is original. In therory it was sized to move air for a 2.5 ton A/C, 1000CFM.
    Now you installed a 3.5 ton A/C, 1400 CFM on the same ductwork.
    Chances are it was only marginal for the 2.5 to begin with. Very common problem.

    Your ductwork is not allowing the proper amount of air flow. So not all the liquid refrigerant boils into a vapor in the evap where it should, because of low air flow. And some of that liquid can make it back to the compressor and damage it.
    Compressors don't do well with liquid refrigerant.

    If your duct was sized proper for the 2.5 tons 1000 CFM. The increase in pressure for the 1400 CFM of the 3.5 ton unit is out of range for a standard residentail blower. So its not moving the air the 3.5 ton needs.
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  7. #7
    So how can I be sure the liquid is backing in to the compressor? Leakage somewhere What should I look for to be sure this is not happening?

  8. #8
    The unit is equipped with a variable speed blower, which if I reduce the blower speed it will reduce the duct velocity.

  9. #9
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    Theres not a lot you can check yourself. You can look to see if the entire compressor is sweating. Thats not always a bad sign, nor is it always a good sign.

    Have your installer do a static pressure test, to see how much air the blower is moving. And how much pressure its working against. VS blowers don't last long working against excessive high static pressures.
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  10. #10
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    Quote Originally Posted by trebor1065 View Post
    The unit is equipped with a variable speed blower, which if I reduce the blower speed it will reduce the duct velocity.
    It will reduce duct velocity, and CFM.
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  11. #11
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    As beenthere has stated,chances are your ducts are undersized for the new larger system.

    High static will also cost more to run the indoor fan,another reason to be sure the ducts are corrected ,if they are undersized.

  12. #12
    To change the duct out will cost thousands due to it being an old house and the duct runs through the walls, My best bet is to get a service tech out to set the fan motor to run at a slower speed..

  13. #13
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    Quote Originally Posted by trebor1065 View Post
    To change the duct out will cost thousands due to it being an old house and the duct runs through the walls, My best bet is to get a service tech out to set the fan motor to run at a slower speed..
    There is a minimum air flow required,don't ask for less,IMHO.

    There are some ways to increase the air flow,that may not be that costly,plus util the test the static pressure,no one knows how high it is.Let us know if you have an interest.

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