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  1. #1
    Join Date
    Jul 2008
    Posts
    23

    Is the existing duct good for a new 4 ton ac

    Currently, we have 14" main flex duct and 8" flex duct for outlets. Measurements were taken from outside sides so probably the inside airflow diameters are little smaller. We are doing an addition and the ac contractor said that we need a 4 ton after doing computer calculations. He also said that the existing duct is not big enough. We are on a tight budget.
    Is the existing duct enough for a 4 ton ac?

    Thanks

  2. #2
    Join Date
    Dec 2007
    Posts
    96
    If your main duct is 14" that is not even close for a 4 ton. That is the reason your contractor told you it was not big enough.

  3. #3
    Join Date
    Jul 2008
    Posts
    23
    Is the 8" outlets ok. We have 10 and adding another 2.

  4. #4
    Join Date
    Dec 2007
    Posts
    96
    That part sounds ok.

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    12 ,8" branch ducts may or may not be okay.Being they are all 5" ,it sounds like the duct system was not designed correctly,seldom if ever would all branch runs be the same size,as they are feeding different rooms/areas.

    What is the physical relationship between the indoor unit,existing home,and the adddition?how large is the addition?

    What size/tons is the old system?

  6. #6
    Join Date
    Jul 2008
    Posts
    23
    The existing house is around 1400-1500 square feet and the existing ac is 3 ton.
    The addition will add another 600-700 square feet.

    Thanks

  7. #7
    Join Date
    Jul 2008
    Posts
    23
    What would happen if I leave the duct work as is with the new 4.0 ton ac?

    Thanks

  8. #8
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    It will be noisy, you risk freezing up of the coil, they may have to overcharge to get the low side pressure high enough to avoid freezing which is hard on the compressor. The efficiency takes a dive so you'll really get much lower than the 13 SEER or whatever you think you are getting, even though you went 33% larger, you may not gain a lot in cooling due to the low airflow.

  9. #9
    Join Date
    Dec 2007
    Posts
    96
    1)could void warranty
    2)poor performance
    3)greatly reduce equipment life

    I would not pay to increase size of equipment if you are not going to correct ductwork. I run into this all the time. I know times are tight but people love to do additions but always seem to want to cut corners on the HVAC. Then they have spent money on something that they are not even comfortable in. Then they complain about it forever.

  10. #10
    Join Date
    Jul 2008
    Posts
    23
    I will try to at least replace the main duct.

    Thanks

  11. #11
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by GG123 View Post
    The existing house is around 1400-1500 square feet and the existing ac is 3 ton.
    The addition will add another 600-700 square feet.

    Thanks
    Ask if they can simply run an additional "trunk duct" from very near the indoor unit,out to the new branch ducts for the addition.

    Also new to provide a return duct or return path for the supply air to the addition.

  12. #12
    Join Date
    Jul 2008
    Posts
    23
    Will do

  13. #13
    Join Date
    Jul 2006
    Location
    NW burbs of Detroit
    Posts
    6,058
    Quote Originally Posted by dash View Post
    Ask if they can simply run an additional "trunk duct" from very near the indoor unit,out to the new branch ducts for the addition.

    Also new to provide a return duct or return path for the supply air to the addition.
    Man you gotta love flex duct...it's more a victim of hacks then metal will ever be.....I feel the OPs pain

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