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  1. #1

    Picking the right CFM for your A/C

    I had a contractor come to the house. I have an old house built in 1974. The ductwork is not the top design. It's a small house roughly 1400 sq ft. One of the major problem is the upstairs is not as cool as the down. It's almost like no air is getting upstairs. You can feel the air comming out of the vents upstairs but it's not as forcefull as down.

    The A/C is relatively new it's a Lennox HS26-261-1P with a matching evaporator coil (I believe this is 1.5 ton). The furnace is however original with the house and needs changing.

    A person that came to the house was telling me when putting in a new furnace with a bigger blower motor will solve a lot of the upstairs heating/cooling issues. However, I dont' know that is a good idea. Isn't the rule of thumb 400CFM for ever ton of cooling. Is it ok to go beyond this?????

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,708
    Bad rule of thumb.
    What says your moving 400 CFM per ton with your current blower.
    You could be moving 340, or 420, unless someone measured it by more then just holding their hand at a register.

    A larger blower couold also give you noise problems.
    Might just be a balance problem.
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  3. #3
    Quote Originally Posted by beenthere View Post
    Bad rule of thumb.
    What says your moving 400 CFM per ton with your current blower.
    You could be moving 340, or 420, unless someone measured it by more then just holding their hand at a register.

    A larger blower couold also give you noise problems.
    Might just be a balance problem.
    So what is the correct method.

  4. #4
    Join Date
    Nov 2007
    Location
    Tucson AZ
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    166

    Here's one plan

    1/ Get someone with a hood to measure the actual airflow coming from each vent when the A/C is running at its highest airflow setting. Write down the results.

    2/ Add up all the numbers to see if you are really getting the 600cfm expected for your 1 1/2 ton unit. If not, find out why.

    3/ Look at the numbers to see if the air distribution is the cause of the temperature extremes in the house. Do some balancing if needed.


    If you have the flow, but it just is not enough, then consider upsizing the equipment to provide more airflow.

    good luck with step 1!

  5. #5
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
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    4,584
    Hs26-261 is a 2.0 ton unit with C22-31 coil would be 2.5 ton , so first thing you should do is check coil for obstructions/like dirt, then check blower wheel and returns, to insure airflow and area of return is correct. Check the amount of ductwork and measure cfm at each register and find out what cfm blower is set.

    You stated these system were new; maybe used five years then installed there, but they are allot older.

    Best would be to look at over all condition and access what new style furnace with maybe variable drive could be install with newer coil.Better make sure of returns and supply air before this is done..
    "Everyday above ground, is a good day".
    "But everyday that you have made a difference in someones life, may insure you stay above ground a little longer".<aircooled>

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,708
    A manual J to find out if you have the correct sized A/C. And to know how many CFM each room needs. Then check how much air the blower is moving. And how many CFMs the rooms are getting.
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  7. #7
    Join Date
    Aug 2006
    Location
    Houston, Texas
    Posts
    21
    You also should have your ac tech ck the overall static pressure
    If he doesnt have a magahelic on his truck he is not a very good technician

  8. #8
    Join Date
    Feb 2006
    Location
    Tn.
    Posts
    1,344
    soounds like a job for the KY clan

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    Most lack of air flow to certain room issues can be solved,you need to call around and find one of the few that know how.

  10. #10
    Join Date
    Jun 2008
    Location
    Buffalo ny
    Posts
    8
    Find a company that will do a system performance test. This includes all the information that has been mentioned in the previous replies. They will verify the epuipment performance compared to design. A big part of this work will focus on the ductwork on the system. You need to verify the air volume and tempertures throught the total system with the proper instrumentation. If this is not done, you just don't know for sure what you really have.

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