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  1. #1

    Blower speed turn down... why?

    Hey Guys, I'm new to the forum ( I've read the rules, I promise! )

    I'm a home owner, and the other day, our HVAC group that installed our new A/C and furnace came out to do a PM checkup on us. The installed the Furnace in 2011 and A/C in 2010 ( same installers ).
    Anyway... the Furnace is a Bryant 315AAV (110,000 BTU 80% variable speed )
    The A/C is a Bryant Preferred Series 127A I think, which I believe is 3.5ton, but I'm not at home right now to be able to be sure.

    We have a 2850 sq foot house above grade. We live in the north of Indiana.

    When he was doing his PM, he saw that we have a 5 ton blower installed, and said that it should be turned down because we only have a 3.5 ton A/C.
    I wasn't home at the time, my wife handled it so I wasn't able to ask any questions.

    He did go ahead and turn it down. I honestly haven't noticed any difference ( well, obviously difference in airflow )

    He said it would cool better, but I'm afraid I don't understand why that would be true. Wouldn't more airflow increase the efficiency of the heat transfer?
    I could see it costing more electricity to run the blower faster, but I don't understand why that would make it cooler.

    I understand why having an oversized A/C is bad, but why is having an oversized Blower bad?
    Can anyone educate me on the plusses and minuses?

    Also, in the winter won't this blower now not be properly sized with the Furnace?

    Thanks very much for any advice and education you can give me!

  2. #2
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,642
    the a/c speed is set separately from the heating speed, to high of a blower speed will not allow the system to dehumidify as well as it should.
    he did the right thing by matching the blower speed to the a/c size.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,808
    Too high of an air flow and it removes less moisture, allowing your indoor humidity to rise.
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    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  4. #4
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,826
    There are two systems to be protected by the blower. The first is the furnace. It has a Btu heat rating and also a minimum and maximum temperature rise. Some different model furnaces will have different blower capacities with the same furnace capacity. So as long as the airflow for heating is proper, that's the first protection needed. The second protection is to have the required airflow for the AC system. Having a 5-ton blower's okay but if the airflow is for 5-tons of cooling and you have only 3.5-tons, then the airflow is excessive and the air fails to stay in contact with the cold indoor coil long enough to get cooled to its dew point, thus failing to remove moisture. Carrier/Bryant defaults their units to 350 CFM/ton of cooling or in the example you've suggested, 1225 CFM. Then generally accepted norm for cooling is 400 CFM/ton or 1400 CFM in your case. The lower of those two volumes would do more moisture removal than the larger.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  5. #5
    Thank you everyone for your explanation. All of you gave great responses that helped me understand the situation. I appreciate it and I'm glad I asked the question!

  6. #6
    Join Date
    Feb 2009
    Location
    Tallahassee, FL
    Posts
    5,821
    There is a step forward you can take to verify the air flow in both heat and cool.

    A static pressure test can be done and compare readings with manufactures data. Also a temp rise test with furnace running

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,960
    The best way to do it is actually measure the airflow. Often ductwork is undersized and the blower needs to be on high speed to help compensate for poor duckwork. Only when ductwork is correctly sized (which is rare, at least in our area) should blower speed be reduced for lower loads. Blower wattage doesn't go down much from the lower speed, might save 5% of the A/C total power. Basically it's a tradeoff of cooling vs. humidity removal. Higher speeds give better cooling, lower speeds give better humidity removal.

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