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  1. #1
    Join Date
    Jun 2006
    A hot topic seems to be replacement unit sizing, and I am wondering how critical size is if you are running a two stage compressor, and 2 stage furnace with a variable speed blower? Don't all those things tend to accommodate a slightly oversized system, maybe even with slightly undersized ducts?

  2. #2
    Join Date
    Jun 2006
    If your spending on that stuff make sure it is properly sized.

  3. #3
    Join Date
    Jun 2004
    4H: Hot, Humid Houston H.O.

    BTUs one thing, ductwork maybe another

    Danglerb, how you figure that VS works better with undersized ducts?

    I can see your point that a two-stage AC if oversized, will continue to spend most of its time on low stage. Just slightly less than if it were properly sized. Given the reluctance of techs to do load calcs, that technology may be the best hope we have in the real world. IMO.

    But if a duct system is dysfunctional, it is dysfunctional. The VS blower will still go up to 0.9 ESP or higher trying to push its set amount of air through the ducts. If the ductwork is too small, it still will be exceeding the Manual D speed limit (700 flex) and make excessive noise. Granted, it will only occur on high stage, whereas a single stage AC will exhibit this dysfunction every minute it is on. Maybe that is what people mean by "working better" with poor ducts.

    It is my opinion that ductwork is not something to be taken lightly. Many homes have undersized ducts and oversized ACs, and that problem can be much reduced by using the correct size AC.

    Hope this helps -- Pstu

  4. #4
    Join Date
    Jun 2006
    Actually I am wondering if some of these advancements aren't in response to so many improperly sized systems.

    I'm not suggesting or advocating anything either, just curious about the consequences of some sizing and replacement choices, not the wisdom behind them or the overall merit.

    Seems like (notice I put in all the weasel words to let people know I am just speculating not stating facts) the variable speed blowers force the air through even if the vents aren't ideally sized, and like you say that can make noise.

    I kind of miss my old dorm room, sounded like somebody was taking a shower when the AC came on. After a while the white noise made it easier to sleep.

  5. #5

    It's all interconnected

    IMO, variable speed blowers and staged compressor speeds are there to save you on electrical and running costs, plus keep you comfortable at lower loads.

    In addition to that, an undersized or oversized AC will only mean more harm than good even with the features mentioned previously. At least your CAPITAL cost will be impacted by oversizing it (e.g. by buying a larger AC but trying to run on low speed all the time) even if your electrical cost won't be.

    Undersizing AC is not good.

    The best bet is to size it properly based on your average load, and you'll be having good night sleep without someone taking shower in your duct

  6. #6

    Too oversized LOWERS seer ?????

    The best argument for 2 stage equipment, slightly oversized for 2nd stage, is the OPTION (if the ducts can handle it) of a faster recovery time with a setback thermostat.

    But there becomes a point of oversizing where if your ducts aren't the correct size, even if you don't burn out your equipment, your SEER will be reduced.

    The SEER calculations assume VERY VERY GOOD (450 cfm ???) airflow.

    Pro's am I correct (generally) on this ?????

  7. #7
    Join Date
    Jun 2001
    Louisville, KY
    Carrier SEER is calc'd at 350 or 400 cfm per ton -- depends upon the model.
    Perhaps you should have read the instructions before calling.

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