Can ducts be too big? - Page 3
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  1. #27
    Join Date
    Aug 2005
    Posts
    52
    [QUOTE]Originally posted by dash
    Somehow what I'm saying is not clear.

    We have hundreds of Carrier two speeds/stages,installed and there is NO PROBLEM.they work like a charm,"oversized" ducts "hehe!",and all.
    Dash,

    You are quite clear, and I believe you . I was just trying to rectify your experience with that of the other respondents who were worried about the inefficiencies of "oversized" ducts.

  2. #28
    Join Date
    Jul 2005
    Posts
    171
    Iginla, you need translation. Some of the pros are telling you that ducts oversized by 30% and more will work just fine. They are also telling you that the other pros (the ones who claim oversize is bad) are full of it. But they are saying it in a way as to not offend their fellow pros.
    Get it now?

  3. #29
    Join Date
    Aug 2005
    Posts
    52
    Dave,

    Got it . Thanks for the translation. I meant no offense to the other respondents, was just trying to understand the reason for differing points of view. Peace.

  4. #30
    Join Date
    Jul 2005
    Posts
    126
    Originally posted by dx
    Iginla, you need translation. Some of the pros are telling you that ducts oversized by 30% and more will work just fine. They are also telling you that the other pros (the ones who claim oversize is bad) are full of it. But they are saying it in a way as to not offend their fellow pros.
    Get it now?
    Thank you DX. You took the words right out of my mouth!

    After reading yesterday's responses, I reviewed Manual D to find out what it says about oversized ducts. The answer - NADA. But there's plenty said about undersized ducts. That's not to say there isn't good reason to 'right size' the ducts (notwithstanding the issue of multi-stage, which isn't addressed in Manual D).

    So I tried to think of every possible impact of having an oversized duct system. There are indeed some issues, but they pale compared to undersized. Here are some additional thoughts on the subject:

    First, understand we're talking about up to 40% oversize, not 100% or 200%

    On the negative side: Since larger ducts have larger surface areas, conductive losses will increase proportionally. If you've done your job right, ducts loses should be relatively small.

    Also, when the system cycles off there's more conditioned air "lost" in a larger duct system (% difference in diameter squared).

    Mitigating on the positive side: larger ducts will have lower velocities and thus lower leakage rates (again, not an issue if you've done your job right)

    THe air handlers used with most two-stage condensers are programmed to remain on for a short period (harvest).

    If the duct system is installed within the thermal envelope (as it _should_ be), none of the above are issues. Ok, I know this isn't often possible, but you can always hope!

    For two-stage systems, it is recommended that room diffusers be sized to the mid-point between the max and minimum airflow rates. The trade-off is throw pattern vs. noise. But keep in mind that throw patterns have become less important as homes have become tighter and better insulated.

    Finally, a side comment about setting system airflow rates for multi-stage systems: In humid climates (most of the country), the latent load will nearly always be higher during part-load conditions. For this reason, the blower should be set so as to increase the latent capacity during 1st-stage operation. In practice, this means a wider spread between the lowest and highest operating speeds than would otherwise be the case. For example, if the capacity is 3 tons on high, and 2 tons on low (33% reduction), the system airflow might be set to 1300 cfm and 750 cfm, respectively (42% reduction). This is only an example to illustrate the point.

    David

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