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  1. #1
    Join Date
    Jul 2006
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    Over size supply\return duct question

    I understand the issues with undersized duct work, based on threads posted here. My question is:

    If a person replaced a residential 5 ton HP with a 4 ton, and the supply and return duct work was properly sized for the 5 ton unit, would there be equipment failure or comfort issues?

    This is just a question...I can't think of a real world scenario where this would happen, I'm just wondering.

  2. #2
    Join Date
    May 2008
    North Texas
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    Would you assume that the same cfm/ton was delivered?

    Same air duct diameter, 4/5 cfm of air, velocity fpm of air goes down, and friction loss goes down also. The fan curve sees less resistance, so equilibrium is reached at a different point on the curve.

    Assuming that fpm velocities have decreased somewhat, then how sensitive are the terminal throws to velocity reduction? If the throws were good before with some to spare, then no change. If marginal before and now drop in the middle of the room, then maybe will be noticeable.

    Further assume that the duct sizing was performed with an equal friction method.

    If 5 tons were oversized to begin with, then a 4 ton properly sized could even feel more comfortable by running for longer periods and removing more humidity.

    This scenario is possible with an energy efficient house where the contractor has sized to square footage without performing a Manual J. A later Manual J could show that 4 tons or even 3 tons is the correct sizing. At that point the ducts are already in. Then it would be time to backtrack to the curves for the air terminals for throw vs velocity.

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