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Avatar photoSylvanLMP
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    Rodney BECAREFUL.

    I normally agree with my good friend Harold but there is a lot more involved then just replacing pipe and fittings.

    Circulating systems.

    The most common failure seen in circulating
    systems is erosion/corrosion. Erosion/corrosion is a mechanically induced
    failure which may be caused by any or all of the following conditions:

    1. Water at high velocity. High water velocity may be caused by an
    undersized piping system or an oversized pump. One of the interesting myths
    here is the notion that a return line failed because Type M copper was used,
    so the repair is to install Type L copper. Since Type L copper has a smaller
    ID than Type M copper, the velocity in the system is actually increased,
    raising both the likelihood and rate of erosion that may take place. The
    real solution is to lower the water velocity. Installation of a smaller
    capacity pump or a throttling bypass on the existing pump should help in
    lowering the velocity of the water in the system. The relationship is as
    follows: Reduce pipe size, increase velocity; increase pipe size, reduce
    velocity. Recommended velocity for hot water in a copper tube system is 4 –
    5 feet per second (fps). If systems are designed to respect these velocities
    the return lines will last as long as the rest of the system. The practice
    of reducing tube diameter for the return line run is counter-productive and
    is the main cause of high velocity in these systems. Since there is no
    sizing guide in the Model Codes for return lines, there is little guidance
    for the plumber on how to size this portion of the system. In the course of
    installing or replacing circulating lines, some contractors have been
    increasing the size of the return loop piping to 1″ or at least the same
    diameter as the hot water supply out of the water heater.

    2. Numerous, abrupt changes in direction in the piping system. Where
    structural conditions cause numerous directional changes, long radius (1.5
    Diameter) fittings should be used to minimize the interruption of laminar
    flow. Pump manufacturers also have recommendations that limit the number of
    changes in direction near the pump and the minimum distances these can be
    installed from the pump. This is to protect the pump from these same
    erosion/corrosion processes (cavitation). This is the main reason why you
    would not use a corrugated connector on these pump installations. Flow
    characteristics, the number and severity of the bends that could be made in
    these connectors may be detrimental to the pump.

    3. Lack of reaming the tube ends. Burrs left on the ID of the tube can
    cause interruption of laminar flow resulting in localized high water
    velocity and cavitation.

    4. Protrusions into the flow stream caused by excessive lumps of
    solder/brazing material, improperly fabricated tees (branch protruding into
    the run pipe), etc. These protrusions can also cause the
    interruption of laminar flow resulting in localized high water velocity and
    cavitation.

    5. Excessive water temperature. Heating water above 140 degrees Fahrenheit
    can accelerate the process of erosion/corrosion. As the temperature
    increases, the velocity should be lowered. At 180 degrees Fahrenheit, the
    velocity should be down around 2 – 3 feet per second (fps).

    Bear in mind that these conditions are not peculiar to copper tube, but can
    affect other materials as well. However, when erosion/corrosion does occur
    in copper tube it is readily identifiable by the horseshoe shaped pitting
    throughout the inside of the tubes. Identification OD these pits or grooved
    corrosion-product-free areas on the inside of the tube can help you
    determine that water velocity is too high and that the reduction of velocity
    can correct the condition.

    I would strongly suggest you vist Harolds web site of contact your local Licensed Master plumber to properly design these systems.

    [Edited by Moderator on 26 September 2000]

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