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Sizing Secondary Systems

  1. Aug 5, 2004 #1
    I would like to get some opinions on secondary circulator and piping design for heating system.

    System Description:

    Two (2) boilers each one is 75% of building total heating load. Each with a secondary circulator pumping to a manifold into a primary loop. The return piping comes from a second manifold adjacent to to supply manifold.

    Primary loop with two (2) parallel circulators moving the heating water 100% of the heating load using a 20 deg F TD to the building and through the equipment.

    Okay, the boiler manufacturer suggests the formula:

    Gross Output BTUH/(Temp Rise x 500) = Secondary gpm

    Temp Rise = 230 deg F - Return Water Temp

    I know that the boiler can handle higher flows than will be obtained through this formula (with a 70 deg Temp Rise and a 515,000 btuh boiler I only get 14.7 gpm).

    So which is better, use less water at higher temperature, or use more water at lower, closer to system, temperature? Or is it best to figure a compromise and go with that? Say a 40 deg F or 50 deg F temperature rise in the boiler?
  2. jcsd
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