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MarketManager » Loads and Sizing
Q: I am modeling a dual duct system. The equation for cooling design loads presented in \'help\' uses the entire fan air volume in calculating cooling coil load, whereas only a portion of the fan air volume passes over the cooling coil in a dual duct system. Why does the program over-estimate the coil size?


We respectfully disagree, though it may over semantics, not engineering. First, it is true that the worst cooling case for the dual duct (and any) system is when all zones simultaneously require cooling at *their* design conditions, plus the amount of energy required to condition outdoor fresh air.

A dual duct system responds to varying zonal heating and cooling demands by adjusting the relative proportions of supply fan air that is cooled or heated. At the extremes, all supply air can be required to be cooled, or all supply air can be required to be heated, with no mixing. Neither extreme occurs frequently in practice. Nevertheless both are possible. Accordingly, MarketManager sizes its cooling coil to be able to cool all supply air to the cold air supply setpoint.

If the dual duct supply fan is oversized with respect to the sum of zonal supply air volume requirements, then, of course, this leads to the cooling coil to be oversized also, since even at design cooling conditions less than 100% of the supply air will be cooled.

In MarketManager, and in engineering practice that we am familiar with, cooling coils are sized to provide a desired supply air temperature drop (e.g., from 78 degF to 55 degF) at the design system air flow volume. In a dual duct system the design air flow in this case is defined as "100% cooling". We agree that, if the supply fan is oversized, then "100% cooling" is more than is required to meet the sum of all zonal design loads plus ventilation. But it reflects engineering practice.

If you disagree or follow a different practice, then we suggest the following steps to resolve the difference:

  1. autosize the supply fan volume on the dual duct system (making it equal to the sum of all zonal air flow requirements). Make sure that all zones' supply air flow requirements are set to their default values.
  2. Let MarketManager auto-size the cooling coil, and consequently the chiller size. Thereafter set the chiller size to "manual" to fix it at this size.
  3. Return the supply air flow volume to whatever it was in the first place (presumably greater than the value in (1)).

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