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Q: Why does my CHW result in a lower heating energy costs (in reheat and DD/MZ systems)?


If the chilled water temperature, during a reset, is too high to produce the desired cold supply air temperature setpoint, then the latter is raised accordingly. To do so, a simple approach is used whereby chilled water and cold air temperatures are never allowed to differ by less than the difference between their design values. For example, if the chilled water setpoint is 45F(7.2C) and the cold supply air temperature setpoint is 55F(12.8C), then during a chilled water reset the cold supply air is always maintained at least 10F(5.6C) above the current chilled water temperature or at the actual cold supply air setpoint, whichever is higher.

If in your project, you have a chilled water setpoint of 40F, and a cold supply air temperature of 55F. That would leave a 15F difference.

If you reset your chilled water temperature to 47, then the lowest your cold supply air temperature is allowed to go is 62F.

As a result, there is less need for reheat in reheat systems during the winter and crossover months, as the temperature of the air stream upstream of the reheats is 62F, rather than 55F, and the DT that the reheats must effect drops as a result.

In a multizone system, since the cold deck temperature increases during reset, less air must pass over the hot deck, thereby decreasing heating coil load.

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