Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
TempFirst will drive down the Supply Air Temperature as the first step in the control strategy.
This will minimize fan energy, at the cost of greater reheat and chiller energy. Use this strategy for a Low temp VAV System. Low temperature refers the supply air temps 10 DBC or less to be true 'Low temp'.
The FlowFirst strategy will offload the chillers, at the expense of the AHU Fans.
The energy comparison you've shown may be skewed by the sizing details of the chilled water plant, cooling coils, AHU Fans, Supply air temperatures in the SetpointManager ranges.
For an effective Low Temp VAV system you need the chillers to be delivering cold water and the cooling coils to be sized for the corresponding temps. The chiller curves used could also confuse things - if you are extrapolating outside of the chilled water temperatures limits in the curves than the COPs could be erroneous. If you have hardsized any of these, than simply switching the SetpointManager may not lead to a fair comparison. Also, if you have autosized these, then you may not be operating the plant in the temperatures ranges to make the strategy efficient.
I would expect that the 2nd and 3rd scenarios should show roughly the same chiller+AHU energy, with chiller energy higher in the 2nd scenario. It would be interesting and perhaps helpful to see a plot (of one zone) Zone Air Temperature, Supply Air Temperature and Supply Air Volume Flow Rate for each scenario. While the Fan energy is lower in (2), as expected, the chiller energy is lower as well. That's the detail to focus on I reckon. The other big variable is the climate. The FlowFirst may make sense where the hours of economy cycle are high, and conversely, it could be a penalty in a hot and humid climate.
