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Metrix » Tuning
Q: Can you have negative constants in your regression equation?


SRC Systems' recommendations regarding tuning have changed in accordance with the latest thinking in ASHAREs GCP-14P Monitoring and Verification Committee.   In the past, we recommended that users not allow negative baseloads.  This is no longer necessary.

If tuned correctly, some buildings will require negative constants.  (The constant is the number multiplied by the number of days in the baseline equation.)  For example, the following building requires a heating balance point around 58 to 60 degrees.   If we respect the heating balance point as shown, then the tuning will yield a negative constant.

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The concern in the past was that when the independent variable is at or near zero, a negative constant could result in predicted negative baseline usage, which is not physically meaningful. 

If you tune with weather only, and have a  negative constant, you should billmatch any deselected points near the y-axis.  This should remove the threat of a negative baseload.

If you are tuning with user-created variables, then you won't have deselected points near the y-axis.  Then you must decide yourself, whether the tuning equation you have can ever yield a negative baseline.  Given the simple example below:>

Baseline kWh = -10 * #Days + 100 * # CDD + 1000 * #Widgets Produced

If there was a strike in the winter (no widgets produced, and no CDD), the Baseline kWh would be negative.   This is what we are trying to avoid.   If it is unlikely that there will be a strike in winter, then this could be an acceptable tuning.

One final note:  Metrix will not allow negative baselines.  If your tuning equation comes up with a negative number, Metrix will report 0 for the baseline amount.

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Otherwise feel free to contact our Tech Support staff at (805) 329-6565, or via email at helpdesk@abraxasenergy.com.

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