FAQ

Last Revised: September 29, 1999

SRC Systems' recommendations regarding tuning have changed in accordance with the latest thinking in ASHRAE's GPC-14P Monitoring and Verification Committee.
Below are an up-to-date set of Tuning Guidelines:

Determination of M&V Approach

1. To use the Whole Building approach (Utility Bill Analysis), expected savings should exceed 10% of measured energy use or demand.

Tuning Periods

1. Tune bills for periods in multiples of roughly 365 days.

Balance Point

1. Use the outdoor temperature graph to determine balance point. Balance point should not deviate by more than a few degrees from the temperature indicated in the outdoor temperature graph.
2. The cooling balance point, however, should still always be below the indoor setpoint or below the economizer setpoint, if any.
3. Energy, Demand, and different TOU categories do not need to have the same balance point temperature.

Minimum Degree Days / Day and Excluded Points

1. Great care should be taken in selecting an appropriate minimum degree-day / day setting.  See the topic:  How do you select an appropriate minimum degree-day / day setting?
2. Tunings that include more than one variable should have minimum degree day / day settings of 0.
3. Deselect as few points as possible. However you may exclude up to 7 points out of 12 (via the minimum degree-day / day setting).
4. If more than 3 points are deselected due to the min DD/day setting, then all excluded points should be billmatched with a replicating modification.
5. If many points are excluded, make sure that the remaining points span both mild and severe seasons.
6. Excluded points should preferably be in sequential order (i.e., Dec., Jan., Feb.).

Manually Excluded Points

1. Points should only be manually excluded if there is a reason for it (one-time, anomalous conditions). Technically, the values of R² & T should not be used as sole criteria to decide if a point is "convenient" or not.
2. If you manually exclude a point, determine and document why the data does not follow the trend, (i.e., bldg. was evacuated, meter was broken). Manually excluded points for one-time, anomalous conditions need not be billmatched with a replicating modification.

Tuning Requirements

1. R² value > 0.75
2. |T statistic| >2
3. |Mean Bias| < 0.5%   (this may change)
4. |CV(RMSE)| < 25% for energy and |CV(RMSE)| < 35% for demand   (this may change)

Baseload

1. For one variable regressions, baseload (constant * #days) values should be close to bill values in which independent variable = 0.
2. Tuning baseload coefficient can be negative (if you use bill-matching modification for deselected points).

Multivariable Regressions

1. When in doubt, include fewer Variables.
2. User-Defined Variables should be normally distributed.
3. User-Defined Variables should be independent of each other.
4. Highs and Lows of a User-Defined Variable should span at least 2 to 1.

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