Calculated demand does not match entered demand

Product(s): WaterCAD, WaterGEMS
Version(s): V8i, CONNECT Edition
Area: Output and Reporting

Problem

After computing a model, the calculated demand is different than the entered fixed demand. For example for a steady state, the total demand shown in the Calculation Summary does not match the sum of the demands in the Demand Control Center.

Solution

1) This could be due to a Demand Adjustment which manipulates the demand in a certain "Scope" (or a selection set). You can check that under Analysis > Calculation Options. Find the option "Demand Adjustments" or "Unit Demand Adjustments" and see if either of these is active. If so, that might be causing the difference in report demand.  For more about active demand adjustments, see "Option 2" in this article: Modeling scenarios with different demand conditions

2) For an EPS (Extended Period Simulation), the calculated demand for a node for a particular timestep is equal to the base demand times the multiplier for that time from the associated pattern (if not "fixed"), in accordance with the following: Demand calculation with Continuous vs Stepwise patterns

3) Check the Daily Factors and Monthly Factors tabs of the patterns assigned to your nodes. These are additional multipliers on top of the hourly multipliers.

4) Check if demands are applied to inactive elements. If you are counting the total demand as seen in the demand control center, some of the nodes may be inactive ("is active" attribute set to "false") which means they will not be considered in the calculations. Try creating a query with Is active = true, select those elements, then filter on selection in the demand control center before adding them up and comparing to your calculation summary. If this is the problem and you used LoadBuilder and wanted to assign the demands only to active elements, re-run Loadbuilder with a selection set of only active elements.

5) Make sure the calculation option "Hydraulic Time Step" is set appropriately. Try lowering this value.

See Also

How to edit demands, unit demands, or a subset of demands globally

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