Why does the DHW consumption predicted by DesignBuilder differ from my calculations?

One reason for differences between DesignBuilder Version 1 DHW output and simple calculations may be down to the way DB calculates the m3/s hot water consumption used as input for E+ from the l/m2-day data provided in the interface. The m3/s value supplied to E+ is calculated as:

m3/s = l/m2-day x 0.001 x Zone floor area / (TypicalHoursPerDay * 3600)

The above m3/s is multiplied by the value of the DHW schedule for each timestep in the simulation.

If you use a Compact schedule to define DHW operation DB cannot correctly calculate TypicalHoursPerDay and so uses a 'typical' assumed value of 8 hours in the above equation. You should be able to get better results by redefining the DHW operation schedule using 7/12 schedules which are processed to accurately determine 'TypicalHoursPerDay'. Note that the TypicalHoursPerDay calc is done for the Monday in January and applied to the whole year so weekends will not be exactly represented in the calculation of maximum m3/s.

The post-process conversion from m3 hot water consumption output by the simulation to kWh displayed in DB is defined at:

https://www.designbuilder.co.uk/helpv5.0/#Calculation_of_DesignBuilder_Output_from_EnergyPlus_Report_Variables.htm

This uses a standard m x cw x dT equation.

The problems described above stem from the way standard hot water consumption rate data is provided in the UK National Calculation Methodology (i.e. l/m2-day) and implemented in DesignBuilder. We plan to include an option for an alternative (more direct) way to define DHW consumption in future versions to avoid this issue.