Underestimated DHW energy consumption

Some care is required when entering data for DHW systems to avoid DHW energy consumption being underestimated. A number of factors can contribute to such issues.

DesignBuilder’s Simple Activity Data mode calculates water flow by dividing daily consumption by an assumed number of typical hours. This can cause peak flow rates to be underestimated if schedules or activity assumptions are inaccurate.

Also, autosizing tends to produce boilers sized to match modelled demand. If demand inputs are too low or schedules underestimate actual DHW draws. This can cause the equipment to run near or at peak load continuously. In these conditions, the efficiency curve applied may reflect ideal or flat efficiencies without capturing realistic part-load degradation, leading to overestimated operating efficiency and so, underestimating DHW energy consumption.

DHW underestimation typically arises primarily because modellers rely on autosizing in conjunction with potentially low or default DHW demand inputs, apply efficiency curves that do not reflect true equipment performance at part load, and use activity templates or schedules that underestimate actual draw volumes or peak loads. Without validation of inputs, equipment sizing, and operational assumptions, the energy use may be underestimated.

In summary, DesignBuilder and EnergyPlus simulate behaviours according to the model inputs and configurations provided. To ensure the best accuracy, modellers should validate autosized equipment against manual calculations, use realistic schedules, and ensure efficiency curves reflect actual equipment behaviour including part-load effects.

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