Why is the heating design load greater than the value given by the simple UAdT equation?

In the most simple heating design calculation, one might use an equation similar to:

Heat load = U x Area x (T_air_int - T_air_ext)

The DesignBuilder Heating design calculations use EnergyPlus running in a steady-state mode and improve on the above basic equation in these ways:

1. DesignBuilder uses an 'environmental' or 'radiant star' inside zone temperature instead of the air temperature to model heat transfer between zones and outside. This will be somewhere between the air and the radiant temperatures..
2. In DesignBuilder, heat is also lost by radiation to the cold night sky and this is not accounted for by the single outside air temperature.
3. Surface heat transfer coefficients for walls, roofs, floors etc and windows are calculated within the EnergyPlus simulation based on inside and outside temperatures, surface orientation and wind speed so the u-values shown in the DesignBuilder constructions and glazing dialogs are illustrative only and are not used in the calculations.